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B.Sc. Sem II
UNIT IV
CARBOXYLIC ACIDS AND THEIR DERIVATIVES
Ku. Suvarna S. Nalge
Defination: - The organic compounds containing carboxyl group (–COOH) in their molecule are called
carboxylic acids.
Carboxyl group contains CARBO nyl group (>C = O) and a hydro XYL group (– OH) and hence the name
carboxyl group.
Classification of carboxylic acid: -
Acids are classified according to the number of carboxyl groups present in their molecules as,
(i) Monocarboxylic acid (ii) Dicarboxylic acid and (iii) Tricarboxylic acid
Monocarboxylic acid: -
Defination: -The compounds which are obtained by replacement of one hydrogen atom of alkane by carboxyl
group (-COOH) are called as carboxylic acid or Monocarboxylic acid.
The aliphatic monocarboxylic acids are known as fatty acids.
Representation: - R – COOH , where R is an alkyl group except in formic acid.
General formula: - CnH2n+1 COOH
Functional group: - The carboxylic acid contain
C O
OH
( - COOH)
as a functional group.
It is situated at the end of hydrocarbon chain as R – CH2 – COOH
Examples: - (i) H – COOH Formic acid
(ii) CH3 – COOH Acetic acid
NOMENCLATURE OF CARBOXYLIC ACIDS:- (Refer to Class 12th text Book)
STRUCTURE AND BONDING OF THE CARBOXYL ACID GROUP:-
Carboxylic acids are organic compounds having a carboxyl functional group -CO2H. The name carboxyl comes
from the fact that a carbonyl and a hydroxyl group are attached to the same carbon.
The carbon and oxygen in the carbonyl group are both sp2 hybridized which give a carbonyl group a basic trigonal
shape.
The carbonyl carbon is SP2 hybridized and, thus it has three hybrid orbitals and one unhybridised p – orbital.
2
It uses SP2 hybrid orbitals to form three sigma bonds, one with oxygen atom and remaining two with two other
atoms or groups.
All these three sigma bonds lie in same plane at an angle of 1200.
The unhybridised p – orbital of carbonyl carbon forms π bond with oxygen atom by sidewise overlapping with
half-filled p – orbital of oxygen atom as shown in fig.
C
(G.S.)
1S 2
2S2
2Px1
2Py1
2Pz0
C
(E.S.)
1S 2
2S 2Px1
2Py1
2Pz1 1
C( H. S.)
1s2
(sp2)1
(sp2)1
(sp2)1
2Pz1
O
(G.S.)
1S 2
2S2
2Px 2Py1
2Pz12
C O
120
120
120
:..
The hydroxyl oxygen is also sp2 hybridized which allows one of its lone pair electrons to conjugate with the pi
system of the carbonyl group. This make the carboxyl group planar and can represented with the following
resonance structure.
ACIDITY OF CARBOXYLIC ACIDS:-
→In aqueous solution, carboxylic acids dissociate as follows:
→Since they liberate hydrogen ions in solution, they are acidic. However they are weaker than mineral acids,
but stronger acids than alcohols and phenols.
3
→Carboxylic acids as well as carboxylate ion both are stabilized by resonance.
→However, carboxylate ion is more stabilized by resonance because its contributing structures are exactly
identical. The contributing structures of carboxylic acid involve charge separation.
→Since carboxylate ion is more stabilized by resonance than carboxylic acid therefore equilibrium in above
reaction lies very much in forward direction i.e., in favor of ionized form. Hence carboxylic acid behave as
strong acids.
→Carboxylic acids are stronger acids than phenols.
→It can be understood by comparing the hybrid structures of carboxylate ion and phenoxide ions.
→In carboxylate ion, the negative charge is equally distributed over two electronegative atoms (oxygen atoms)
while in phenoxide ion, it is present only on one oxygen.
→Thus, carboxylate ion is more stabilized as compared to phenoxide ion.
→ Hence, carboxylic acids ionize to the greater extent than phenols furnishing higher concentration of H+ ions.
Therefore carboxylic acids behave as stronger acids than phenols.
Resonating structures of carboxylate ion
O..
: O
-
O
-
-
OO..
::-
:-
Resonating structures of phenoxide ion
EFFECT OF SUBSTITUENTS ON THE ACIDITY OF CARBOXYLIC ACIDS
→The factors, which increase the stability of carboxylate ion more than the carboxylic acids, increase the acidic
strength of acid and the factors that decrease the stability of carboxylate ion decrease the acid strength.
