Name Reactions Part 1 SEM IV POC III

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Pinacol-Pinacolone, Hofmann, Baeyer-Villiger oxidation, Benzilic acid rearrangement reaction, Beckmann’s rearrangement and Schmidt rearrangement,

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REACTION SEM- IV NAME Compiled By- Dr. Atul Bendale © Dr. Atul R. Bendale

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PINACOL PINACOLONE REARRANGEMENT 1 © Dr. Atul R. Bendale

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Step 1:  Since the reaction is carried out in an acidic medium, the hydroxide group of the pinacol is protonated by the acid. Step 2:  Water is now removed from the compound, leaving behind a carbocation . This carbocation is tertiary and therefore stable. Step 3:  The methyl group shifts to the positively charged carbon in a rearrangement of the compound. Step 4:  The oxygen atom which is doubly bonded to the carbon is now deprotonated , giving rise to the required pinacolone . Mechanism Reaction © Dr. Atul R. Bendale

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HOFMANN REARRANGEMENT 2 © Dr. Atul R. Bendale

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Hofmann rearrangement , also known as Hofmann degradation [ and not to be confused with Hofmann elimination ] is the reaction of a primary amide with a halogen (chlorine or bromine) in strongly basic (sodium or potassium hydroxide) aqueous medium, which converts the amide to a primary amine, which is one carbon less than the amide. For example The carbamic acid spontaneously loses CO 2 , yielding the amine product. Base abstracts an acidic N-H proton, yielding an anion. The anion reacts with bromine in an α-substitution reaction to give an  N - bromoamide . Base abstraction of the remaining amide proton gives a bromoamide anion. The bromoamide anion rearranges as the R group attached to the carbonyl carbon migrates to nitrogen at the same time the bromide ion leaves, giving an isocyanate . The isocyanate adds water in a nucleophilic addition step to yield a  carbamic acid (aka  urethane ). Amide Amine The reaction actually uses Br₂ + NaOH , but this is equivalent to using NaOBr , because the reaction produces NaOBr   in situ . Mechanism Reaction © Dr. Atul R. Bendale

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BAEYER VILLIGER OXIDATION 3 © Dr. Atul R. Bendale

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BENZILIC ACID REARRANGEMENT 4 © Dr. Atul R. Bendale

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0 Benzilic acid rearrangement converts a 1,2-diketone into an α- hydroxycarboxylic acid containing a rearranged carbon skeleton. This reaction receives its name from the reaction of  benzil  with potassium hydroxide to form  benzilic acid. It can be viewed as an  intramolecular   disproportionation  reaction , as one carbon center is oxidized while the other is reduced. The reaction has been shown to work in aromatic, semi-aromatic, aliphatic, and heterocyclic substrates. The reaction works best when the ketone functional groups have no adjacent  enolizable  protons , as this allows  aldol condensation to compete. It has been found that aryl groups more readily migrate than alkyl groups, and that aryl groups with electron-withdrawing groups migrate the fastest. Mechanism Reaction Addition of OH – to the carbonyl carbon as a  nucleophilic addition to form the  alkoxide . The next step requires a bond rotation to conformer which places the migrating group in position for attack on the second carbonyl group The  carboxylic acid  in intermediate is less basic than the alkoxide and therefore reversible proton transfer takes place favoring intermediate  it is protonated on acidic workup to the final α- hydroxy – carboxylic acid Animation © Dr. Atul R. Bendale

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BECKMANN REARRANGEMENT 5 © Dr. Atul R. Bendale

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0 The acid-catalyzed conversion of an  oxime  into an amide is known as  Beckmann rearrangement : The reaction begins by protonation of the alcohol group forming a better leaving group. Its mechanism follows the same pattern as a pinacol reaction – acid converts the oxime OH into a leaving group, and an alkyl group migrates on to the nitrogen as water departs. The product cation is then trapped by water to give an amide. Mechanism Reaction ( a )  Protonation of oxime ( b ) rearrangement of imine (Rate determining step) ( c ) Attack of carbocation ( d )  Deprotonation © Dr. Atul R. Bendale

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SCHMIDT REACTION 6 © Dr. Atul R. Bendale

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0 The Schmidt reaction  is an organic reaction in which an  azide  reacts with a carbonyl derivative, usually a aldehyde , ketone , or carboxylic acid, under acidic conditions to give an  amine or amide , with expulsion of nitrogen Schmidt Reaction Mechanism for Producing Amines The carbamate is now deprotonated . The subsequent removal of CO 2  yields the required amine. This mechanism begins with the formation of an acylium ion from the protonation of the carboxylic acid followed by the removal of water. This acylium ion is now reacted with hydrazoic acid, leading to the formation of a protonated azido ketone . Now, the protonated azido ketone and the R group undergo a rearrangement reaction, resulting in the migration of the carbon-nitrogen bond and the removal of dinitrogen leading to the formation of a protonated isocyanate . Now, a carbamate is formed when water is introduced to attack the protonated isocyanate . Mechanism Reaction Acid Amine © Dr. Atul R. Bendale

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Schmidt Reaction Mechanism for Producing Amides The relocation of a proton belonging to the tautomer of the amide gives the final amide product. Mechanism Reaction This Mechanism begins with the protonation of the ketone , leading to the formation of an O-H bond. The subsequent nucleophilic addition of the azide leads to the formation of an intermediate. Water is now removed from this intermediate via an elimination reaction, forming a temporary imine . An alkyl group which was a part of the original ketone now migrates from the carbon to the  nitrogen  belonging to the imine . This results in the elimination of dinitrogen . Now, water is used to attack the resulting compound, and the subsequent deprotonation yields a tautomer of the required amide. Ketone Amide © Dr. Atul R. Bendale

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