RUDIMENTS OF ORGANIC CHEMISTRY

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some fundamentals of chemistry

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Rudimentary concepts in organic chemistry : 

Rudimentary concepts in organic chemistry Professor S.Perumal School of Chemistry Madurai Kamaraj University Madurai 625 021 subbu.perum@gmail.com

Let us learn the fundamentals and make small daily progress to realize great dreams! : 

Let us learn the fundamentals and make small daily progress to realize great dreams! The way we do small things determines the way that we do everything. If we execute our minor tasks well, we will also excel at our larger efforts. Mastery then becomes our way of being. But more than this – each tiny effort builds on the next, so that brick by brick, magnificient things can be created, great confidence grows and uncommon dreams are realized. The truly wise recognize that small daily improvements always lead to exceptional results over time. - Robin Sharma Go as far as you can see. When you get there, you’ll be able to see farther - Thomas Carlyle

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FACTORS THAT INFLUENCE ORGANIC REACTIONS Polarity, bond strength, polarizability Electronic, steric and stereoelectronic effects Orbital symmetry Solvation Catalysis Reaction conditions: temperature, pressure etc.

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4 Nucleophiles and bases are structurally similar: both have a lone pair or a  bond. They differ in what they attack. Nucleophiles and bases

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5 In a nucleophilic substitution reaction of R—X, the C—X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:¯. The more stable the leaving group X:¯, the better able it is to accept an electron pair. For example, H2O is a better leaving group than HO¯ because H2O is a weaker base. Nucleophilic Substitution

Why do SN2 reactions occur at the rear side? : 

Why do SN2 reactions occur at the rear side? sp2 hybridized

Slide 7: 

Interaction between one filled (HOMO) and unfilled orbital (LUMO) leads to lower energy transition state

Slide 8: 

Why are hard or strong bases poor leaving groups?

Why soft bases or more polarizable nucleophiles react readily in SN2 reactions? : 

Why soft bases or more polarizable nucleophiles react readily in SN2 reactions? PhSH better nucleophile than PhOH Ph3P better nucleophile than Ph3N

Catalyst-mediated inversion of polarity and its influence on reactivity : 

Catalyst-mediated inversion of polarity and its influence on reactivity Electrophilic catalysis Opposite charges on carbon and OH; Stronger bond Positive charge on carbon and OH; weaker bond OH- less stable than H2O

Nucleophilic catalysis : 

Nucleophilic catalysis

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Why E2 eliminations prefer antiperiplanar geometry? synperiplanar Electron flow from s to s*: filled and unfillled orbitals interact Electron flow from s to s?

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Brominations of alkenes give trans-dibromoalkene Epoxide ring opening gives trans-diols filled bromide orbital and unfillled s* orbital of C-Br bond interact Hence rear attack and trans-dibromide formed Further examples of HOMO-LUMO interactions and stereochemistry

Polarity and chemoselectivityspecies of opposite electronic nature (nucleophile and electrophile) react;not two species of the same kind : 

Polarity and chemoselectivityspecies of opposite electronic nature (nucleophile and electrophile) react;not two species of the same kind In the first step oxygen behaves as a nucleophile; In the second step oxygen acts as the electrophilic centre repulsion between the four lone pairs of oxygens weakens the bond

HSAB THEORYHard base prefers to react with hard acid;soft base prefers to react with soft acid : 

HSAB THEORYHard base prefers to react with hard acid;soft base prefers to react with soft acid High +ve charge and small size (or charge/surface area high)– hard acid High –ve charge and small size – hard base F- - hard base; I- - soft base; Cl- - borderline EtO- - hard base; EtS- - soft base H+ - hard acid; C+ - soft acid EtO- prefers to react with proton – good base EtS- prefers to react with carbon – good nucleophile

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Polarity (HSAB) and product-selectivity Small, highly electronegative, highly positively charged species, (hard acid) prefers to react with a highly electronegative, highly negatively charged, low polarizable site (hard base) or a weak acid and weak base react (HSAB Concept).