→Electron withdrawing groups such as halo group, -NO2, -CN etc increase the acidity of carboxylic acids.
These electron withdrawing groups stabilize the carboxylate anion by dispersal of the negative charge and
increase the strength of the acid.
→Electron releasing groups like alkyl groups causes increase of negative charge, destabilize the carboxylate
anion and decrease the strength of the acid.
4
→The more powerful is the electron withdrawing substituents, the greater is the acidity.
FCH2COOH > ClCH2COOH > Br CH2COOH > ICH2COOH
→More is the number of electron withdrawing substituents, more is the strength of the acid
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH
→More closer is the electron withdrawing substituent, more is the strength of acid.
CH3CH2CH(Cl)COOH > CH3CHCl CH2 COOH > ClCH2CH2CH2COOH
The inductive effect decreases rapidly with increasing distance from the carboxylic group and so does the acid
strength.
METHODS OF PREPARATION: -
A) From primary alcohols or aldehydes (Oxidation): -
a) From primary alcohol: A primary alcohol on oxidation with acidified potassium dichromate (K2Cr2O7) or
acidified KMnO4 gives corresponding aldehyde which on further oxidation gives respective acid.
C
H
H
OH + [O]K2Cr2O7
dil. H2SO4
R C
H
OR
aldehyde
+ H2O
10 alcohol
R C
H
O
aldehyde
[O]K2Cr2O7
dil. H2SO4
R COOH+
carboxylic acid
5
e.g. (i)
+
CH3 CH2 OH + [O] CH3 C
H
O +K2Cr2O7
dil. H2SO4
H2O
ethyl alcohol acetaldehyde
CH3 C
H
O
acetaldehyde
[O]K2Cr2O7
dil. H2SO4
CH3 C O
OHAcetic acid
b) From aldehydes: - When an aldehyde is oxidised by acidified K2Cr2O7, the corresponding carboxylic acid is
obtained.
R C
H
O
aldehyde
[O]K2Cr2O7
dil. H2SO4
R COOH+
carboxylic acid
e.g. (i)
+CH3 C
H
O
acetaldehyde
[O]K2Cr2O7
dil. H2SO4
CH3 C O
OHAcetic acid
B) From alkyl cyanides ( by hydrolysis): -
Monocarboxylic acids are prepared by the hydrolysis of alkyl cyanides either by using
(i) dil. Acid or (ii) aq. alkali.
a) By using dil. Acid (Acid hydrolysis): -Alkyl cyanide (alkyl nitrile) on boiling with dil. mineral acids (dil.
HCl, dil. H2SO4) gives carboxylic acid. R C N + 2 H2O + HCl R COOH
carboxylic acid+ NH4Cl
Alkyl cyanide
e.g. (i) CH3 C N + 2 H2O + HCl CH3 COOH
acetic acid+ NH4Cl
Methyl cyanide
b) By using aq. alkali (Alkaline hydrolysis): -When alkyl cyanide is boiled with aq. alkali like NaOH or KOH
gives first alkali salt of corresponding carboxylic acid, which
on acidification gives carboxylic acid. R C N + H2O + NaOH R COONa
Sodium carboxylate
+ NH3Alkyl cyanide R COONa + HCl R COOH
carboxylic acid
+ NaCl
e.g. CH3 C N + H2O + NaOH CH3 COONa
Sodium acetate
+ NH3Methyl cyanide
CH3 COONa + HCl CH3 COOHAcetic acid
+ NaCl
(acetonitrile)
C) From dry ice (solid CO2) and Grignard reagent: -
When Grignard reagent is treated with solid carbon dioxide in presence of dry ether gives Mg – complex which
on acid hydrolysis gives carboxylic acid.
6
C
O
O + R MgXdry ether
R C
O
O MgX
Mg - complexdry ice
R C
O
O MgX + H2O R C
O
O H + Mg
X
OH
dil. HCl
caroxylic acid
(Formic acid cannot be prepared by this method.)
e.g. (i)
C
O
O + CH3 MgIdry ether
CH3C
O
O MgX
Mg - complex
dry ice methyl mag. iodide
CH3 C
O
O MgX + H2O CH3C
O
O H + Mg
I
OH
dil. HCl
Acetic acid
(Ethanoic acid)
C
O
O + CH2 MgIdry ether
CH2C
O
O MgX
Mg - complex
dry ice
H2O CH2C
O
O H + Mg
I
OH
dil. HCl
Propionic acid
ethyl mag. iodide
CH3 CH3
CH2C
O
O MgXCH3 + CH3
(Propanoic acid)
D) From Alkenes: -
R - CH = CH - R' R - COOH + R'COOHAcidifed or Alk. KMnO4
CH3- CH = CH - CH
3 2 CH
3 - COOH
Acidifed or Alk. KMnO4e.g.
acidic KMnO4
COOH
Benzoic acid
CH = CH2
phenyl ethene
+ CO2 + H
2O
E) From alkyl benzene:- Side chain alkyl group on oxidation gives benzoic acid.
