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Edit Comment Close Premium member Presentation Transcript FUNDAMENTALS OF Reaction MECHANISM : FUNDAMENTALS OF Reaction MECHANISM CLEAVAGE OF COVALENT BONDS1.Homolytic Cleavage : The bond breaks in such a way that each atom departs with one electron of the shared pair . A .. B → A . + B . Free Radicals Br .. Br → Br . + Br . Homolytic cleavage is possible only when the electronegativity of both the bonded atoms is same. CLEAVAGE OF COVALENT BONDS1.Homolytic Cleavage 2.Heterolytic cleavage : The covalent bond is broken in such a way that one of the atoms carries away both the electrons of the shared pair.This results in the formation of a cation and anion A .. B → :A- + B+ (When A is more electronegative than B) A .. B → A+ + B-: (When B is more electronegative than A) CH3 CH3 CH3___ C ___ Cl → CH3___ C+ + Cl-: CH3 CH3 2.Heterolytic cleavage TYPES OF ATTACKING REAGENTS : 1.Nucleophilic Reagents are the reagents having at least one lone pair of electrons.They are nucleus loving in nature and are called nuleophiles.Nucleophiles are capable of donating electron pairs,hence they are Lewis bases.They are of two types:- Negative nucleophiles (:Nu-) They carry an excess of electron pair and are negatively charged.e.g. F-,Cl-,Br-,OH-,CN-, R3C-,COO-,NH2-,R- TYPES OF ATTACKING REAGENTS Neutral nucleophiles(:Nu) : They also carry an excess of electron pair but are neutral in nature.e.g. R–OH,H–OH,RSH,ROR,NH3,RNH2 Neutral nucleophiles(:Nu) 2.Electrophilic Reagents : A positively charged or neutral species which are electron deficient and can accept a pair of electrons is called electrophilic reagent. They are electron loving in nature.Electrophiles are capable of accepting electron pairs hence they are considered as Lewis acids. Types: Positive electrophile(E+): They carry positive charge. e.g. H+,Br+,Cl+,NO2+,NH4+,H3O+ 2.Electrophilic Reagents (b) Neutral Electrophiles E: : Neutral Electrophiles are electron deficient species which do not carry positive charge. e.g.BF3,AlCl3,ZnCl2,SO3 (b) Neutral Electrophiles E: Inductive Effect : Definition:- Permanent displacement of bonded electron pair of a covalent bond towards the more electronegative element,whereby polarity is induced on the carbon atom and substituent attached to it. I I ı ___Cδ+''___ Cδ+'___Cδ+___Clδ- I I I Inductive Effect -I Effect or electron withdrawing effect : This effect is exerted by the groups attached to the carbon chain having high electronegativity than carbon.Some of the examples of the groups having –I effect are:- NO2 >CN>COOH>F>Cl>Br>I>OCH3 >OH -I Effect or electron withdrawing effect + I Effect or electron releasing effect : This effect is exerted by the atoms or groups attached to the carbon chain having lesser electronegativity than carbon. Some of the examples of groups having +I effect are : C6 H5 O- >COO ->CR3 >CHR2 >CH2 R>CH3 >H + I Effect or electron releasing effect Applications of Inductive effect : 1. Dipole effect:- Inductive effect leads to dipolar character in the molecule.More is the inductive effect,greater will be the dipole moment.Example CH3 ,CH3 Br , CH3 Cl -I effect increases Dipole moment increases Applications of Inductive effect 2. Bond length : Usually the bond length of organic molecules increases with decrease in inductive effect CH3F,CH3 Cl,CH3 Br,CH3I - I effect decreases Bond length increases 2. Bond length 3.Reactivity of Alkyl halides : Alkyl halides undergo nucleophilic substitution reactions.Due to –I effect of halogen atom,the carbon attached to the halogen acquires a partial positive charge and hence is readily attacked by a nucleophilie (negatively charged ion) CH3 →Cl or CH3δ+ Clδ- Attack by Nucleophile 3.Reactivity of Alkyl halides 4.Relative strength of the Organic acids : Carboxylic acids have the tendency to release a proton and change into carboxylate anion. O O R C OH R C O- + H+ The acidic strength of carboxylic acid depends upon the ease with which it ionises to gives proton . Any factor which stabilises the carboxylate ion will increase the acidic strength of the carboxylic acid 4.Relative strength of the Organic acids (a) Effect of +I groups : O O H C OH CH 3 C OH (I) Formic Acid (II) Acetic acid CH3 group in (II) has +I effect (electron releasing effect) and it will destabilise the acetate ion because it increases the negative charge on it.where as no such destabilisation is present in formate ion from (I).Hence formic acid is stronger acid than acetic acid. Therefore the acidic strength decreases in the order:-HCOOH>CH3 COOH> CH3CH2COOH> CH3CH2CH2COOH (a) Effect of +I groups Slide 16: Therefore the acidic strength decreases in the order:- HCOOH>CH3 COOH>CH3CH2COOH>(CH3 )2CHCOOH> (CH3 )3 CCOOH Effect of -I groups : The groups having –I effect will stabilise the carboxylate ion by dispersing the negative charge on it.for example:- The acidic strength of chloroacetic acid decreases in the order:- Cl3CCOOH>Cl2CHCOOH>ClCH2COOH>CH3COOH More is the electronegativity of halogen more is the acidic strength FCH2COOH>ClCH2 COOH>BrCH 2COOH>ICH2 COOH Effect of -I groups Relative Basic strength of amines : +I Effect:-The electron donating(+I) groups present on nitrogen increases the electron density on nitrogen.As a result nitrogen releases the electron pair more easily,increasing the basic srength. H H H N H < R N H Relative Basic strength of amines Examples : On the basis of inductive effect alone the strength of the amines should follow the order:- NH3<RNH2<R2NH<R3N But the actual order of basicity is:- 2o Amine > 1o Amine > 3o Amine Thus apart from inductive effect,other factors such as steric factor,stability of conjugate acid formed also play their role in determining the basicity. Examples ELECTROMERIC EFFECT : Electromeric effect is a temporary effect, which involves the complete transfer of π electron pair of a multiple bond(double or triple bond) to one of the bonded atoms under the influence of an attacking agent.As soon as the attacking agent is removed the electromeric effect disappears. -E Effect C-N + C = O nucleophile C+ O- CN ELECTROMERIC EFFECT Types of Electromeric effect : +E effect:-If the transference of electrons takes place towards the atom to which attacking agent is finally attached,the effect is called +E