IIT JEE , AIEEE Forum - Articles Category , Community shelf - an overview of ALDEHYDES AND KETONES......................................................

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Aldehydes and Ketones.

Reduction to Hydrocarbons
(review of Chapter 12)

Clemmensen Reduction (acidic conditions)

Wolff-Kishner Reduction (basic conditions)

NH₂NH₂ / KOH / ethylene glycol (a high boiling solvent) reduces the C=O into -CH₂-

Overview

These reduction methods do not reduce C=C, C?C or -CO₂H
The choice of method should be made based on the tolerance of other functional groups to the acidic or basic reaction conditions.

Reactions of RLi and RMgX with Aldehydes and Ketones

Reactions usually in Et₂O or THF followed by H₃O⁺ work-ups

Reaction type: Nucleophilic Addition

Summary

Organolithium or Grignard reagents react with the carbonyl group, C=O, in aldehydes or ketones to give alcohols.
The substituents on the carbonyl dictate the nature of the product alcohol.
Addition to methanal (formaldehyde) gives primary alcohols.
Addition to other aldehydes gives secondary alcohols.
Addition to ketones gives tertiary alcohols.
The acidic work-up converts an intermediate metal alkoxide salt into the desired alcohol via a simple acid base reaction.

Hydride Reductions of Aldehydes and Ketones
(review of Chapter 15)

Reactions usually in Et₂O or THF followed by H₃O⁺ work-ups

Reaction type: Nucleophilic Addition

Summary

Aldehydes and ketones are most readily reduced with hydride reagents.
The reducing agents LiAlH₄ and NaBH₄ act as a source of 4 x H^- (hydride ion).
Overall 2 H atoms are added across the C=O to give H-C-O-H.
Hydride reacts with the carbonyl group, C=O, in aldehydes or ketones to give alcohols.
The substituents on the carbonyl dictate the nature of the product alcohol.
Reduction of methanal (formaldehyde) gives methanol.
Reduction of other aldehydes gives primary alcohols.
Reduction of ketones gives secondary alcohols.
The acidic work-up converts an intermediate metal alkoxide salt into the desired alcohol via a simple acid base reaction.

Reactions of Alcohols to give Acetals

hemi-acetal acetal

Reaction type: Nucleophilic Addition then nucleophilic substitution

Summary

Typical reagents : excess ROH, catalytic p-toluenesulfonic acid (often written as TsOH) in refluxing benzene.
Aldehydes and ketones react with two moles of an alcohol to give 1,1-geminal diethers more commonly known as acetals.
The term "acetal" used to be restricted to systems derived from aldehydes and the term "ketal" applied to those from ketones, but chemists now use acetal to describe both.
Acetals are biologically important due to their role in the chemistry of carbohydrates.
The equilibrium is shifted towards the acetal by using an excess of the alcohol and/or removing water as it forms.
It is also possible to use 1,2- or 1,3-diols to form cyclic acetals, two common examples are shown below:

Acetals can be readily converted back to the aldehyde or ketone by heating with aqueous acid.
The mechanism for this is the reverse of that shown below for acetal formation.

The Wittig Reaction

ylide aldehyde or ketone alkene

Reaction type: Nucleophilic Addition then Elimination

Summary

The Wittig reaction is an important method for the formation of alkenes.
The double bond forms specifically at the location of the original aldehyde or ketone.
Ylides are neutral molecules but have +ve and -ve centers on adjacent atoms that are connected by a s bond.
The ylid is prepared via a two step process:

Cyanohydrin Formation

Reaction type: Nucleophilic Addition
Summary

Cyanide adds to aldehydes and ketones to give a cyanohydrin.
The reaction is usually carried out using NaCN or KCN with HCl.
HCN is a fairly weak acid, but very toxic.
The reaction is useful since the cyano group can be converted into other useful functional groups (-CO₂H or -CH₂NH₂)

NUCLEOPHILIC ADDITION OF CYANIDE TO AN ALDEHYDE
Step 1: The nucleophilic C in the cyanide adds to the electrophilic C in the polar carbonyl group, electrons from the C=O move to the electronegative O creating an intermediate alkoxide.
Step 2: An acid/base reaction. Protonation of the alkoxide oxygen creates the cyanohydrin prod

NUCLEOPHILIC ADDITION OF LiAlH₄ TO AN ALDEHYDE
Step 1: The nucleophilic H in the hydride reagent adds to the electrophilic C in the polar carbonyl group in the aldehyde, electrons from the C=O move to the O creating an intermediate metal alkoxide complex. (note that all 4 of the H atoms can react)
Step 2: This is the work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the primary alcohol product from the intermediate complex.