CH2-CH
2-CH
2-CH
3
acidic KMnO4
COOH
n-butyl benzene Benzoic acid
7
F) From derivatives of carboxylic acids (Hydrolysis): -
Derivatives of carboxylic acids on hydrolysis gives carboxylic acids.
R - C - Cl
OH2O / H+
R - C - OH + HCl
O
Acid Chloride Carboxylic acid
R - C - NH2
OH2O / H+
R - C - OH + NH3
O
Acid amide Carboxylic acid
R - C - OR'
OH2O / H+
R - C - OH + R'OH
O
Ester Carboxylic acid
R - C - O - C -R
OH2O / H+
2 R - C - OH
O
Acid anhydride Carboxylic acid
O
Physical Properties of Carboxylic Acids
1. Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature with
unpleasant odours. The higher acids are wax like solids.
2. The lower carboxylic acids are freely miscible with water due to the presence of intermolecular hydrogen
bonding with H2O molecules. However, the solubility in water decreases gradually due to increase in the size of
alkyl group.
3. Monocarboxylic acids have higher boiling points as compared to the alcohols of comparable molecular
masses due to the presence of stronger intermolecular hydrogen bonding as shown below.
4. Melting points of aliphatic monocarboxylic acids shows alternation or oscillation effect, i.e., the m.p. of an
acid with even number of carbon atoms is higher than the next lower and next higher homologue containing odd
number of carbon atoms. This is because, in case of acids with even number of carbon atoms, the terminal -
CH3 and -COOH groups lie on the opposite sides of the zig-zag chain. As a result, they get closely packed in the
crystal lattice.
5. Glacial acetic acid is completely pure acetic acid and represents the solid state of acetic acid. Below 16.6°C
temperature pure acetic acid is converted into ice like solid hence it is called glacial acetic acid.
8
REACTIONS OF CARBOXYLIC ACIDS: -
1) ACIDIC NATURE: - Carboxylic acids are quite strong acids because of the presence of polar O – H group.
They ionize to give hydrogen ions and hence, behave as acids. These are however much weaker than mineral
acids like HCl, H2SO4 and sulphonic acids
(R – SO3H)
R C
O
OH R C
O
O-
+ H+
Carboxylic acid Carboxylate ion
a) Action of active metals: -
Carboxylic acids react with the strongly electropositive metals like Na, K, Ca, Zn etc to form the corresponding
metal salts with liberation of hydrogen gas.
R C
O
OH R C
O
ONa + H2
Carboxylic acid sodium salt of acid
+ 2 Na2 2
e.g. (i)
CH3C
O
OH CH3C
O
ONa + H2
Acetic acid sodium acetate
+ 2 Na2 2
(ii)
CH3C
O
OH CH3C
O
OK + H2
Acetic acid potassium acetate
+ 2 K2 2
(iii)
CH3C
O
OH ( CH3C
O
O)2 +
Acetic acid Calcium acetate
+ Ca2 Ca H2
(iv)
CH3C
O
OH ( CH3C
O
O)2 +
Acetic acid Zinc acetate
+ Zn2 Zn H2
b) Action of alkali: -
When aqueous solution of caustic soda (NaOH) or caustic potash (KOH) is reacted with
carboxylic acid corresponding salts are obtained.
(i)
R C
O
OH R C
O
ONa + H2O
Carboxylic acid sodium salt of acid
+ NaOH
e.g.
CH3C
O
OH CH3C
O
ONa + H2O
Acetic acid sodium acetate
+ NaOH
9
(ii)
R C
O
OH R C
O
OK + H2O
Carboxylic acid potassium salt of acid
+ KOH
e.g.