effect. C = C + H+ C C+ H Types of Electromeric effect -E Effect : If the electrons of this double bond are transferred to an atom of the double bond other than the one to which the attacking agent is finally attached , the effect is called as -E Effect. Nucleophile C = O + CN- C O- (-E Effect) CN -E Effect Mesomeric Effect or Resonance Effect : Mesomeric effect takes place in conjugated systems . Conjugated systems are the systems having alternate single and double bonds ex. CH2= CH CH = CH2 Mesomeric effect is a permanent effect which refers to the polarity which is produced in the conjugated system as a result of transference of π electrons between two two π-bonds or a π bond and a lone pair . Mesomeric Effect or Resonance Effect Examples : CH2= CH Cl: :CH2- CH = Cl+ I II (the movement of the electrons from atom having lone pair to the single covalent bond) CH2= CH C = N CH2+ CH = C = N:- I II (The movement of the electrons from multiple bond to the single covalent bond and also from multiple bond to the atom) Examples Types of Mesomeric Effect : -M or –R effect: A group or atom exhibits –M or –R effect if it withdraws electrons from the double bond or from a conjugated system.In such cases ,electrons move towards the atom or group in conjugated system.Examples:- -NO2,-CN, C=O,-SO3H CH2= CH – C = N: CH2– CH = C = N:- Types of Mesomeric Effect +M Effect : A group is said to exhibit +M or +R effect,if it donates the electrons to a conjugated system.In such cases the transference of electrons is away from the atom or group in conjugated system.Examples:- -Cl,-Br,-I,-NH2,-NR2,-OH,-OCH3 CH2= CH – Cl: CH2- – CH= Cl+: +M Effect Hyperconjugation or Baker Nathan Effect : Hyperconjugation involves the delocalisation of σ electrons through overlapping of π orbitals of C=C double bond or p orbital with σ orbital of the adjacent C-H bond in unsaturated system. H H+ H – C – CH = CH2 H – C = CH –CH2- H H I Hyperconjugation or Baker Nathan Effect Examples Contd. : H H H+ C = CH – CH2- H – C = CH – CH-2 H H+ II III Three hyperconjugative forms Examples Contd. Reaction Intermediates : Free Radicals :A free radical may be defined as any species having an odd or unpaired electron. Free radicals are electically neutral.Due to the presence of unpaired electron, they have strong tendency to pair up and hence these are highly reactive species. Reaction Intermediates Carbon atom is sp2 hybridised : Free radicals are formed by thermal or photolytic decomposition of compounds which involve the homolytic cleavage of a covalent bond.Examples:- O O O CH3– C – O-- O - C–CH3 2CH3–C –O. Acetyl peroxide Acetyl free radical Carbon atom is sp2 hybridised Relative Stability of Free Radicals : Benzyl>Allyl>Tertiary>Secondary>Primary>Methyl=vinyl Relative Stability of Free Radicals Carbanion : They are represented as R- An organic species with three covalent bonds, a pair of unshared electrons and a negative charge on the central atom is called a carbanion. It is formed by the heterolytic cleavage of a single covalent bond in which carbon atom is attached to the less electronegative element. Carbanion Relative stability of carbanion : Any factor which tends to disperse the negative charge of the electron rich carbon will enhance the stability of carbanion and vice versa. Therefore electron attracting groups exhibiting –I effect (-CN,-NO2,X-) will enhance the stability of carbanion and the electron releasing groups exhibiting +I effect will decrease the stability of carbanion by lntensifying the negative charge. Relative stability of carbanion In carbanion, carbon atom is sp3- hybridised : Examples:- H H H – C –- X H – C- + X+ H H Formation of carbanion:- O O NaOH + H-CH2-C –H C-H2–C –H + Na+ + H2O In carbanion, carbon atom is sp3- hybridised Carbanion may be produced by the cleavage of carbon metal bond in oranometallic compounds : H H – C –Na C-H3 + Na+ H Stability of carbanion follows the order:- CH3–C-H2 > (CH3)2 C-H > (CH3)3 C- Primary > secondary > Tertiary (C6H5)3C-> (C6H5)2C-H> C6H5-C-H2 > CH2=CH-C-H2 No. of resonationg structures: 10 7 4 2 Carbanion may be produced by the cleavage of carbon metal bond in oranometallic compounds Carbonium ion or Carbocation : A carbocation is an organic species which has only six electrons(3 pairs of electrons) and a positive charge at its carbon centre. They are represented as R+ In carbocation carbon is sp2 hybridised. Carbocations are formed by heterolytic cleavage of covalent bond in which carbon is bonded to a more electronegative atom. Carbonium ion or Carbocation Examples : H H H – C – X H – C+ + X- H H CH3 CH3 CH3 C – Cl CH3 – C+ + Cl- CH3 CH3 Examples Stability of Carbocation : Any factor which tends to disperse the positive charge of the electron deficient carbon will enhance the stability of carbocation and vice versa.Therefore the electron releasing groups exhibiting +I effect like alkyl groups will enhance the stability of carbocation. (C6H5)3C+ > (C6H5)2C+H2 > C6H5C+H2> CH2=CH– C+H2 >(CH3)3C+> (CH3)2C+H > CH3CH+2>C+H3 Stability of Carbocation Types of Organic Reactions : 1.Electrophilic Addition Reactions:-When an an addition reaction is started by the addition of an electrophile followed by the addition of nucleophile,it is called Electrophilic addition reaction.e.g. H2C=CH2 + HBr H3C-CH2-Br Types of Organic Reactions Addition Reactions:-are those in which atoms or groups are added to a molecule without elimination of any atom or group.e.g. H H H A H C=C + A-B C – C H H H B H Nucleophilic Addition Rection:-When an addition reaction is started by the addition of an nucleophile followed by the addition of an electrophile.e.g. O OH CH3CCH3 + HCN CH3- C- CH3 CN Markonikov Rule : The negative part of the addendum will go to the carbon atom that contains least number of hydrogen atoms.e.g. Br CH3-CH-CH3 CH2=CH-CH3+HBr 2-bromopropane(major) CH2-CH2-CH3 Br 1-bromopropane(minor) Markonikov Rule Anti-markonikov Rule : This is also known as Kharasch or Peroxide effect.It states that in presence of peroxide the addition of HBr to unsymmetrical alkene takes place opposite to the Markonikov rule ,i.e.,the negative part of the addendum goes to that carbon which has more number of hydrogen atoms.e.g. Br No peroxide CH3-CH-CH3 isopropyl bromide CH3-CH=CH2 peroxide CH3-CH2-CH2-Br n-propyl