Carboxylic Acids | Organic Chemistry 2
Naming the Carboxylic Acids
Order of Precedence of Functional Groups:
The -ane ending of the corresponding alkane is replace by -oic acid.
Same rules as alcohols (except that the position of carboxylic acid is always 1 ⇒ do not specified it).
When -COOH attached to a ring = cycloalkane-carboxylic acid (and carbon bearing the -COOH is C1).
The aromatic counterperts are the benzoic acids.
Properties of Carboxylic Acids
C=O bond: short, strong and very polar; C-O bond: polar
Hybridization: sp2 ⇒ trigonal planar
-COOH forms hydrogen bonds (with other -COOH or polarized molecules).
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
Synthesis of Carboxylic Acids
Oxidation of Alcohols and Aldehydes (see Chapter 8 - Organic Chemistry 1):
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
For -COOH, the process is complicated by the poor leaving group (HO-)
⇒ Convert -COOH to carboxylic acid derivatives (e.g. Acyl Halides).
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.
Preparation of Carboxylic Acid Derivatives
Step 1: Addition
Step 2: Elimination
Acid-Catalyzed (Fischer) Esterification. This reaction is reversible ⇒ ester hydrolysis.
Step 1: Protonation of the carboxy group
Step 2: Attack by alcohol
Step 3: Elimination of water
Mechanism: Addition-Elimination process
1st step: nucleophilic attack of the amine on -COOH.