Chapter 9

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

Geometry:

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