CH3C
O
OH CH3C
O
OK + H2O
Acetic acid potassium acetate
+ KOH
c) Action of sodium carbonate (washing soda) and sodium bicarbonate (baking soda): -
Carboxylic acids react with aqueous sodium carbonate or sodium bicarbonate to give corresponding sodium salts
with liberation of carbon dioxide gas.
(i)
R C
O
OH R C
O
ONa + H2O
Carboxylic acid sodium salt of acid
+ Na2CO3 + CO22 2
e.g.
2 CH3C
O
OH CH3C
O
ONa + H2O
Acetic acid sodium acetate
+ Na2CO3 + CO22
(ii)
R C
O
OH R C
O
ONa + H2O
Carboxylic acid sodium salt of acid
+ NaHCO 3 + CO2
e.g.
CH3C
O
OH CH3C
O
ONa + H2O
Acetic acid sodium acetate
+ NaHCO 3 + CO2
d) Action of ammonia (Formation of amide): -
Carboxylic acids react with ammonia to give the corresponding ammonium salts, which on heating decomposes
to form amides.
R C
O
OH R C
O
ONH4+ H2O
Carboxylic acid ammonium salt of acid
+ NH3R C
O
NH2
amide
e.g.
CH3C
O
OH CH3C
O
ONH4+ H2O
Acetic acid ammonium acetate
+ NH3CH3
C
O
NH2
acetamide
2) Action of SOCl2, PCl3 or PCl5 (Formation of acid chloride): -
Carboxylic acids react with Thionyl chloride or phosphorus chloride or phosphorus
trichloride to form acid chloride by replacement of – OH group by – Cl atom.
10
(i)
R C
O
OH R C
O
Cl + HCl
Carboxylic acid acid chloride
+ SOCl2 + SO2
thionyl chloride e.g.
CH3C
O
OH CH3C
O
Cl + HCl
Acetic acid acetyl chloride
+ SOCl2 + SO2
thionyl chloride (ii)
R C
O
OH R C
O
Cl + H3PO3
Carboxylic acid acid chloride
+ PCl33 3
e.g.
CH3C
O
OH CH3C
O
Cl + H3PO3
Acetic acid acetyl chloride
+ PCl33 3
(iii)
R C
O
OH R C
O
Cl + POCl3Carboxylic acid acid chloride
+ PCl5 + HCl
e.g.
C2H5 C
O
OH C2H5C
O
Cl + POCl3Propionic acid Propionyl chloride
+ PCl5 + HCl
3) Action of alcohols (Formation of esters): -
When carboxylic acids are heated with alcohol in presence of strong dehydrating agent such as conc. H2SO4 or
dry HCl or anhydrous ZnCl2, gives an ester and water. It is called Fischer’s esterification.
R C + H O R'OH
O
R C O
O
R' + H2O
carboxylic acid alcohol ester
esterificationH+
hydrolysis
If dry hydrogen chloride gas is used the reaction is called Fischer – Spier esterification.
e.g. When acetic acid is heated with ethyl alcohol at 413 K in the presence of dehydrating agent conc. H2SO4, it
gives ethyl acetate.
CH3 C + H O C2H5OH
O
CH3C O
O
C2H5 + H2O
Acetic acid ethyl alcohol ethyl acetate
conc. H2SO4
413 K
4) Action of P2O5 (Formation of anhydrides): -
When a carboxylic acid is heated with strong dehydrating agent like P2O5 gives an acid anhydride by removal of
water molecule from two molecules of acids.
It is an intermolecular dehydration of carboxylic acid.
11
R C O
O
+C R
O
H + HO R C O
O
C R
O
H2O
carboxylic acid acid anhydride
P2O5
P2O5 reacts with water and gives metaphosphoric acid to prevent backward reaction. P2O5 + H2O 2 HPO 3
metaphosphoric acid
e.g.(i)
CH3 C O
O
+C CH3
O
H + HO CH3 C O
O
C CH3
O
H2O
acetic acid Acetic anhydride
P2O5
(ethanoic anhydride)
(ii)
C2H5 C O
O
+C C2H5
O
H + HO C2H5 C O
O
C C2H5
O
H2O
propionc acid Propionc anhydride
P2O5
(Propanoic anhydride)
Mixed anhydride can be prepared by heating acid chloride with sodium salt of the acid
e.g.
R C +Cl
O
R C
O
+ NaCl
Acid chloride acid anhydride
C R'
O
NaO O C R'
O
sodium carboxylate
5) Reduction of carboxylic acid:-
Carboxylic acids on reduction with LiAlH4 gives primary alcohol.
