Carboxylic Acids - Practice Questions & MCQ

From Esters
Esters on acidic hydrolysis give acids, while on basic hydrolysis give carboxylates, which on acidification give corresponding acids. The reaction occurs as follows:

From Grignard reagents
Grignard reagents with dry ice forms salts of carboxylic acids which after acidification give corresponding carboxylic acids. The reaction occurs as follows:

From Nitriles and Amides
Nitriles are hydrolysed in acidic or basic medium first to amides and then to acids. The reaction occurs as follows:

Formation of Anhydride
Carboxylic acid on heating with H₂SO₄ or P₂O₅ gives corresponding anhydride. The reaction occurs as follows:

Reaction with Ammonia
On heating with NH₃, carboxylic acid gives ammonium salt which on further heating at high temperature gives amides. The reaction occurs as follows:

Reduction
Carboxylic acids are reduced to 1^o alcohol by LiALH₄ or better with B₂H₆. B₂H₆ does not easily reduce esters, nitro, and halo groups, and NaBH₄ does not reduce (-COOH) group. The reaction occurs as follows:

Decarboxylation
When sodium salts of carboxylic acids are heated with soda lime (NaOH and CaO in 3:1 ratio), they form hydrocarbons by losing CO₂. This is known as decarboxylation. The reaction occurs as follows:

Kolbe's electrolysis
The electrolysis of aqueous solution of sodium or potassium salts of carboxylic acid makes the carboxylic acid to undergo decarboxylation to form alkane. The reaction occurs as follows:

Halogenation
The method of preparing α-chloro or α-bromo acid is by Hell-Volhard-Zelinsky reaction, which is carried out by treating the acid with Cl₂ or Br₂ in the presence of a small amount of red phosphorous. The reaction occurs as follows:

Ring substitution
Aromatic carboxylic acid undergoes substitution electrophilic reactions in which the (-COOH) group acts as a deactivating and meta-directing group. It does not undergo Friedel-Crafts reaction because the (-COOH) group is deactivating and the catalyst AlCl₃ gets bonded to the (COOH) group. The reactions occur as follows:

Carboxylic acids are weaker than mineral acids but they are stronger than alcohols, phenols and peroxy acids.

• Phenols are stronger acids than alcohols.
• Carboxylic acids are stronger acids than phenols.
• Carboxylic acids are stronger than peroxy acids.
• Formic acid is stronger is a stronger acid than benzoic acid.

Aromatic aldehydes when heated with the anhydride of an aliphatic acid (containing two -H atoms) in the presence of its sodium or potassium salt result in condensation to form α,β-unsaturated acid. 

Mechanism

For example,

Ketones and aldehydes react with α-bromoesters(BrCHRCOOEt) and Zn in benzene to form β-hydroxy esters. First the zinc organometallic BrZnCHRCOOEt is formed and then it adds to the (C=O).

Mechanism

For example, 

Benzoin condensation
When benzaldehyde is refluxed with aq. alcoholic KCN solution to give benzoin(-hydroxy ketone), the process is called benzoin condensation.

Mechanism

For example,

Benzil-Benzilic acid rearrangement
-Diketones undergo a rearrangement when treated with base (NaOH) to give -hydroxy acids.

Mechanism

For example,