bromide Anti-markonikov Rule Substitution Reaction : The replacement of an atom or group from a molecule by other atom or group is known as substitution reaction.e.g. CH3-OH + HBr CH3-Br + H2O NO2 + NO2+ +H+ Substitution Reaction 1.Free Radical Substitution : Substitution reaction which proceeds through free radical formation are known as free radical substitution reactions.e.g. Cl-Cl 2Cl. CH3-H +2Cl. hν CH3-Cl + HCl Methane Methyl chloride 1.Free Radical Substitution Nucleophilic Substitution Reaction : Substitution Reactions brought about by nucleophiles are known as nucleophilic substitution reactions.They are denoted by sN e.g. CH3-Br + NaOH CH3-OH + NaBr Nucleophilic Substitution Reaction SN1Reaction : When the rate of nucleophilic substitution reaction depends upon the concentration of alkyl halide only and is independent of the concentration of nucleophile,the reaction is said to proceed through unimolecular mechanism and is known as SN1reaction. SN1 reaction involves two steps:- (i) Slow step-Bond breaking step (ii) Fast step-formation of bond takes place This reaction becomes faster in presence of strong ionising solvents. SN1Reaction Mechanism : e.g. (CH3)3C-Br + OH- (CH3)3COH + Br- Step 1: H3C CH3 CH3 H3C C-Br slow C+ + Br- H3C CH3 Tertiary butyl Planar carbocation Bromide central carbon is sp2-hybridised Mechanism Step 2. : OH- CH3 CH3 CH3 H3C C+ Fast HO-C CH3+ H3C-C-OH CH3 CH3 H3C Inversion Retention SN1 reaction in optically active compounds is often accompanied by racemisation i.e. a 50/50 mixture of enantiomers(equimolar mixture of two optically active isomers) Rate=dx/dt=K (CH3)3 -C-Br Order=1 Step 2. SN2 Reaction : When the rate of nucleophilic substitution reaction depends on the concentration of both alkyl halide as well as the nucleeophile,the reaction is said to proceed via bimolecular mechanism and is known as SN2 reaction. It proceeds in a single step. CH3-Br +OH- CH3-OH +Br- SN2 Reaction Mechanism : H H H OH-+ H-C-Br Hoδ-- -C- -Brδ- H H Pentavalent Transition state H HO-C- H + Br- Inversion H Mechanism Rate=dx/dt =K CH3-Br OH- order=2 : Elimination Reaction An elimination reaction is one in which two atoms or groups are completely removed from a molecule . If two atoms are removed from adjacent carbon atoms it will lead to the formation of pi bond and is known as 1-2 elimination or β-elimination. e.g. Dehydrohalogenation of ethyl chloride CH3-CH2-Cl Alc.KOH H2C=CH2+HCl Rate=dx/dt =K CH3-Br OH- order=2 When two atoms are removed from the same carbon atom it is known as α-eliminatione.g.CHCl3 OH- CCl2: +HCl : Chloroform Dichloro carbene When two atoms are removed from the same carbon atom it is known as α-eliminatione.g.CHCl3 OH- CCl2: +HCl Saytzeff Rule : In the dehydrohalogenation of alkyl halide and dehydration of alcohol the most substituted alkene is the main product. e.g. (CH3)2C=CH-CH3(Major) (CH3)2CH-CH-CH3 I 2-iodo-3-metyl butane (CH3)2CH-CH=CH2(Minor) Saytzeff Rule Aldol Condensation : Condensation between two molecules of an aldehyde or a ketone having at least one α-hydrogen atom in presence of dilute base to form a β-hydroxy aldehyde or β-hydroxy ketone is known as aldol condensation. H O H O H H O H-C-C +H-C-C OH- CH3-C- C-C H H H H OH H H α-hydrogen Acetaldehyde β-hydroxyaldehyde 3-hydroxy butanal Aldol Condensation Slide 54: O O CH3- C-CH3 + H-CH2-C-CH3 OH- Acetone (two molecules) OH H CH3-C – C – C – CH3 CH3H O Diacetone alcohol 4-hydroxy 4-methyl 2-pentanone Mechanism : Step I:- Base abstracts the α-hydrogen in the form of a proton from α-carbon atom to form carbanion. H O O O- H-C-C + OH- -CH2-C CH2=C +H2O H H H H Acetaldehyde Carbanion Resonance stabilised enolate ion Mechanism Step II. : The carbanion generated in step I attacks other aldehyde molecule to form alkoxide ion. CH3 O O- O C=O + -CH2-C CH3 -C-CH2-C H H H H Aldehyde carbanion alkoxide ion Step II. Step III : The alkoxide ion thus formed abstracts proton from water to form aldol and OH- is regenerated. O- O OH O CH3-C-CH2-C + H2O CH3-C-CH2-C +OH- H H H H Alkoxide ion Aldol Step III Crossed Aldol Condensation : If aldol condensation takes place between (a) Two different aldehydes (b) Two different ketones (c) An aldehyde and a ketone,then it is called crossed aldol condensation. O O OH O CH3-C +CH3-C-CH3 OH- CH3-C-CH2-C-CH3 + H H Acetaldehyde Acetone 4-hydroxy pentanone Crossed Aldol Condensation CONTD. : OH CH3-C-CH2CHO H 3-hydroxy butanal In this case (aldehyde and ketone having α-H atom) only two products are formed since ketone itself does not undergo aldol condensation because of +I effect of methyl group and partly because of steric hinderance. CONTD. Applications of Aldol condensation : 1.Synthesis of saturated alcohol:-Aldol on dehydration gives α,β unsaturated aldehyde when heated alone or with a trace of acid or alkali,which on reduction gives saturated alcohol. OH H O 2-butenal(crotonaldehyde) CH3–C – C-C ∆ CH3–CH=CH-CHO H H H Ni/H2 CH3–CH2–CH2–CH2OH Aldol n-butanol Applications of Aldol condensation 2.Synthesis of Sorbic acid:- : Sorbic acid is used as food preservative and is obtained by the condensation of crotonaldehyde with acetaldehyde followed by oxidation. O OH O 2CH3–C CH3–C-CH2–C ∆ H H H -H2O Acetaldehyde Aldol ∆-H2O CH3 –CH=CH-CHO 2.Synthesis of Sorbic acid:- Slide 62: OH H CH –CH=CH-C – C –CHO -H2O H H CH3–CH=CH-CH=CH-CHO mild oxidation CH3–CH=CH-CH=CH-COOH Sorbic acid The aldehydes or ketones having no α-hydrogen atom,do not undergo aldol condensation however they undergo cannizzaro reaction. : Cannizzaro,sReaction is that in which Aldehydes containing no α-hydrogen,on treatment with strong base undergo disproportionation reaction to form salt of an acid and a primary alcohol. In this reaction one molecule of aldehyde is oxidised into acid(which is converted into salt) and the other molecule is reduced to alcohol i.e. ,it is a self oxidation-reduction reaction. The aldehydes or ketones having no α-hydrogen atom,do not undergo aldol condensation however they undergo cannizzaro reaction. Slide 64: 2C6H5-CH=O NaOH C6H5-CH2-OH+C6H5COONa Benzaldehyde Benzyl alcohol Sodium Benzoate 2HCHO Base CH3-OH + HCOONa Formaldehyde Methyl alcohol Sodium formate Mechanism : Step I OH C6H5-C=O + O-H C6H5-C-O- H H Benzaldehyde Hydroxy alkoxide ion Nucleophilic addition of hydroxyl