Carboxylic Acids and Derivatives

Overview

Description

Carboxylic acids are carbonyls with an ${-}{\rm{OH}}$ group on one side and an R group on the other side. They can also be a diacid, a molecule with carboxylic acids at either end of a straight chain. Using the IUPAC naming system, carboxylic acids are named by adding -anoic acid to the end of the root alkane name. The carboxylic acid is the highest-priority group, and the numbering of the carbon chain starts with the carbon in the carboxyl group. Carboxylic acids are generally acidic because of inductive and resonance effects. These effects stabilize the conjugate base, making the carboxylic acid stronger. Carboxylic acid derivatives include acid halides, acid anhydrides, esters, amides, and nitriles, in order of reactivity. Each has specific naming conventions in the IUPAC system: acid halides use the ending -oyl chloride, acid anhydrides use the suffix -oic anhydride, esters use a prefix for the alkyl group and the suffix -oate, amides have the -oic or -ic substituted with -amide, and nitriles end in -nitrile. Reactivity is governed by the structure of the derivative and the ability of the leaving group to leave. Acid halides are the most reactive, followed by acid anhydrides, esters, amides, and nitriles. Because of their stability, amides are an important component of proteins found in the human body.

At A Glance

• Carboxylic acids contain carbonyls with an ${-}{\rm{OH}}$ on one side and an R group on the other side. In IUPAC nomenclature, carboxylic acids are named by changing the suffix to -anoic acid.
• Carboxylic acids usually have a pKa around 4.5. The acidity of carboxylic acids results from both resonance and inductive effects that help stabilize the conjugate base, making it more stable and less likely to undergo reprotonation. The hydrogen of a ${-}{\rm{COOH}}$ shows up around 10–12 ppm in proton nuclear magnetic resonance (1H NMR), and the hydrogen of a carbon next to the carbonyl shows up between 2 and 3 ppm in 1H NMR.
• Carboxylic acid derivatives include acid halides, acid anhydrides, esters, amides, and nitriles. Acid halides have the ending -oyl followed by the name of the halide, such as chloride. Anhydrides have the ending -oic anhydride, esters have a prefix for the alkyl group and the ending -oate, and nitriles have the ending -nitrile.
• Reactivity depends on the ability of the derivative to act as a leaving group. Therefore, acid halides are the fastest-reacting derivatives, followed closely by anhydrides. Then there is a significant drop in reactivity for esters, amides, and nitriles. In 1H NMR the hydrogen atoms on the alpha carbon of a carboxylic acid derivative have a peak between 2 and 3 ppm.