Summary of alpha carbon compounds PDF

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Summary of alpha substitution and condensation of carbonyl compounds Formation of enolates Acidic nature of alpha hydrogen in carbonyl compounds Alpha-carbon hydrogen is acidic due to stabilization by resonance An enolate anion is formed in the reaction

Enol-keto tautomerism Treatment of enolate with acid generates keto and enol

Generally, keto is more stable than enol

Alpha substitution of carbonyl compounds Alpha-halogenation of ketones Does not work well for aldehydes because they are subject to oxidation by halogens. Acid catalyzed halogenation

No multiple halogenation for acid-catalyzed halogenation

Base promoted halogenation

Multiple halogenation occurs for base-promoted halogenation

Alpha-alkylation Problem of alkylation using enolate anion: 1. The base would compete with enolate ion for SN2 reaction. Remedy : Use LDA 2. Multiple alkylation would occur 3. Aldol condensation would occur for aldehydes 4. E2 reaction rather than SN2 may take place

Enamine replaces enolate for substitution reactions Remedy: Instead of having an enolate anion, use enamine for reaction. Hydrolysis after reaction gives back the carbonyl compound Enamine reactions

For example:

Compared with:

Advantages: 1. No multiple alkylation 2. Can do with an aldehyde

Alpha condensation of carbonyl compounds Aldol condensation Treatment of an aldehyde or ketone with a strong base O

OH H

O

NaOH 2

CH3CH2CH2C

H

CH3CH2CH2C H

C

CH3CH2CH2CH

C

C

H

+

H

CH2CH3

O

Heat

C

Reaction is good for aldehydes. For ketones, the yield is low unless special arrangement in synthesis

H2O

On heating, dehydration yields α,β-unsaturated carbonyl compounds

CH2CH3

Problem with crossed-aldol reaction: More than one condensed product is formed

Plus the following self-aldol products:

Claisen condensation Treatment of an ester with a strong base

Application of Claisen condensation To synthesize substituted (Both mono and disubstituted) ketones

Conjugate addition to  -unsaturated aldehydes and ketones: Michael addition For  -unsaturated aldehydes and ketone, 1,4- addition takes place

Robinson Annulation Michael addition followed by intramolecular aldol condensation

Reasons: • The α-hydrogen is very acidic • Esters can be hydrolyzed to carboxylic acids • Β-keto acid can be decarboxylated by heating

Mannich reaction

Significance of the Mannich reaction To form α, β-unsaturated carbonyl compound with one carbon which cannot be synthesized by aldol condensation.

For aldol condensation with one carbon aldehyde: Will not work by condensation with formaldehyde....