ion(OH-) on carbonyl carbon. Mechanism Step II : OH O O O- C6H5-C-O- + C-C6H5slowC6H5-C-OH+C6H5-C-H H H H Hydroxy alkoxide ion Benzaldehyde Benzoic acid Alkoxide ion Hydroxy alkoxide ion produced in step I acts as a hydride donor to another benzaldehyde molecule,resulting in the formation of benzoic acid and alkoxide ion. Step II Step III : O-H O- O- OH C6H5-C=O+C6H5-C-H FastC6H5-C=O+C6H5-C-H H H Benzoic acid Alkoxide ion Benzoate ion Benzyl alcohol There is a proton transfer between the acid and alkoxide ion formed to get more stable benzoate ion. Step III Cross Cannizaro Reaction : Cannizaro rection taking place between two different aldehydes is known as Cross Cannizaro reaction.e.g. C6H5– CHO+HCHO C6H5CH2-OH+HCOONa Benzaldehyde Formaldehyde Benzyl alcohol Sodium formate Cross Cannizaro Reaction Applications of Cannizaro Reaction : (i) Synthesis of alcohols and acids:- 2COOH CH2OH + COONa CHO COONa COONa Glyoxalic acid Sodium glycolate Sodium oxalate (ii) Applications of Cannizaro Reaction Diels-Alder Reaction : The addition of a conjugated diene to an unsaturated molecule generally known as Dienophile to form a six membered cyclic adduct is known as Diels-Alder reaction. CH2 CH + CH2 200o CH CH2 CH2 Ethylene(dienophile) Cyclohexene(Adduct) 1,3-Butadiene Diels-Alder Reaction Components of Diels-Alder Reaction : Dienophile: are unsaturated olefinic or acetylenic compounds,having –M group in conjugation with the unsaturated double bond.e.g. C6H5-CH=CH-CHO C6H5CH=CHNO2 Cinnamaldehyde O CH2=CH-CHO CH-C Acraldehyde O Maleic anhydride CH-C O Components of Diels-Alder Reaction -M may be C=O,-CHO,-COOH,-NO2,-CN etc : CH CHO CH C CH (ii) Diene: Diene may be an open chain or cyclic conjugated compound.The reaction rate is enhanced when electron donating groups are present in diene. CH2=CH-CH=CH2 1,3-butadiene O Furan -M may be C=O,-CHO,-COOH,-NO2,-CN etc Mechanism : (i) Heterolytic mechanism: According to this mechanism,diene and the dienophile undergo the electromeric effect and the dienophile attaches itself to the 1,4 position of the diene to form six membered adduct. CH2 -CH2 CH Electromeric CH CH shift CH CH2 +CH2 Mechanism Maleic Anhydride : O O CH – C +CH - C O O CH – C -CH - C O O Maleic Anhydride Slide 75: -CH2 O CH2 O CH +CH-C CH CH-C CH + O Slow O Fast +CH2 -CH-C CH -CH-C O +CH2 O Slide 76: CH2 O CH CH C O CH CH C CH2 O Adduct Tetra hydro phthalic anhydride (ii) Homolytic mechanism : CH2 COOH CH2 COOH CH CH CH CH CH + CH CH .CH CH C6H5 .CH C6H5 CH3 CH3 Penta-1,3 diene Cinnamic Diradical acid This mechanism proceeds via diradical (ii) Homolytic mechanism Slide 78: CH2 COOH CH CH CH CH CH C6H5 CH3 Adduct (iii) One step mechanism : CH2 CH CH2 CH + CH2 CH2 1,3-butadiene Ethylene Cyclohexene In this mechanism it is assumed that a cyclic six centered transition state Is formed and there is no intermediate transition state. (iii) One step mechanism Applications of Diels Alder Reaction : It is used to detect the conjugated double bonds in various natural products like β-carotene,vitamin A etc. (ii) A variety of natural products can be synthesised with the help of this reaction like Morphene,Camphene,Cholestrol. Applications of Diels Alder Reaction Beckmann’s Rearrangement : Rearrangement of oximes into substituted amides in presence of acidic reagents is known as Beckmann,s Rearrangement. Examples:- R C=NOH H+ R – C – NHR’ or R’ – C -NHR R’ O O Ketoxime N-substituted amide Beckmann’s Rearrangement Examples : R C=N-OH H H – C -NHR H O Aldoxime N-substituted amide Due to the presence of C=N bond,oximes exhibit geometrical isomerism as shown below:- C6H5 OH CH3 OH C=N C=N CH3 (i) C6H5 (ii) The migration of group from carbon to nitrogen is always anti,hence the rearranged products of isomers (i) and (ii) would be: Examples Slide 83: O O C6H5– C – NHCH3 CH3 – C – NHC6H5 From (i) From (ii) Mechanism R R C=N H+ C=N: -H2O R’-C=N+-R R’ OH R’ O+H2 Nitrilium ion CONTD. : H2O R’-C=N-R R’-C-NH-R Hydrolysis -H+ OH O N-substituted amide R-COOH+ RNH2 Acid Amine CONTD. Slide 85: Mechanism :- (i) Protonation of leaving –OH group. (ii) The alkyl or aryl group migrates intramolecularly . (iii) Intermediate nitrilium ion is formed. Applications:- 1.It is used to determine the configuration of ketoximes. 2.It is used to prepare E-caprolactum,which is used to prepare Nylon-6 on heating. Slide 86: OH O O N H2SO4 NH Heat -NH-(CH2)5 –C n E-caprolactum Nylon-6 Cyclohexanone (cyclic amide) oxime Hoffmann Rearrangement or Hoffmann Degradation of Amides : The conversion of unsubstituted amide to primary amine,having one carbon atom less than the starting amide,on treatment with hypohalite (NaOH solution+Br2or Cl2) is known as Hoffmann rearrangement.e.g. RCONH2+Br2+4KOH RNH2+ K2CO3 + 2KBr+2H2O CH3CONH2+Br2+4KOH CH3NH2+K2CO3 Methyl amine +2KBr+2H2O Hoffmann Rearrangement or Hoffmann Degradation of Amides Mechanism : Step 1.Formation of Bromamide takes place by the substitution of H which is attached to the N-atom of the amide by Br atom. O O R-C-NH2 + Br2 R-C-NHBr + HBr N-bromamide Mechanism Step2. : Base abstracts a proton from the nitrogen atom of bromamide to form anion of N-bromamide. R-C-N-Br OH- R-C-N--Br O H -H2O O N-bromamide Anion of N-bromamide Step2. Step III : Separation of the halide ion gives an electron deficient nitrogen atom in acyl nitrene. R-C-N--Br -Br- R-C-N O O Anion of N-bromamide Acyl Nitrene (unstable species) Step III Step IV : Migration of alkyl/aryl group from carbonyl carbon to electron deficient nitrogen. R-C-N C=N-R O O Acyl Nitrene Isocyanate Step V C=N-R HO-C-N-R RNH2+CO2 O O H Isocyanate Carbamic acid Primary amine Hydrolysis of isocyanate gives primary amine. Step IV Applications:- : 1.If this reaction is carried out in alcoholic solution the isocyanate is converted into urethane .Urethane on polymerisation gives poly urethane which can be used for surface coating,in making foams,adhesives etc. H O R-N=C=O R’OH R-N-C-O-R’ Polyme- Polyurethane Isocyanate urethane risation Applications:- Contd. : (ii) Aliphatic and aromatic primary amines from amides containing upto seven carbon atoms can be prepared from Hoffmann reaction. C6H5–CONH2 Br2+KOH C6H5-NH2 Benzamide Aniline (iii) Hydrazine can be prepared by the action of Br2and KOH on urea. NH2– CO-NH2 Br2+KOH H2N-NH2 Urea Hydrazine Contd. 