R - C - OH
O
R - CH2 - OH
Carboxylic acid 10 alcohol
i) LiAlH4
ii) H2O
CH3 - C - OH
O
CH3 - CH
2 - OH
Acetic acid ethyl alcohol
i) LiAlH4
ii) H2Oe.g.
12
HELL – VOLHARD – ZELINSKY REACTION (HVZ): -
→It is α halogenation of carboxylic acid.
→It is a characteristic reaction of carboxylic acid having α hydrogen.
→It is an organic reaction in which a carboxylic acid having α hydrogen when heated with halogen in presence
of red phosphorous gives α halo acid.
R - CH2 - C - OH
O
+ X2
Red P R - CH - C - OH
O
X
HX
carboxylic acidalpha halo acid
+
→e.g. Acetic acid when heated with Br2 / red P gives bromo acetic acid
CH3 - C - OH
O
+ Br2
Red P CH
2 - C - OH
O
Br
HBr
acetic acidbromo acetic acid
+
CH3 - C - OH
O
+ Br2
Red P CH
2 - C - OH
O
Br-HBr
acetic acidbromo acetic acid
Br2 / red P
- HBr
Br2/Red P Br -CH - C - OH
O
Brdibromo acetic acid
-HBr
tribromo acetic acid
Br- C- C - OH
O
Br
Br
DECARBOXYLATION: -
a) From calcium salt of carboxylic acid (decarboxylation) (By dry distillation): -
→Calcium salts of carboxylic acids on heating give aldehydes or ketones
→Dry distillation of equimolar mixture of calcium formate and calcium salt of monocarboxylic acid gives an
aldehyde.
e.g. (i) An equimolar mixture of calcium formate and calcium acetate on dry distillation gives acetaldehyde.
CH3 C
O
O
CH3 C
O
O
Ca +
H C
O
O
H C
O
O
Cadry distillation
CH3 C
H
O
Acetaldehyde
+ 2 CaCO 3
calcium acetate calcium formate
2
ii) When calcium formate is dry distilled alone, it gives formaldehyde.
H C
O
O
H C
O
O
Cadry distillation
H C
H
O
Formaldehyde
+ CaCO3
calcium formate → Simple ketones are obtained by dry distillation of calcium salts of mono carboxylic acids, other than formic
acid.
R C
O
O
R C
O
O
Cadry distillation
R C
R
O
ketone
+ CaCO3
calcium salt of
monocarboxylic acid
13
e.g. i) Calcium acetate on dry distillation give acetone.
CH3 C
O
O
CH3 C
O
O
Cadry distillation
CH3 C
CH3
O
Acetone
+ CaCO3
calcium acetate
ii) Mixed ketones are obtained by the dry distillation of equimolar mixture of calcium salts of two different
appropriate carboxylic acids (other than formic acid).
CH3 C
O
O
CH3 C
O
O
Ca +
C2H5 C
O
O
C2H5 C
O
O
Cadry distillation
CH3 C
C2H5
O
Ethyl methyl ketone
+ 2 CaCO3
calcium acetate calcium Propionate
2
b) By Kolbe’s electrolytic decarboxylation: -
The aqueous solution of sodium or potassium salts of carboxylic acids on electrolysis give alkanes at anode. This
reaction is called Kolbe’s electrolysis.
R - COONa (aq.)
R - COO- + Na+
at anode: 2 R-COO-
R - R + CO2 + 2e-
alkane
at cathode: 2 H2O + 2e- H
2 + 2OH-
c) By soda lime decarboxylation: -
Sodium or potassium salts of carboxylic acids on heating with soda lime (NaOH and CaO) give alkanes with one
carbon less than the parent acids.
R - COONa + NaOHCaO
R -H + Na2CO
3heat
alkanesodium salt of carboxylic acid
CH3 - COONa + NaOH
CaOCH
4 + Na
2CO
3heatmethanesodium acetate
d) By Hunsdiecker reaction: -
→When silver salt of carboxylic acid react with halogen undergoes decarboxylation to give alkyl halide. This
reaction is known as Hunsdiecker reaction.
