Mark Bishop

Analytical Chemistry and

Signal Acquisition/Analysis



Reactions involving carbanions

Carboxylic acids are acidic due to the stability of the resonance hybrid carbanion.

image not available

Hydrogens attached to carbon atoms immediately adjacent to a carbonyl carbon (a hydrogens) are acidic for the same reason.

Halogenation of ketones

Base promoted

image not available

Acid catalyzed

In contrast, acid catalyzed halogenation does not involve a carbanion but rather a carbonium ion. not involve a carbanion but rather a carbonium ion.

image not available

The Aldol condensation

The carbonyl carbon is subject to nucleophillic attack due to the ability of the intermediate to accommodate a negative charge.

image not available

Carbonyl compounds under basic conditions form a carbanion with nucleophillic character.

image not available

This nucleophile can now add to the carbonyl carbon to form a b hydroxy carbonyl compound.

image not available

So, acetone gives 4-hydroxy-4-methyl-2-pentanone also known as diacetone alcohol.

The Aldol product can undergo dehydration.

image not available

So, diacetone alcohol gives 4-methyl-3-penten-2-one (mesityl oxide).

The Wittig Reaction

Triphenylphosphine reacts with alkyl halides to form an ylide.

image not available image not available

The ylide can be represented by the following two resonance structures and has a carbanion like character.

image not available

The ylide can react as a nucleophile towards a carbonyl carbon to form a betaine.

image not available

The betaine goes on to form a new compound where the original =O group has been replaced by a = CR1R2 group.

image not available

The Claisen condensation

In the presence of an alkoxide ion the a hydrogen of a carboxylic acid ester is abstracted to form a carbanion.

image not available

This carbanion is a nucleophile which can in turn attack the carbonyl carbon of the same or different carboxylic acid ester

image not available

The resulting intermediate loses an alkoxide ion. In the reaction mixture (strongly basic due to the presence of the alkoxide ion) the product is present as the following very stable carbanion.

image not available

Upon acidification a b-keto ester is formed.

image not available

Acetoacetic ester is one important b-keto ester.

The Reformatsky reaction

In this reaction Zinc is reacted with an a bromo ester to form a nucleophile

image not available

This intermediate can then react with the carbonyl carbon of an aldehyde or ketone to form an intermediate which upon acidification yields a b-hydroxy ester.

image not available

The reaction product can be dehydrated to form a, b unsaturated esters which can be hydrogenated and then hydrolyzed to form carboxylic acids.

Malonic ester synthesis of carboxylic acids

image not available

Reaction of sodiomalonic ester with an alkyl halide gives an alkylmalonic ester.

image not available

The alkylmalonic ester can be further reacted with sodium ethoxide and the resulting salt reacted with an alkyl halide to form a dialkylmalonic ester.

The esters can be hydrolyzed to the malonic acids. Acid and heat forms a substituted acetic acid.

image not available

Acetoacetic ester synthesis of ketones

image not available

Reaction of sodioacetoacetic ester with an alkyl halide gives an alkylacetoacetic ester (aka an ethyl alkylacetoacetate).

image not available

The alkylacetoacetic ester can be further reacted with sodium ethoxide and the resulting salt reacted with an alkyl halide to form a dialkylacetoacetic ester.

The esters can be hydrolyzed to the corresponding acids acids. Acid and heat (sometimes not necessary) causes decarboxylation to form ketones

image not available

Synthesis of acids and esters via 2-oxazolines

A protective group for the carboxyl functionality.

image not available

Organoborane synthesis of acids and ketones

image not available image not available

The best base is

image not available

A good alkylborane can be prepared by the following scheme:

image not available

Alkylation of carbonyl compounds via enamines

image not available
ID may be exposed