Important Concepts
1. Carboxylic acids are named as alkanoic acids. The carbonyl carbon is
numbered 1 in the longest chain incorporating the carboxy group. Dicarboxylic
acids are called alkanedioic acids. Cyclic
and aromatic systems are called cycloalkane
carboxylic and benzoic acids, respectively. In these systems the ring carbon bearing the
carboxy group is assigned the number 1.
2. The carboxy
group is approximately trigonal
planar. Except in very dilute solution, carboxylic acids form dimers by
hydrogen bonding.
3. The carboxylic acid proton chemical shift is variable but relatively high (δ = 10 – 13), because of hydrogen bonding. The carbonyl carbon is also relatively
deshielded but not as much as in aldehydes and ketones, because of the
resonance contribution of the hydroxy group. The carboxy function shows two
important infrared bands, one at about 1710 cm-1 for the C=O bond
and a very broad band between 2500 and 3300 cm-1 for the O – H
group. The mass spectrum of carboxylic
acids
shows facile fragmentation in three ways.
4. The carbonyl group in carboxylic acids
undergoes nucleophilic displacement via the addition – elimination pathway. Addition of a nucleophile gives an
unstable tetrahedral intermediate that
decomposes by elimination of the hydroxy group to give a carboxylic acid derivative.
5. Lithium
aluminum hydride is a strong enough nucleophile to add to the carbonyl
group of carboxylate ions. This process allows the reduction of carboxylic
acids to primary alcohols.
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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