α−PROTON CHEMISTRY
When a carboxylic acid with an α−proton is brominated in the presence of a small amount of PBr3 you will get an α−bromo-carboxylic acid instead of an acyl bromide.
The carboxylic acid will first undergo a nucleophilic substitution as discussed above, to give the acyl bromide.
However the α−proton of the acyl bromide is slightly acidic and it will execute a keto-enol equilibrium. The enol can then be brominated.
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The hydrogen bromide can then protonate the hydroxyl group of the carboxylic acid making it a good leaving group, so that it can be substituted by the bromide.
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The acyl bromide will again participate in the keto-enol equilibrium until it is converted to an α−bromo acyl bromide.
Simultaneously another reaction is taking place. The water produced can react with an acyl bromide to give α−bromo carboxylic acid.
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The overall reaction can then be represented by;
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REDUCTION CHEMISTRY
Carboxylic acida can be reduced to the alcohols. A system generally used in the laboratory is LiAH4 in ether.
LiAH4 is so reactive that it can even reduce a carboxylate salt to the alcohol.
REPLACEMENT of −COOH with a HALIDE
The −COOH can be replaced by a chloride or a bromide. What is significant about these reactions is the size of the hydrocarbon chain is decreased by one-carbon unit.
R−COOH � R−X
HUNSDIECKER REACTION
The silver salt of a carboxylic acid when treated with bromine (and also iodine), with carbon tetrachloride as the solvent, can be degraded to a bromide.
The yield with primary acids is about 65%.
For secondary and tertiary acids a mixture of lead tetraacetate and lithium chloride is used.
This is known as the Kochi Reaction, and the yield can be as high as 90%.