RCOO-Ag++ Br
2 R - Br + CO2 + AgBr
silver salt of carboxylic acid
Mechanism: -
→The reaction involve a free radical chain mechanism
Initiation: -
→Bromine reacts with the silver carboxylate to give an unstable acyl hypobromite
RCOO-Ag++ Br
2
silver salt of carboxylic acid
RCOOBr + AgBr
acyl hypobromite
→The weak O – Br bond undergoes homolytic cleavage to form an acyl radical RCOO - Br RCOO. + Br.
acyl radical
14
Propagation: -
→The acyl radical loses a molecule of CO2 to form an alkyl radical
RCOO. R. + CO2
→The alkyl radical reacts with the acyl hypobromite to form an alkyl bromide and generate another acyl radical
R. + RCOO - Br R - Br + RCOO.
METHODS OF PREPARATION OF α, β UNSATURATED CARBOXYLIC ACIDS:-
PERKIN REACTION (PERKIN CONDENSATION): -
→The reaction between aromatic aldehyde and aliphatic acid anhydride in presence of sodium or potassium salt
of corresponding carboxylic acid to give α, β unsaturated acid is known as perkin reaction.
→Thus it is an organic reaction used to convert an aromatic aldehyde and an anhydride to an α,β-unsaturated
carboxylic acid using sodium acetate, a base
→This reaction can also be carried out by using other bases like triethyl amine, pyridine etc
C6H
5 - C - H + CH
3 - C - O - C - CH
3
O O OCH
3 - C - ONa
O
C6H
5 - CH = CH - C - OH
O
acetic anhydride alpha - beta unsaturated acid (cinnamic acid)
Benzaldehyde
REFORMATSKY REACTION:-
Condensation reaction of carbonyl compounds with alpha haloester in presence of zinc metal is known as
Reformatsky reaction.
e.g. When benzaldehyde is treated with α–bromo ester in presence of zinc and product obtained on hydrolysis
gives β – hydroxy ester which on dehydration gives α,βunsaturated carboxylic acid.
DICARBOXYLIC ACIDS:-
PHTHALIC ACID:-
Synthesis:-
1) From Naphthalene:-
2) From O-Xylene :-
Xylene on oxidation with acidified K2Cr2O7, acidified or alkaline KMnO4 gives Phthalic acid
O
C6H
5 - C
O
H
+ Br - CH2 - C - OC
2H5 C
6H
5 - C
OZnBr
H
- CH2 - C - OC
2H5
O
C6H
5 - C
OH
H
- CH2 - C - OH
O
Hydrolysis
DehydrationC
6H
5 - CH=CH -COOH
cinnamic acid
15
CH3
CH3
O- Xylene
Acidified K2Cr2O7
COOH
COOH
Phthalic acid
Chemical Reactions: -
1) Action of heat/ Dehydrating agent: - Phthalic acid on heating or on treatment with dehydrating agent
like conc. H2SO4 undergoes dehydration to give Phthalic anhydride
2) Action of ammonia:-
SUCCINIC ACID:-
Synthesis:-
1) From Ethylene dibromide
2) From Maleic acid:-
CH-COOH
CH-COOH+ H
2
Maleic acid Succinic acid
CH2-COOH
CH2-COOH
Ni
Chemical Reactions:-
1) Action of heat/ Dehydrating agent: - Succinic acid on heating or on treatment with dehydrating agent
like conc. H2SO4 undergoes dehydration to give Succinic anhydride
16
2) Action of ammonia:-
Succinic acid
CH2-COOH
CH2-COOH
+ NH3
CH2-COONH
4
CH2-COONH
4
CH2-CONH
2
CH2-CONH
2
CH2-CO
CH2-CO
NH
Succinimide
heatheat
- NH3- 2H2O
CARBOXYLIC ACID DERIVATIVES:-
Compounds obtained from carboxylic acid by replacing – OH of – COOH group by another group (Z) are called
derivatives of carboxylic acid.
There are four carboxylic acid derivatives. These are generally represented as, where Z is halogen
(usually Cl), OCOR’, OR’ or NH2 (or NHR’ or NR2‘).
(a) When Z is halogen (usually Cl), the derivatives are called as acid chlorides.
(b) When Z is -OR’, the derivatives are called as esters.
(c) When Z is, the derivatives are called carboxylic anhydrides.
(d) Where Z is – NH2, the derivatives are called amides. When Z is -NHR’ or -NR2 they are called N –
substituted amides.