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Edit Comment Close Premium member Presentation Transcript FUNDAMENTALS OF Reaction MECHANISM : FUNDAMENTALS OF Reaction MECHANISM CLEAVAGE OF COVALENT BONDS1.Homolytic Cleavage : The bond breaks in such a way that each atom departs with one electron of the shared pair . A .. B → A . + B . Free Radicals Br .. Br → Br . + Br . Homolytic cleavage is possible only when the electronegativity of both the bonded atoms is same. CLEAVAGE OF COVALENT BONDS1.Homolytic Cleavage 2.Heterolytic cleavage : The covalent bond is broken in such a way that one of the atoms carries away both the electrons of the shared pair.This results in the formation of a cation and anion A .. B → :A- + B+ (When A is more electronegative than B) A .. B → A+ + B-: (When B is more electronegative than A) CH3 CH3 CH3___ C ___ Cl → CH3___ C+ + Cl-: CH3 CH3 2.Heterolytic cleavage TYPES OF ATTACKING REAGENTS : 1.Nucleophilic Reagents are the reagents having at least one lone pair of electrons.They are nucleus loving in nature and are called nuleophiles.Nucleophiles are capable of donating electron pairs,hence they are Lewis bases.They are of two types:- Negative nucleophiles (:Nu-) They carry an excess of electron pair and are negatively charged.e.g. F-,Cl-,Br-,OH-,CN-, R3C-,COO-,NH2-,R- TYPES OF ATTACKING REAGENTS Neutral nucleophiles(:Nu) : They also carry an excess of electron pair but are neutral in nature.e.g. R–OH,H–OH,RSH,ROR,NH3,RNH2 Neutral nucleophiles(:Nu) 2.Electrophilic Reagents : A positively charged or neutral species which are electron deficient and can accept a pair of electrons is called electrophilic reagent. They are electron loving in nature.Electrophiles are capable of accepting electron pairs hence they are considered as Lewis acids. Types: Positive electrophile(E+): They carry positive charge. e.g. H+,Br+,Cl+,NO2+,NH4+,H3O+ 2.Electrophilic Reagents (b) Neutral Electrophiles E: : Neutral Electrophiles are electron deficient species which do not carry positive charge. e.g.BF3,AlCl3,ZnCl2,SO3 (b) Neutral Electrophiles E: Inductive Effect : Definition:- Permanent displacement of bonded electron pair of a covalent bond towards the more electronegative element,whereby polarity is induced on the carbon atom and substituent attached to it. I I ı ___Cδ+''___ Cδ+'___Cδ+___Clδ- I I I Inductive Effect -I Effect or electron withdrawing effect : This effect is exerted by the groups attached to the carbon chain having high electronegativity than carbon.Some of the examples of the groups having –I effect are:- NO2 >CN>COOH>F>Cl>Br>I>OCH3 >OH -I Effect or electron withdrawing effect + I Effect or electron releasing effect : This effect is exerted by the atoms or groups attached to the carbon chain having lesser electronegativity than carbon. Some of the examples of groups having +I effect are : C6 H5 O- >COO ->CR3 >CHR2 >CH2 R>CH3 >H + I Effect or electron releasing effect Applications of Inductive effect : 1. Dipole effect:- Inductive effect leads to dipolar character in the molecule.More is the inductive effect,greater will be the dipole moment.Example CH3 ,CH3 Br , CH3 Cl -I effect increases Dipole moment increases Applications of Inductive effect 2. Bond length : Usually the bond length of organic molecules increases with decrease in inductive effect CH3F,CH3 Cl,CH3 Br,CH3I - I effect decreases Bond length increases 2. Bond length 3.Reactivity of Alkyl halides : Alkyl halides undergo nucleophilic substitution reactions.Due to –I effect of halogen atom,the carbon attached to the halogen acquires a partial positive charge and hence is readily attacked by a nucleophilie (negatively charged ion) CH3 →Cl or CH3δ+ Clδ- Attack by Nucleophile 3.Reactivity of Alkyl halides 4.Relative strength of the Organic acids : Carboxylic acids have the tendency to release a proton and change into carboxylate anion. O O R C OH R C O- + H+ The acidic strength of carboxylic acid depends upon the ease with which it ionises to gives proton . Any factor which stabilises the carboxylate ion will increase the acidic strength of the carboxylic acid 4.Relative strength of the Organic acids (a) Effect of +I groups : O O H C OH CH 3 C OH (I) Formic Acid (II) Acetic acid CH3 group in (II) has +I effect (electron releasing effect) and it will destabilise the acetate ion because it increases the negative charge on it.where as no such destabilisation is present in formate ion from (I).Hence formic acid is stronger acid than acetic acid. Therefore the acidic strength decreases in the order:-HCOOH>CH3 COOH> CH3CH2COOH> CH3CH2CH2COOH (a) Effect of +I groups Slide 16: Therefore the acidic strength decreases in the order:- HCOOH>CH3 COOH>CH3CH2COOH>(CH3 )2CHCOOH> (CH3 )3 CCOOH Effect of -I groups : The groups having –I effect will stabilise the carboxylate ion by dispersing the negative charge on it.for example:- The acidic strength of chloroacetic acid decreases in the order:- Cl3CCOOH>Cl2CHCOOH>ClCH2COOH>CH3COOH More is the electronegativity of halogen more is the acidic strength FCH2COOH>ClCH2 COOH>BrCH 2COOH>ICH2 COOH Effect of -I groups Relative Basic strength of amines : +I Effect:-The electron donating(+I) groups present on nitrogen increases the electron density on nitrogen.As a result nitrogen releases the electron pair more easily,increasing the basic srength. H H H N H < R N H Relative Basic strength of amines Examples : On the basis of inductive effect alone the strength of the amines should follow the order:- NH3<RNH2<R2NH<R3N But the actual order of basicity is:- 2o Amine > 1o Amine > 3o Amine Thus apart from inductive effect,other factors such as steric factor,stability of conjugate acid formed also play their role in determining the basicity. Examples ELECTROMERIC EFFECT : Electromeric effect is a temporary effect, which involves the complete transfer of π electron pair of a multiple bond(double or triple bond) to one of the bonded atoms under the influence of an attacking agent.As soon as the attacking agent is removed the electromeric effect disappears. -E Effect C-N + C = O nucleophile C+ O- CN ELECTROMERIC EFFECT Types of Electromeric effect : +E effect:-If the transference of electrons takes place towards the atom to which attacking agent is finally attached,the effect is called +E