PREPARATIONS OF CARBOXYLIC ACIDS
Primary alcohols and aldehydes are converted to carboxylic acids by oxida-tion. Acid chlorides, acid anhydrides, esters, and amides can be hydrolyzed to their parent carboxylic acids, but only the hydrolysis of esters serves a useful synthetic role if the ester is being used as a protecting group.
Aromatic carboxylic acids are obtained by the oxidation of alkyl benzenes.
Alkyl halides can be converted to carboxylic acids where the carbon chain has been extended by one carbon unit. Two methods are possible. The alkyl halide can be converted to a cyanide which is then hydrolyzed. Alterna- tively, the alkyl halide can be converted to a Grignard reagent then treated with carbon dioxide. A range of carboxylic acids can be prepared by alkyl- ating diethyl malonate, then hydrolyzing and decarboxylating the product.
Alkenes can be cleaved across the double bond by potassium perman- ganate. Carboxylic acids are formed if a vinylic proton is present.
Carboxylic acids can be obtained by the oxidation of primary alcohols or aldehydes, the hydrolysis of nitriles , or the hydrolysis of esters which can be used as protecting groups for carboxylic acids. Amides can also be hydrolyzed to carboxylic acids. However, fiercer reaction conditions are required due to the lower reactivity of amides and so amides are less useful as carboxylic acid protecting groups.
Although acid chlorides and anhydrides are easily hydrolyzed to carboxylic acids, the reaction serves no synthetic purpose since acid chlorides and acid anhydrides are synthesized from carboxylic acids in the first place. They are also too reactive to be used as protecting groups.
Aromatic carboxylic acids can be obtained by oxidation of alkyl benzenes. It does not matter how large the alkyl group is, since they are all oxidized to a benzoic acid structure.
There are two methods by which alkyl halides can be converted to a carboxylic acid and in both cases, the carbon chain is extended by one carbon. One method involves substituting the halogen with a cyanide ion , then hydrolysing the cyanide group. This works best with primary alkyl halides.
The other method involves the formation of a Grignard reagent which is then treated with carbon dioxide (Fig. 1b).
The mechanism for the Grignard reaction is similar to the nucleophilic addition of a Grignard reagent to an aldehyde or ketone.
A range of carboxylic acids can be prepared by alkylating diethyl malonate, then hydrolyzing and decarboxylating the product.
Alkenes can be cleaved with potassium permanganate to produce carboxylic acids (Fig. 4). A vinylic proton has to be present, that is a proton directly attached to the double bond.
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