Carboxylic Acids | Organic Chemistry 2

Carboxylic acids are studied in this chapter: name, properties and synthesis of carboxylic acids, reduction and addition-elimination reactions, preparation of carboxylic acid derivatives: acyl halides, carboxylic anhydrides, esters, amides

Nomenclature of Carboxylic Acids

Order of precedence of functional groups:


Name: the -ane ending of the corresponding alkane is replace by -oic acid

The rules for naming alcohols  also apply to carboxylic acids, except that it is not necessary to specify their position ⇒ always carbon 1. Aromatic counterparts are called benzoic acids
Cycloalkane-carboxylic acid: carboxylic acid functional group is attached to a ring and the carbon of -COOH is carbon 1


Properties of Carboxylic Acids

Carboxylic acids bonds:

C=O bond: short, strong and very polar; C-O bond: polar
-COOH forms hydrogen bonds with other -COOH or polarized molecules ⇒ carboxylic acids have high melting and boiling points


The carbon of the C=O bond of carboxylic acids are sp2 hybridized ⇒ trigonal planar geometry

Absorption spectroscopy:

NMR: 1H δ ~ 10-13 ppm (O-H); 13C δ ~ 180 ppm
IR: O-H stretching ⇒ broad band ~ 2500-3300 cm-1; C=O stretching ⇒ intense band ~ 1710 cm-1

Reactivity of carboxylic acids:

Synthesis of Carboxylic Acids

Oxidation of alcohols and aldehydes:


Carbonation of organometallic reagents:


Hydrolysis of nitriles:

Reduction of Carboxylic Acids

The Addition-Elimination Mechanism

Substitution at the carboxy carbon occurs by addition-elimination (with Nu = Nucleophile and L = Leaving group):

Nucleophilic Attack on C=O of carboxylic acid derivatives is a key step of the Addition-Elimination process. It can be catalyzed by both base and acid

Synthetic strategy:

For -COOH, the addition-elimination process is complicated because of the poor leaving group (HO-). To favor this process, carboxylic acids can be converted to one of its derivatives (e.g. acyl halides) with a better leaving group

Preparation of Carboxylic Acid Derivatives

Acyl Halides:


  1. Addition reaction

  2. Elimination reaction


Carboxylic Anhydrides:




Acid-catalyzed (Fischer) esterification. This reaction is reversible ⇒ ester hydrolysis

  1. Protonation of the carboxy group:

  2. Attack by an alcohol

  3. Elimination of water





Addition - elimination process
1st step: nucleophilic attack of the amine on -COOH