Lesson 9 of 13 · 5 min
Carboxylic acids: names and structure
NCERT §8.6
The ingredients board lists acetic acid, citric acid and sodium benzoate. Tara notices that half the acid names come from Latin words for where they were first found.
The lesson in notes
In short
A carboxylic acid carries the carboxyl group, –COOH: a carbonyl joined to a hydroxyl, which gives the name. Acids are aliphatic (RCOOH) or aromatic (ArCOOH).
Higher aliphatic acids with C₁₂–C₁₈ chains, the fatty acids, occur in natural fats as glycerol esters. Acids are the starting point for anhydrides, esters, acid chlorides and amides.
Many acids were isolated early and keep common names ending in '-ic acid' taken from their source: formic acid from red ants (Latin formica), acetic acid from vinegar (acetum), butyric acid from rancid butter (butyrum).
IUPAC names change the '-e' of the alkane to '-oic acid', with the carboxyl carbon as C-1. For two or more –COOH groups the chain without them is named and 'dicarboxylic acid', 'tricarboxylic acid' is added, with numbers for the positions.
Name pairs: methanoic (formic), ethanoic (acetic), propanoic (propionic), butanoic (butyric) and 2-methylpropanoic (isobutyric) acids; ethanedioic (oxalic), propanedioic (malonic), butanedioic (succinic), pentanedioic (glutaric) and hexanedioic (adipic) acids; benzenecarboxylic acid (benzoic acid), 2-phenylethanoic (phenylacetic) acid and benzene-1,2-dicarboxylic (phthalic) acid.
The carboxyl carbon is planar, its three bonds roughly 120° apart. The –OH oxygen shares a lone pair with C=O by resonance, so this carbon is a weaker electrophile than the carbon of an aldehyde or ketone.