17
NOMENCLATURE OF DERIVATIVES: -
Derivative of Carboxylic acid
Acid Chloride
Common name
Replace ic acid by oyl chloride
CH3COCl (Acetyl Chloride)
IUPAC Name
Replace oic acid by oyl chloride
CH3COCl (Ethanoyl Chloride)
Acid amide Replace ic acid by amide
CH3CONH2 (Acetamide)
Replace oic acid by amide
CH3CONH2 (Ethanamide)
Acid anhydride Replace acid by anhydride
CH3COOCOCH3 (Acetic
anhydride)
Replace acid by anhydride
CH3COOCOCH3 (Ethanoic
anhydride)
Ester Replace ic acid by ate
CH3COOCH3 (Ethyl acetate)
Replace oic acid by ate
CH3COOCH3 (Ethyl ethanoate)
Reactivity and stability of acyl derivatives:- →Stability and reactivity have an inverse relationship, which means that the more stable a compound, generally the
less reactive - and vice versa.
→Acyl derivatives gives nucleophilic substitution reaction as follows
General reaction
General mechanism 1) Nucleophilic attack on the carbonyl
18
2) Leaving group is removed
→Acid halides are the most reactive towards nucleophiles, followed by anhydrides, esters,and amides.
→A major factor in determining the reactivity of acyl derivatives is leaving group ability to leave the molecule.
→ Weak bases are better leaving groups than strong bases. Thus, chloride ion is a better leaving group.
→The reactivity of acyl compounds towards nucleophiles decreases as the basicity of the leaving group
increases.
→The order of the reactivity of different acyl derivatives is as follows
→Since acyl halides are the least stable group listed above, it makes sense that they can be chemically changed
to the other types. Since the amides are the most stable type listed above, it should logically follow that they
cannot easily changed into the other molecule types, and this is indeed the case.
SYNTHESIS:-
Acid Amide:- Carboxylic acids react with ammonia to give the corresponding ammonium salts, which on heating decomposes
to form amides.
R C
O
OH R C
O
ONH4+ H2O
Carboxylic acid ammonium salt of acid
+ NH3R C
O
NH2
amide
19
e.g.
CH3C
O
OH CH3C
O
ONH4+ H2O
Acetic acid ammonium acetate
+ NH3CH3
C
O
NH2
acetamide
Urea:- Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two
–NH2 groups joined by a carbonyl (C=O) group.
NH2 - C - NH
2
O
Methods of preparation:-
1) Industrial method: Industry uses liquid ammonia and carbon dioxide as raw materials to directly
synthesize urea under high temperature and pressure conditions.
2) From Ammonium cyanate:-
3) From phosgene and ammonia:-
The reaction between phosgene (Carbonyl chloride) and ammonia gases produces urea and ammonium
chloride.
O = CCl
Cl
+ H - NH2
+ H - NH2
NH2 - C - NH
2 + 2 HCl
O
Phogene Urea
Chemical Reactions:-
1) Hydrolysis:-
20
2) Action of heat:-
Acid chloride: - Carboxylic acids react with Thionyl chloride or phosphorus chloride or phosphorus trichloride to form acid
chloride by replacement of – OH group by – Cl atom.
(i)
R C
O
OH R C
O
Cl + HCl
Carboxylic acid acid chloride
+ SOCl2 + SO2
thionyl chloride
e.g.
CH3C
O
OH CH3C
O
Cl + HCl
Acetic acid acetyl chloride
+ SOCl2 + SO2
thionyl chloride (ii)
R C
O
OH R C
O
Cl + H3PO3
Carboxylic acid acid chloride
+ PCl33 3
e.g.
CH3C
O
OH CH3C
O
Cl + H3PO3
Acetic acid acetyl chloride
+ PCl33 3
(iii)
R C
O
OH R C
O
Cl + POCl3Carboxylic acid acid chloride
+ PCl5 + HCl
e.g.
C2H5 C
O
OH C2H5C
O
Cl + POCl3Propionic acid Propionyl chloride
+ PCl5 + HCl
Esters: -
When carboxylic acids are heated with alcohol in presence of strong dehydrating agent such as conc. H2SO4 or
dry HCl or anhydrous ZnCl2, gives an ester and water. It is called Fischer’s esterification.
21
R C + H O R'OH
O
R C O
O
R' + H2O
carboxylic acid alcohol ester
esterificationH+
hydrolysis
If dry hydrogen chloride gas is used the reaction is called Fischer – Spier esterification.
e.g.1) When acetic acid is heated with ethyl alcohol at 413 K in the presence of dehydrating agent conc. H2SO4,
it gives ethyl acetate.
