Carboxylic Acids

What a carboxylic acid is

• Carboxylic acids are a homologous series of organic compounds containing the carboxyl functional group, −COOH
• The carboxyl group is what makes these compounds acidic — it is the proton-donor end of the molecule (compare with acids in topic 16)
• Common examples include vinegar (an aqueous solution of ethanoic acid, about 5% by volume), the methanoic acid in nettle stings, and the long-chain fatty acids that make up animal and plant fats

The carboxyl group up close

• The −COOH group consists of:
  • A central carbon
  • A C=O double bond to one oxygen
  • A single bond to a separate O−H group
• The whole group is written as −COOH for short, or drawn out as −C(=O)−O−H when every bond is shown
• The hydrogen of the O−H is the acidic hydrogen — it is the proton released into solution that gives a carboxylic acid its acid character

General formula

• The general formula of the carboxylic acid series is:

C_nH_2n+1COOH

• where n counts only the carbons sitting in the alkyl chain bonded to the −COOH group — it does not count the carbon already inside the carboxyl group itself
• A molecule with n alkyl carbons therefore has (n + 1) carbon atoms in total, including the one in the −COOH
• Special case: when n = 0, the alkyl chain is just a single hydrogen atom and the formula reduces to HCOOH (methanoic acid). This is the smallest member of the series

Naming carboxylic acids

• The name follows the standard stem-plus-ending convention from topic 21, with the ending "-anoic acid":
  • 1 carbon → methan- + oic acid → methanoic acid
  • 2 carbons → ethan- + oic acid → ethanoic acid
  • 3 carbons → propan- + oic acid → propanoic acid
  • 4 carbons → butan- + oic acid → butanoic acid

The first four carboxylic acids

The four shortest members of the series, all liquids at room temperature with sharp, vinegar-like smells:

Every formula in this table contains the −COOH group. That group is what gives the molecules their acidic behaviour: in water the O–H end releases a hydrogen ion (H⁺), leaving a negatively-charged carboxylate ion behind.