effect. C = C + H+ C C+ H Types of Electromeric effect -E Effect : If the electrons of this double bond are transferred to an atom of the double bond other than the one to which the attacking agent is finally attached , the effect is called as -E Effect. Nucleophile C = O + CN- C O- (-E Effect) CN -E Effect Mesomeric Effect or Resonance Effect : Mesomeric effect takes place in conjugated systems . Conjugated systems are the systems having alternate single and double bonds ex. CH2= CH CH = CH2 Mesomeric effect is a permanent effect which refers to the polarity which is produced in the conjugated system as a result of transference of π electrons between two two π-bonds or a π bond and a lone pair . Mesomeric Effect or Resonance Effect Examples : CH2= CH Cl: :CH2- CH = Cl+ I II (the movement of the electrons from atom having lone pair to the single covalent bond) CH2= CH C = N CH2+ CH = C = N:- I II (The movement of the electrons from multiple bond to the single covalent bond and also from multiple bond to the atom) Examples Types of Mesomeric Effect : -M or –R effect: A group or atom exhibits –M or –R effect if it withdraws electrons from the double bond or from a conjugated system.In such cases ,electrons move towards the atom or group in conjugated system.Examples:- -NO2,-CN, C=O,-SO3H CH2= CH – C = N: CH2– CH = C = N:- Types of Mesomeric Effect +M Effect : A group is said to exhibit +M or +R effect,if it donates the electrons to a conjugated system.In such cases the transference of electrons is away from the atom or group in conjugated system.Examples:- -Cl,-Br,-I,-NH2,-NR2,-OH,-OCH3 CH2= CH – Cl: CH2- – CH= Cl+: +M Effect Hyperconjugation or Baker Nathan Effect : Hyperconjugation involves the delocalisation of σ electrons through overlapping of π orbitals of C=C double bond or p orbital with σ orbital of the adjacent C-H bond in unsaturated system. H H+ H – C – CH = CH2 H – C = CH –CH2- H H I Hyperconjugation or Baker Nathan Effect Examples Contd. : H H H+ C = CH – CH2- H – C = CH – CH-2 H H+ II III Three hyperconjugative forms Examples Contd. Reaction Intermediates : Free Radicals :A free radical may be defined as any species having an odd or unpaired electron. Free radicals are electically neutral.Due to the presence of unpaired electron, they have strong tendency to pair up and hence these are highly reactive species. Reaction Intermediates Carbon atom is sp2 hybridised : Free radicals are formed by thermal or photolytic decomposition of compounds which involve the homolytic cleavage of a covalent bond.Examples:- O O O CH3– C – O-- O - C–CH3 2CH3–C –O. Acetyl peroxide Acetyl free radical Carbon atom is sp2 hybridised Relative Stability of Free Radicals : Benzyl>Allyl>Tertiary>Secondary>Primary>Methyl=vinyl Relative Stability of Free Radicals Carbanion : They are represented as R- An organic species with three covalent bonds, a pair of unshared electrons and a negative charge on the central atom is called a carbanion. It is formed by the heterolytic cleavage of a single covalent bond in which carbon atom is attached to the less electronegative element. Carbanion Relative stability of carbanion : Any factor which tends to disperse the negative charge of the electron rich carbon will enhance the stability of carbanion and vice versa. Therefore electron attracting groups exhibiting –I effect (-CN,-NO2,X-) will enhance the stability of carbanion and the electron releasing groups exhibiting +I effect will decrease the stability of carbanion by lntensifying the negative charge. Relative stability of carbanion In carbanion, carbon atom is sp3- hybridised : Examples:- H H H – C –- X H – C- + X+ H H Formation of carbanion:- O O NaOH + H-CH2-C –H C-H2–C –H + Na+ + H2O In carbanion, carbon atom is sp3- hybridised Carbanion may be produced by the cleavage of carbon metal bond in oranometallic compounds : H H – C –Na C-H3 + Na+ H Stability of carbanion follows the order:- CH3–C-H2 > (CH3)2 C-H > (CH3)3 C- Primary > secondary > Tertiary (C6H5)3C-> (C6H5)2C-H> C6H5-C-H2 > CH2=CH-C-H2 No. of resonationg structures: 10 7 4 2 Carbanion may be produced by the cleavage of carbon metal bond in oranometallic compounds Carbonium ion or Carbocation : A carbocation is an organic species which has only six electrons(3 pairs of electrons) and a positive charge at its carbon centre. They are represented as R+ In carbocation carbon is sp2 hybridised. Carbocations are formed by heterolytic cleavage of covalent bond in which carbon is bonded to a more electronegative atom. Carbonium ion or Carbocation Examples : H H H – C – X H – C+ + X- H H CH3 CH3 CH3 C – Cl CH3 – C+ + Cl- CH3 CH3 Examples Stability of Carbocation : Any factor which tends to disperse the positive charge of the electron deficient carbon will enhance the stability of carbocation and vice versa.Therefore the electron releasing groups exhibiting +I effect like alkyl groups will enhance the stability of carbocation. (C6H5)3C+ > (C6H5)2C+H2 > C6H5C+H2> CH2=CH– C+H2 >(CH3)3C+> (CH3)2C+H > CH3CH+2>C+H3 Stability of Carbocation Types of Organic Reactions : 1.Electrophilic Addition Reactions:-When an an addition reaction is started by the addition of an electrophile followed by the addition of nucleophile,it is called Electrophilic addition reaction.e.g. H2C=CH2 + HBr H3C-CH2-Br Types of Organic Reactions Addition Reactions:-are those in which atoms or groups are added to a molecule without elimination of any atom or group.e.g. H H H A H C=C + A-B C – C H H H B H Nucleophilic Addition Rection:-When an addition reaction is started by the addition of an nucleophile followed by the addition of an electrophile.e.g. O OH CH3CCH3 + HCN CH3- C- CH3 CN Markonikov Rule : The negative part of the addendum will go to the carbon atom that contains least number of hydrogen atoms.e.g. Br CH3-CH-CH3 CH2=CH-CH3+HBr 2-bromopropane(major) CH2-CH2-CH3 Br 1-bromopropane(minor) Markonikov Rule Anti-markonikov Rule : This is also known as Kharasch or Peroxide effect.It states that in presence of peroxide the addition of HBr to unsymmetrical alkene takes place opposite to the Markonikov rule ,i.e.,the negative part of the addendum goes to that carbon which has more number of hydrogen atoms.e.g. Br No peroxide CH3-CH-CH3 isopropyl bromide CH3-CH=CH2 peroxide CH3-CH2-CH2-Br n-propyl