Tuesday, February 5, 2008

Carboxylic acids - Characteristic reactions

Grouping of the reactions

A. Reactions due to hydrogen atom of carboxyl group
B. Reactions due to OH part of carboxyl group
C. Reactions due to carboxyl group
D. Reactions due to alkyl group and benzene ring

A. Reactions due to hydrogen atom of carboxyl group

1. Acidic character

a. Action with blue litmus
all carboxylic acids turn blue litmus red.

b. Reaction with metals: liberation of hydrogen
Carboxylic acids react with active metals such as Na, K, Ca, Mg, Zn, etc., to form their salts with the liberation of hydrogen.

c. Action with alkalies: formation of salts
Carboxylic acids neutralize alkalies forming salts and water.

d. Action with carbonates and bicarbonates: evolving carbon dioxide
Carboxylic acids decompose carbonates and bicarbonates evolving carbon dioxide with brisk effervescence.

Mechanism of acidic character

In the COOH group due to resonance in the OH, O acquires some positive charge the electron pair of OH is drawn towards O. This displacement of electrons causes the release of a proton and a carboxylate ion, RCOO- is formed. This is the reason for acidic character of carboxylic acids.

The strength of acids can be expressed in terms of dissociation constant Ka or Ph number of PKa number which is pKa = -log Ka

A stronger acid will have a higher Ka value but smaller PKa value.

Effects of substituents on acidic strength of acids

Electron releasing substituents: Alkyl is an electron releasing group. If the H atom of formic acid (HCOOH) is replaced by CH3 group to form acetic acid (CH3COOH) the alkyl group will tend to increase the electron density on the oxygen atom of the O-H bond. This increase will make removal H+ ion difficult in comparison to formic acid.

Acetic acid is a weaker acid in comparison to formic acid.

The electron release effect is called +I effect. As +I effect increases, acidic strength will go down. As more alkyl groups are there +I effect increases
Therefore acidic property is stronger or more for CH3COOH.

Acidic strength is in the following order
acidic strength of HCOOH>CH3COOH>CH3CH2COOH>(CH3)2CHCOOH

Electron withdrawing substituents: Substituents like halogens tend to withdraw the electron charge. Halogens are electron attracting atoms(-I inductive effect). They withdraw the electrons from the carbon to which they are attached and this effect is transmitted throught the chain. The increases positive charge on O atom in the O-H bond and dissociation of H+ ion or proton takes place more easily.

Hence chloroacetic acid is stronger acid than acetic acid.

B. Reactions due to OH part of carboxyl group

The -OH group of carboxylic acids can be replaced by a number of groups such as, -Cl, -NH2, OR, -OOCR’ to form chlorides, amides, esters ad anhydrides.

Formation of acid chlorides
Acetic acid + PCl5  Acetyl chloride + POCL3 + HCl

Formation of esters
When carboxylic acids are heated with alcohols in the presence of concentrated sulphuric acid, esters are formed.

Formation of amides
Carboxylic acids react with ammonia to form ammonium salts.
Acetic acid + Ammonia  Ammonium acetate
Ammonium acetate on heating gives acetamide plus water.

Formation of acid anhydrides
Carboxylic acids on heating in the presence of a strong dehydrating agent such as phosphorous pentoxide form acid anhydrides.

C. Reactions due to carboxyl group

Salts of carboxylic acids get decarboxylated – lose carbon dioxide in some reactions.

a. Sodium or potassium salts of carboxylic acids on heating with soda lime give alkanes.
b. Electrolysis of acqueous solutions of sodium or potassium salts of carboxylic acids gives alkanes due to decarboxylation.
c. When calcium salts of monocarboxylic acids (fatty acids) are heated aldehydes or ketones are formed.


a. Partial reduction: Carboxylic acids on reduction with lithium aluminium hydride or with hydrogen in the presence of copper chromite are reduced to alcohols.
b. Complete reduction to alkanes: Carboxylic acids on reduction with HI and red P give alkanes.

Action of bromine on silver salt of acid:
The silver saltsof the carboxylic acid on treatment with Br2 in the presence of CCl4 give alkyl halides having one carbon atom less than the parent acid.

This reaction is called Hunsdiecker reaction.

D. Reactions due to alkyl group and benzene ring


Carboxylic acids react with chlorine on bromine in the presence of a small amount of phosphorus to give halogenated compounds. The reaction is called Hell Volhard-Zelinksy reaction.

Ring substitution in aromatic acids:
In substitution reactions with aromatic carboxylic acids, carboxyl group is an electron withdrawing group and therefore it favours meta position for the substituent.

Hence benzoinc acid + bromine gives 3-Bromobenzoic acid

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