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Interaction between p-bonds   Positive charge at carbonyl carbon (C-2) is more than that at b-carbon (C-4); therefore C-2 is hard acid and C-4 is soft acid. Nucleophiles which are hard bases react at C-2 while nucleophiles which are soft bases react at C-4. Hard and soft carbons in the same molecule

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hard nucelophile soft nucelophiles Polarity (HSAB) and product-selectivity

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Catalysts facilitate reactions, which are otherwise sluggish or don’t occur

Electrophilic catalysis – another example

Catalysts turn ‘soft acid’ into a ‘hard acid’ and reverses regioselectivity : 

Catalysts turn ‘soft acid’ into a ‘hard acid’ and reverses regioselectivity Electron deficiency more on carbon when silver Pulls a pair of electrons of bromine. Hence carbon in this case becomes a hard electrophile

HSAB theory helps in understanding Solvation : 

HSAB theory helps in understanding Solvation H-bonds; solvation strong Solvation weak

Hard acid-soft base hydrogen bonding weaker – facilitates reaction! : 

Hard acid-soft base hydrogen bonding weaker – facilitates reaction!

Slide 24: 

Solvation controls regioselectivity in ambident nucleophile

Regioselectivity of ambident nucleophilemanipulation by altering relative amount of reactants/reagents – statistical control : 

Regioselectivity of ambident nucleophilemanipulation by altering relative amount of reactants/reagents – statistical control Statistical control

Statistical control not possible! : 

Statistical control not possible! Statistical control not possible!

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Chemoselectivity – ambident nucleophile When polarity is introduced into a molecule in the transition state, the system usually prefers to take up positive charge on a less electronegative atom and negative charge on a more electronegative atom

Chemoselectivity - ambident electrophile : 

Chemoselectivity - ambident electrophile

Slide 29: 

Polarity and Diels-Alder reactions

Inverting classical polarity of functionalities – the concept of umpolung : 

Inverting classical polarity of functionalities – the concept of umpolung Carbonyl carbon

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UMPOLUNG IN SYNTHESIS! - REACTIONS OF DITHIANE ANION

Synthesis based on classical polarity and umpolung : 

Synthesis based on classical polarity and umpolung 1,2-dicarbonyl

Umpolung at a-carbon of carbonyls : 

Umpolung at a-carbon of carbonyls Synthesis of 1,4-dicarbonyl

Imaginary or Latent polarity at b-carbon of carbonyl systems synthesis of 3-substituted carbonyl systems : 

Imaginary or Latent polarity at b-carbon of carbonyl systems synthesis of 3-substituted carbonyl systems

Umpolung at alkyl carbon : 

Umpolung at alkyl carbon

Umpolung at nitrogen(Woodward 1945) : 

Umpolung at nitrogen(Woodward 1945)

REGIOSELECTIVITY IN CYCLOADDITION : 

REGIOSELECTIVITY IN CYCLOADDITION

Geometry of reactive intermediates : 

Geometry of reactive intermediates

Slide 43: 

Selectivity

Slide 44: 

What selectivity?

Slide 45: 

What selectivity?

Slide 46: 

Sharpless asymmetric dihydroxylation In the example below the achiral alkene yields only one of possible 4 stereoisomers.

Slide 47: 

Solvent-free and solution reactions show different product-selectivity Solution and solvent-free reactions Complimentary nature thiazine thiazole

Selectivities : 

Selectivities

Slide 49: 

THANK YOU

Slide 50: 

Enantioselective synthesis from achiral molecules

Slide 51: 

Diastereoselective synthesis involving one enantiomer of chiral molecules (S)-3-methylcyclo hexanone Diastereoselective Enantio- and diastereoselective

Slide 52: 

Enantio- and diastereoselective Mannich reaction using proline derivative Org. Lett., 2008, 10 (1), pp 21–24

Kinetic and thermodynamic controls - regioselectivity : 

Kinetic and thermodynamic controls - regioselectivity

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