Chemical properties of carboxylic acids.
Carboxylic acid do not give the characteristic reaction of carbonyl group C = O as given by the aldehydes and ketones. as the carbonyl group of carboxylic acid is involved in resonance:
The reactions of carboxylic acids can be classified as follows:
A) Reactions involving cleavage of O – H bond.
B) Reactions involving cleavage of C – OH bond.
C) Reactions involving – COOH group.
D) Substitution reactions involving hydrocarbon part.
A) Reactions involving cleavage of O – H bond.
1) Reactions with metals
Carboxylic acid react with active metals like Na, Mg, Zn etc to form corresponding salts with the liberation of hydrogen.
Example
Acetic acid Sodium acetate
C OH + 2Na C ONa + H2
OO
2 CH3 2 CH3
2) Reaction with alkalies
Carboxylic acid reacts with alkalies to neutralise them and form salts.
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Example
CH3 C OH + NaOH CH3 C ONa + H2O
O O
Accteic acid Sodium acetate
3) Reaction with carbonates and bicarbonates (Test for carboxylic acid group)
Carboxylic acids decompose carbonates and bicarbonates evolving carbondioxide gas with effervescence.
Example
Acetic acid Sodium acetate
C OH + Na2CO3 C ONa + CO + H O2 2
OO
2CH3 2CH3
4) All Carboxylic acids turn blue litmus red
B) Reactions involving cleavage of C-OH bond
1) Reactions with PCl5, PCl3 and SOCl2
The hydroxyl group of carboxylic acids behaves like that of an alcoholic group and is easily replaced by chlorine atom on treating with PCl5, PCl3 or SOCl2.
Example
O
C OH + PCl5 C Cl + POCl3 + HCl
O
CH3 CH3
Acetic acid Acetyl Chloride
O
C OH + SOCl2 C Cl + SO2 + HCl
O
C6H5 C6H5
Benzoic acid Benzoyl chloride
2) Reactions with alcohols ( Esterification) When carboxylic acids are heated with alcohols in the presence of conc. H2SO4 or dry HCl
gas, esters are formed. The reaction is reversible and is called esterification. Example
O
C OH + C 2 H
5 OH C OC
2 H
5 + H
2 O
O
C6H5 C6H5
Benzoic acid ethyl benzoate
H+
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Mechanism of esterification:
The Mechanism of esterification involves the following steps.
H3C OH C
O
H
H3C OH C
O H
C2H5 H O
H3C C
O
OH
O H
C2H5
H
H3C C
O
OH
O C2H5
H
H
H3C C
O
OH
O C2H5
HH H3C
C
O
O C2H5
H
C2H5 H OH3C
C
O
O C2H5
C2H5 H O
H
C) Reactions involving – COOH group
1) Reduction
i) Partial reduction to alcohols
Carboxylic acids are reduced to primary alcohols by LiAlH4 or with hydrogen in the presence of copper chromite as catalyst. Sodium borohydride does not reduce the – COOH group.
Example
C OH
O
CH3 CH3CH2OH + H2O LiAIH4
ethanoicacid ethanol 4(H)
ii) Complete reduction to alkanes
When treated with HI and red phosphorous, carboxylic acid undergoes complete reduction to yield alkanes containing the same number of carbon atoms.
Example
C OH + 6 HI
O
CH3 Red P
CH3 + 3I2 + 2H2OCH3
Acetic acid Ethane 473 K
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2) Decarboxylation
Removal of CO2 from carboxyl group is called as decarboxylation. Carboxylic acids lose carbon di oxide to form hydrocarbon when their sodium salts are heated with soda lime (NaOH and CaO in the ratio 3: 1)
Example
C ONa + NaOH
O
CH3 CH4 + Na2CO3
Sodium acetate Methane
CaO
3) Kolbe’s electrolytic decarboxylation
The aqueous solutions of sodium or potassium salts of carboxylic acid on electrolysis gives alkanes at anode. This reaction is called kolbes electrolysis.
CH3COONa
CH3
CH3CH3 COONa
+ 2CO2 + 2Na Electrolysis
Sodium acetate Anode Cathode
Sodium formate solution on electrolysis gives hydrogen
4) Reactions with ammonia
Carboxylic acids react with ammonia to form ammonium salt which on further heating at high temperature gives amides.
Example
C OH + NH3
O
CH3 C O - NH
4
O
CH3 C NH2 H 2O
O
CH3
Acetic acid Ammonium acetate Acetamide
5) Action of heat in the presence of P2O5
Carboxylic acid on heating in the presence of a strong dehydrating agent such as P2O5 forms acid anhydride.
Example
C OH
O
CH3 + C OH
O
CH3
P2O5 C
O
CH3
C
O
CH3
O + H2O
Acetic acid Acetic anhydride
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D) Substitution reactions in the hydrocarbon part
1) α - Halogenation Carboxylic acids having an α - hydrogen are halogenated at the α - position on
treatment with chlorine or bromine in the presence of small amount of red posphorus to form α halo carboxylic acids. This reaction is known as Hell – Volhard – Zelinsky reaction (HVZ reaction) The α - Halogenated acids are convenient starting materials for preparing α - substituted acids.
COOHCH3 Cl2 / red P4
H2O COOHCH2
Cl
Acetic acid Mono Chloro acetic acid
2) Electrophilic substitution in aromatic carboxylic acids
Aromatic carboxylic acid undergoes electrophilic substitution reactions. The carboxyl group is a deactivating and meta directing group. Some common electrophilic substitution reactions of benzoic acid are given below
i) Halogenation
COOH COOH
Br
Br2
FeBr3 + HBr
Benzoic acid m - bromo benzoic acid
ii) Nitration
+ H2O
Benzoic acid m-Nitrobenzoic acid
COOH COOH
NO2
Conc. HNO3
Conc. H2SO4
iii) Sulphonation
+ H2O
Benzoic acid
COOH COOH
SO3H
Fuming
Conc. H2SO4
m-Sulphobenzoic acid
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iv) Benzoic acid does not undergo friedal craft’s reaction. This is due to the strong deactivating nature of the carboxyl group.
E) Reducing action of Formic acid Formic acid contains both an aldehyde as well as an acid group. Hence, like other aldehydes,
formic acid can easily be oxidised and therefore acts as a strong reducing agent
Aldehyde group Carboxylic acid group
H C OH
O
H C OH
O
i) Formic acid reduces Tollens reagent (ammonical silver nitrate solution) to metallic silver. HCOO - + 2Ag+ + 3OH- 2 Ag + CO3
2 - + 2H2O
(Tollens reagent) Silver mirror
ii) Formic acid reduces Fehlings solution. It reduces blue coloured cupric ions to red coloured cuprous ions.
HCOO**-** + 2Cu2+ + 5 OH - Cu2O + CO3 2 - + 3 H2O
(Fehlings solution) red precipitate
Tests for carboxylic acid group i) In aqueous solution carboxylic acid turn blue litmus red. ii) Carboxylic acids give brisk effervescence with sodium bicarbonate due to the evolution
of carbon-di -oxide. iii) When carboxylic acid is warmed with alcohol and Con H2SO4 it forms an ester, which
is detected by its fruity odour.