Carboxylic Acids | Organic Chemistry 2

The carboxylic acids are studied in this chapter: the naming and properties of carboxylic acids, their synthesis, their reactivity: reduction and addition-elimination reactions, the preparation of carboxylic acid derivatives: acyl halides, carboxylic anhydrides, esters, amides.

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


Reactivities:

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

Acyl Halides:
 


 

Mechanism:

Step 1: Addition


Step 2: Elimination

 

Carboxylic Anhydrides:
 

 

Ester:
 

 

Mechanism:

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

 

Amides:
 

 

Mechanism: Addition-Elimination process
1st step: nucleophilic attack of the amine on -COOH.