CH3 C + H O C2H5OH
O
CH3C O
O
C2H5 + H2O
Acetic acid ethyl alcohol ethyl acetate
conc. H2SO4
413 K
2)
Acid anhydrides: -
When a carboxylic acid is heated with strong dehydrating agent like P2O5 gives an acid anhydride by removal of
water molecule from two molecules of acids.
It is an intermolecular dehydration of carboxylic acid.
R C O
O
+C R
O
H + HO R C O
O
C R
O
H2O
carboxylic acid acid anhydride
P2O5
P2O5 reacts with water and gives metaphosphoric acid to prevent backward reaction. P2O5 + H2O 2 HPO 3
metaphosphoric acid
e.g.(i)
CH3 C O
O
+C CH3
O
H + HO CH3 C O
O
C CH3
O
H2O
acetic acid Acetic anhydride
P2O5
(ethanoic anhydride) (ii)
C2H5 C O
O
+C C2H5
O
H + HO C2H5 C O
O
C C2H5
O
H2O
propionc acid Propionc anhydride
P2O5
(Propanoic anhydride)
Mixed anhydride can be prepared by heating acid chloride with sodium salt of the acid
e.g.
R C +Cl
O
R C
O
+ NaCl
Acid chloride acid anhydride
C R'
O
NaO O C R'
O
sodium carboxylate
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CHEMICAL REACTIONS:-
1) All acid derivatives yield carboxylic acids on hydrolysis. The hydrolysis can either be acid or base
catalyzed.
2) Nucleophilic Substitution reactions:- Derivatives of carboxylic acid undergoes nucleophilic substitution
reactions in which – Z group of derivatives is replaced by another nucleophile.
One derivative of carboxylic acid can be converted into another derivative by this reaction
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Ester:-
Acid Chloride:-
Acid anhydride:-
Acid amide:-
1) Dehydration:-
2) Reduction:-
R - C - NH2
O
R - CH2 - NH
2
Acid amide 10 amine
i) LiAlH4
ii) H2O
24
3) Hydrolysis:-
Acidic Hydrolysis:-
R - C - NH2
O
R - C - OH + NH4Cl
O
+ H2O
HCl
Alkaline hydrolysis of Amides:-
R - C - NH2
O
R - C - ONa + NH3
O
+ H2O
NaOH
R - C - ONa
O
R - C - OH + NaCl
OHCl
e.g.
CH3 - C - NH
2
O
CH3 - C - ONa + NH
3
O
+ H2O
NaOH
CH3 - C - ONa
O
CH3 - C - OH + NaCl
OHCl
4) Hoffmann bromamide degradation:- When an amide is treated with bromine in an aqueous or ethanolic
solution of sodium hydroxide, degradation of amide takes place leading to the formation of primary
amine. This reaction involving degradation of amide is known
as Hoffmann bromamide degradation reaction.
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MECHANISM OF HYDROLYSIS OF ESTERS:-
A) ACID CATALYSED: -
R - C - OH + H+
O
R - C - OH
+OH
R - C - OH
OH
+
R' - OH
R - C - OH
O - R'
H
OH
+
R - C - OH
O - R'
OH- H+
+ H+
+OH
R - C - OH
O - R'
OH- H+
R - C - OH2
O - R'
OH+
R - C+
O - R'
OH
R - C - OR'- H2O
O
R - C - OR'
....
B) BASE CATALYZED: -
R C + H O R'OH
O
R C O
O
R' + H2O
carboxylic acid alcohol ester
OH-
Mechanism: -
R - OH + OH- RO- + H2O
R C OH
O
R C OH+ OR-
O-
OR
R C
O
R C OH
O-
OR
OR + OH-
MECHANISM OF HYDROLYSIS OF ESTERS:-
A) ACID CATALYSED: -
+ H+
+OH - H+
OHR - C O - R'
OH
R - C - OR'
+ H2O O
R - C - OR'+
R - C O - R'
OH
+OH2
R - C O - R'
OH
..
..
R - C - OH+ H+
O
R - C - OH
+OH
R - C - OH
OH
+
- R' - OHR - C - OH
O - R'
H
OH
+
R - C - OH
O - R'
OH- H+
B) BASE CATALYZED: -
OH-
R C + H O R'
O
R C O
O
R' + H2Oalcoholester
ONa
sodium carboxylate
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Mechanism: -
R C
OH
O-
ORR C O
O
R' + OH-
R C OH
O
R C
OH
+ OR-
O-
OR
