Carboxylic Acids | Organic Chemistry 2
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
Preparation of Carboxylic Acid Derivatives
Acyl Halides:
Mechanism:
- Addition reaction
- Elimination reaction
Carboxylic Anhydrides:
Ester:
Mechanism:
Acid-catalyzed (Fischer) esterification. This reaction is reversible ⇒ ester hydrolysis
- Protonation of the carboxy group:
- Attack by an alcohol
- Elimination of water
Amides:
Mechanism:
Addition - elimination process
1st step: nucleophilic attack of the amine on -COOH