bromide Anti-markonikov Rule Substitution Reaction : The replacement of an atom or group from a molecule by other atom or group is known as substitution reaction.e.g. CH3-OH + HBr CH3-Br + H2O NO2 + NO2+ +H+ Substitution Reaction 1.Free Radical Substitution : Substitution reaction which proceeds through free radical formation are known as free radical substitution reactions.e.g. Cl-Cl 2Cl. CH3-H +2Cl. hν CH3-Cl + HCl Methane Methyl chloride 1.Free Radical Substitution Nucleophilic Substitution Reaction : Substitution Reactions brought about by nucleophiles are known as nucleophilic substitution reactions.They are denoted by sN e.g. CH3-Br + NaOH CH3-OH + NaBr Nucleophilic Substitution Reaction SN1Reaction : When the rate of nucleophilic substitution reaction depends upon the concentration of alkyl halide only and is independent of the concentration of nucleophile,the reaction is said to proceed through unimolecular mechanism and is known as SN1reaction. SN1 reaction involves two steps:- (i) Slow step-Bond breaking step (ii) Fast step-formation of bond takes place This reaction becomes faster in presence of strong ionising solvents. SN1Reaction Mechanism : e.g. (CH3)3C-Br + OH- (CH3)3COH + Br- Step 1: H3C CH3 CH3 H3C C-Br slow C+ + Br- H3C CH3 Tertiary butyl Planar carbocation Bromide central carbon is sp2-hybridised Mechanism Step 2. : OH- CH3 CH3 CH3 H3C C+ Fast HO-C CH3+ H3C-C-OH CH3 CH3 H3C Inversion Retention SN1 reaction in optically active compounds is often accompanied by racemisation i.e. a 50/50 mixture of enantiomers(equimolar mixture of two optically active isomers) Rate=dx/dt=K (CH3)3 -C-Br Order=1 Step 2. SN2 Reaction : When the rate of nucleophilic substitution reaction depends on the concentration of both alkyl halide as well as the nucleeophile,the reaction is said to proceed via bimolecular mechanism and is known as SN2 reaction. It proceeds in a single step. CH3-Br +OH- CH3-OH +Br- SN2 Reaction Mechanism : H H H OH-+ H-C-Br Hoδ-- -C- -Brδ- H H Pentavalent Transition state H HO-C- H + Br- Inversion H Mechanism Rate=dx/dt =K CH3-Br OH- order=2 : Elimination Reaction An elimination reaction is one in which two atoms or groups are completely removed from a molecule . If two atoms are removed from adjacent carbon atoms it will lead to the formation of pi bond and is known as 1-2 elimination or β-elimination. e.g. Dehydrohalogenation of ethyl chloride CH3-CH2-Cl Alc.KOH H2C=CH2+HCl Rate=dx/dt =K CH3-Br OH- order=2 When two atoms are removed from the same carbon atom it is known as α-eliminatione.g.CHCl3 OH- CCl2: +HCl : Chloroform Dichloro carbene When two atoms are removed from the same carbon atom it is known as α-eliminatione.g.CHCl3 OH- CCl2: +HCl Saytzeff Rule : In the dehydrohalogenation of alkyl halide and dehydration of alcohol the most substituted alkene is the main product. e.g. (CH3)2C=CH-CH3(Major) (CH3)2CH-CH-CH3 I 2-iodo-3-metyl butane (CH3)2CH-CH=CH2(Minor) Saytzeff Rule Aldol Condensation : Condensation between two molecules of an aldehyde or a ketone having at least one α-hydrogen atom in presence of dilute base to form a β-hydroxy aldehyde or β-hydroxy ketone is known as aldol condensation. H O H O H H O H-C-C +H-C-C OH- CH3-C- C-C H H H H OH H H α-hydrogen Acetaldehyde β-hydroxyaldehyde 3-hydroxy butanal Aldol Condensation Slide 54: O O CH3- C-CH3 + H-CH2-C-CH3 OH- Acetone (two molecules) OH H CH3-C – C – C – CH3 CH3H O Diacetone alcohol 4-hydroxy 4-methyl 2-pentanone Mechanism : Step I:- Base abstracts the α-hydrogen in the form of a proton from α-carbon atom to form carbanion. H O O O- H-C-C + OH- -CH2-C CH2=C +H2O H H H H Acetaldehyde Carbanion Resonance stabilised enolate ion Mechanism Step II. : The carbanion generated in step I attacks other aldehyde molecule to form alkoxide ion. CH3 O O- O C=O + -CH2-C CH3 -C-CH2-C H H H H Aldehyde carbanion alkoxide ion Step II. Step III : The alkoxide ion thus formed abstracts proton from water to form aldol and OH- is regenerated. O- O OH O CH3-C-CH2-C + H2O CH3-C-CH2-C +OH- H H H H Alkoxide ion Aldol Step III Crossed Aldol Condensation : If aldol condensation takes place between (a) Two different aldehydes (b) Two different ketones (c) An aldehyde and a ketone,then it is called crossed aldol condensation. O O OH O CH3-C +CH3-C-CH3 OH- CH3-C-CH2-C-CH3 + H H Acetaldehyde Acetone 4-hydroxy pentanone Crossed Aldol Condensation CONTD. : OH CH3-C-CH2CHO H 3-hydroxy butanal In this case (aldehyde and ketone having α-H atom) only two products are formed since ketone itself does not undergo aldol condensation because of +I effect of methyl group and partly because of steric hinderance. CONTD. Applications of Aldol condensation : 1.Synthesis of saturated alcohol:-Aldol on dehydration gives α,β unsaturated aldehyde when heated alone or with a trace of acid or alkali,which on reduction gives saturated alcohol. OH H O 2-butenal(crotonaldehyde) CH3–C – C-C ∆ CH3–CH=CH-CHO H H H Ni/H2 CH3–CH2–CH2–CH2OH Aldol n-butanol Applications of Aldol condensation 2.Synthesis of Sorbic acid:- : Sorbic acid is used as food preservative and is obtained by the condensation of crotonaldehyde with acetaldehyde followed by oxidation. O OH O 2CH3–C CH3–C-CH2–C ∆ H H H -H2O Acetaldehyde Aldol ∆-H2O CH3 –CH=CH-CHO 2.Synthesis of Sorbic acid:- Slide 62: OH H CH –CH=CH-C – C –CHO -H2O H H CH3–CH=CH-CH=CH-CHO mild oxidation CH3–CH=CH-CH=CH-COOH Sorbic acid The aldehydes or ketones having no α-hydrogen atom,do not undergo aldol condensation however they undergo cannizzaro reaction. : Cannizzaro,sReaction is that in which Aldehydes containing no α-hydrogen,on treatment with strong base undergo disproportionation reaction to form salt of an acid and a primary alcohol. In this reaction one molecule of aldehyde is oxidised into acid(which is converted into salt) and the other molecule is reduced to alcohol i.e. ,it is a self oxidation-reduction reaction. The aldehydes or ketones having no α-hydrogen atom,do not undergo aldol condensation however they undergo cannizzaro reaction. Slide 64: 2C6H5-CH=O NaOH C6H5-CH2-OH+C6H5COONa Benzaldehyde Benzyl alcohol Sodium Benzoate 2HCHO Base CH3-OH + HCOONa Formaldehyde Methyl alcohol Sodium formate Mechanism : Step I OH C6H5-C=O + O-H C6H5-C-O- H H Benzaldehyde Hydroxy alkoxide ion Nucleophilic addition of hydroxyl ion(OH-) on carbonyl carbon. Mechanism Step II : OH O O O- C6H5-C-O- + C-C6H5slowC6H5-C-OH+C6H5-C-H H H H Hydroxy alkoxide ion Benzaldehyde Benzoic acid Alkoxide ion Hydroxy alkoxide ion produced in step I acts as a hydride donor to another benzaldehyde molecule,resulting in the formation of benzoic acid and alkoxide ion. Step II Step III : O-H O- O- OH C6H5-C=O+C6H5-C-H FastC6H5-C=O+C6H5-C-H H H Benzoic acid Alkoxide ion Benzoate ion Benzyl alcohol There is a proton transfer between the acid and alkoxide ion formed to get more stable benzoate ion. Step III Cross Cannizaro Reaction : Cannizaro rection taking place between two different aldehydes is known as Cross Cannizaro reaction.e.g. C6H5– CHO+HCHO C6H5CH2-OH+HCOONa Benzaldehyde Formaldehyde Benzyl alcohol Sodium formate Cross Cannizaro Reaction Applications of Cannizaro Reaction : (i) Synthesis of alcohols and acids:- 2COOH CH2OH + COONa CHO COONa COONa Glyoxalic acid Sodium glycolate Sodium oxalate (ii) Applications of Cannizaro Reaction Diels-Alder Reaction : The addition of a conjugated diene to an unsaturated molecule generally known as Dienophile to form a six membered cyclic adduct is known as Diels-Alder reaction. CH2 CH + CH2 200o CH CH2 CH2 Ethylene(dienophile) Cyclohexene(Adduct) 1,3-Butadiene Diels-Alder Reaction Components of Diels-Alder Reaction : Dienophile: are unsaturated olefinic or acetylenic compounds,having –M group in conjugation with the unsaturated double bond.e.g. C6H5-CH=CH-CHO C6H5CH=CHNO2 Cinnamaldehyde O CH2=CH-CHO CH-C Acraldehyde O Maleic anhydride CH-C O Components of Diels-Alder Reaction -M may be C=O,-CHO,-COOH,-NO2,-CN etc : CH CHO CH C CH (ii) Diene: Diene may be an open chain or cyclic conjugated compound.The reaction rate is enhanced when electron donating groups are present in diene. CH2=CH-CH=CH2 1,3-butadiene O Furan -M may be C=O,-CHO,-COOH,-NO2,-CN etc Mechanism : (i) Heterolytic mechanism: According to this mechanism,diene and the dienophile undergo the electromeric effect and the dienophile attaches itself to the 1,4 position of the diene to form six membered adduct. CH2 -CH2 CH Electromeric CH CH shift CH CH2 +CH2 Mechanism Maleic Anhydride : O O CH – C +CH - C O O CH – C -CH - C O O Maleic Anhydride Slide 75: -CH2 O CH2 O CH +CH-C CH CH-C CH + O Slow O Fast +CH2 -CH-C CH -CH-C O +CH2 O Slide 76: CH2 O CH CH C O CH CH C CH2 O Adduct Tetra hydro phthalic anhydride (ii) Homolytic mechanism : CH2 COOH CH2 COOH CH CH CH CH CH + CH CH .CH CH C6H5 .CH C6H5 CH3 CH3 Penta-1,3 diene Cinnamic Diradical acid This mechanism proceeds via diradical (ii) Homolytic mechanism Slide 78: CH2 COOH CH CH CH CH CH C6H5 CH3 Adduct (iii) One step mechanism : CH2 CH CH2 CH + CH2 CH2 1,3-butadiene Ethylene Cyclohexene In this mechanism it is assumed that a cyclic six centered transition state Is formed and there is no intermediate transition state. (iii) One step mechanism Applications of Diels Alder Reaction : It is used to detect the conjugated double bonds in various natural products like β-carotene,vitamin A etc. (ii) A variety of natural products can be synthesised with the help of this reaction like Morphene,Camphene,Cholestrol. Applications of Diels Alder Reaction Beckmann’s Rearrangement : Rearrangement of oximes into substituted amides in presence of acidic reagents is known as Beckmann,s Rearrangement. Examples:- R C=NOH H+ R – C – NHR’ or R’ – C -NHR R’ O O Ketoxime N-substituted amide Beckmann’s Rearrangement Examples : R C=N-OH H H – C -NHR H O Aldoxime N-substituted amide Due to the presence of C=N bond,oximes exhibit geometrical isomerism as shown below:- C6H5 OH CH3 OH C=N C=N CH3 (i) C6H5 (ii) The migration of group from carbon to nitrogen is always anti,hence the rearranged products of isomers (i) and (ii) would be: Examples Slide 83: O O C6H5– C – NHCH3 CH3 – C – NHC6H5 From (i) From (ii) Mechanism R R C=N H+ C=N: -H2O R’-C=N+-R R’ OH R’ O+H2 Nitrilium ion CONTD. : H2O R’-C=N-R R’-C-NH-R Hydrolysis -H+ OH O N-substituted amide R-COOH+ RNH2 Acid Amine CONTD. Slide 85: Mechanism :- (i) Protonation of leaving –OH group. (ii) The alkyl or aryl group migrates intramolecularly . (iii) Intermediate nitrilium ion is formed. Applications:- 1.It is used to determine the configuration of ketoximes. 2.It is used to prepare E-caprolactum,which is used to prepare Nylon-6 on heating. Slide 86: OH O O N H2SO4 NH Heat -NH-(CH2)5 –C n E-caprolactum Nylon-6 Cyclohexanone (cyclic amide) oxime Hoffmann Rearrangement or Hoffmann Degradation of Amides : The conversion of unsubstituted amide to primary amine,having one carbon atom less than the starting amide,on treatment with hypohalite (NaOH solution+Br2or Cl2) is known as Hoffmann rearrangement.e.g. RCONH2+Br2+4KOH RNH2+ K2CO3 + 2KBr+2H2O CH3CONH2+Br2+4KOH CH3NH2+K2CO3 Methyl amine +2KBr+2H2O Hoffmann Rearrangement or Hoffmann Degradation of Amides Mechanism : Step 1.Formation of Bromamide takes place by the substitution of H which is attached to the N-atom of the amide by Br atom. O O R-C-NH2 + Br2 R-C-NHBr + HBr N-bromamide Mechanism Step2. : Base abstracts a proton from the nitrogen atom of bromamide to form anion of N-bromamide. R-C-N-Br OH- R-C-N--Br O H -H2O O N-bromamide Anion of N-bromamide Step2. Step III : Separation of the halide ion gives an electron deficient nitrogen atom in acyl nitrene. R-C-N--Br -Br- R-C-N O O Anion of N-bromamide Acyl Nitrene (unstable species) Step III Step IV : Migration of alkyl/aryl group from carbonyl carbon to electron deficient nitrogen. R-C-N C=N-R O O Acyl Nitrene Isocyanate Step V C=N-R HO-C-N-R RNH2+CO2 O O H Isocyanate Carbamic acid Primary amine Hydrolysis of isocyanate gives primary amine. Step IV Applications:- : 1.If this reaction is carried out in alcoholic solution the isocyanate is converted into urethane .Urethane on polymerisation gives poly urethane which can be used for surface coating,in making foams,adhesives etc. H O R-N=C=O R’OH R-N-C-O-R’ Polyme- Polyurethane Isocyanate urethane risation Applications:- Contd. : (ii) Aliphatic and aromatic primary amines from amides containing upto seven carbon atoms can be prepared from Hoffmann reaction. C6H5–CONH2 Br2+KOH C6H5-NH2 Benzamide Aniline (iii) Hydrazine can be prepared by the action of Br2and KOH on urea. NH2– CO-NH2 Br2+KOH H2N-NH2 Urea Hydrazine Contd.