Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
1
Carboxylic Acids and their Derivatives
I.
Introduction
Carboxylic acids are those organic compounds that contain the carboxylic group
, which is the combination of the carbonyl group >C=O and the hydroxyl
group –OH.
O
C
OH
Example
(1)
Aliphatic carboxylic acids:
H
H3C
C
CH3CH2CH2
C
________________
OH
Br
OH
C
CH2 CH2
HO
OH
________________
_____________________
O
O
C
OH
_________________
Aromatic carboxylic acids:
COOH
COOH
COOH
O2N
________________
II.
OH
Cl
OH
_________________
O
O
CH3 CH CH2 CH CH2 C
CH3 CH CH CH2 C
________________
(2)
_____________________
O
CH3 CH2 CH CH2 C
HO
OH
_________________
O
O
C
OH
OH
C
O
O
O
COOH
_________________
_____________________
Formation of Carboxylic Acids
(A)
Hydrolysis of Nitriles
1.
Hydrolysis of cyanohydrins or -hydroxy nitriles (prepared from aldehydes and
ketones) results in -hydroxy carboxylic acids.
OH
R
C
H
2.
O
+
HCN
R
C
H
OH
H3O+
CN
R
C
COOH
H
Hydrolysis of nitriles (prepared from nucleophilic substitution of alkyl halides with
sodium cyanide) by a base, or by prolonged refluxing in acid solution produces the
acid salt and ammonia or the acid.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
2
Examples:
NaCN
BrCH2CH2CH2Br
H3O+
Note: This method is limited to the use of primary alkyl halides. Use of secondary or tertiary
halide often lead to elimination to form alkenes rather than substitution because the cyanide ion is
a strong base.
(B)
Oxidation of Alkanols and Aldehydes
Oxidizing agents used : KMnO4 / H+ or K2Cr2O7 / H+
+
R-CHO
KMnO4 / H
+
KMnO4 / H
R-CH2OH
(C)
Oxidation of Aromatic side chains
The side chains of alkylbenzenes are almost easily oxidized by strong oxidizing agents
such as hot potassium manganate (VII) to give the acids.
CH3
KMnO4 /
+
H
HEAT
Note: (1) The oxidation take place at the benzylic carbon. Alkylbenzenes with alkyl groups
greater than methyl are also degraded to benzoic acid.
R
KMnO4 /
+
H
HEAT
(2)
The oxidation begins at the benzylic hydrogen. Those do not possess any benzylic
hydrogen are resistant to side chain oxidation.
CH3
CH2
CH3
CH3
+
KMnO4 /
H
HEAT
NO REACTION
(3)
If the aromatic side chain contains an aldehyde or ketone group, the oxidation
product is determined by the strength of the oxidizing agents.
CH2CHO
KMnO4 /
HEAT
CH2=CH2
+
H
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
3
CH2CHO
K2Cr2O7 /
+
H
HEAT
III.
Acidity and Influence of substituents on acidity
Carboxylic acids are rater strong acids when compared with alcohols but are weak acids when
compared with mineral acids.
The acid strength of carboxylic acid is sufficient enough to liberate carbon dioxide from
metallic carbonate , metallic hydrogencarbonate and to liberate hydrogen from some reactive
metals.
CH3COOH + Na2CO3
CH3COOH + Mg


Note: The acids are displaced by stronger acids from their salts.
RCOO-Na+ + HCl

RCOOH + NaCl
Reason for acidity
(A)
Acidity
The strength of an acid HA, i.e. the extent to which it is dissociated in the aqueous
phase, is determined by the following equilibrium:
H2O + HA
H3O+ + A-
Ka = _________________
Ka is the acidity constant. For weak acids, the value of Ka is very small. It is more
convenient to express Ka by pKa where pKa = - log10Ka .
The smaller the value of pKa , the ___________________ is the strength of that
acid.
(B)
Factors affecting Acidity
1.
The strength of the H-A bond.
2.
The electronegativity of A
3.
Factors stabilizing its conjugated anion A- with respect to HA.
4.
The nature of the solvent.
Example: Compare the acidity of the following compounds:
OH
O
CH3C H
A
O
CH3C OH
B
CH3CH2 OH
C
D
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
4
Interpretation:
(1)
A has the lowest acidity
It is because the carbon atom which is attached to the H atom has lower
electronegativity than the oxygen atom in the rest compounds. This make the O-H
weaker than the C-H bond and thus it is easier to break.
(2)
C has lower acidity than B and D.
The anion for B ( a carboxylate ion) and D (a phenoxide ion) are more stable than
that of C (an alkoxide ion ) due to the stabilization of the resonance effect.
(aromaticity)
OH
+
H2O
O
CH3C OH
these
overall
+
H2O
(3) B has higher acidity than D
It is due to the fact that delocalization of the negative charge in the carboxylate anion
involves resonance structures of identical energy content and involves two highly
electronegative oxygen atoms.
In the phenoxide anion, the resonace structure involving negative charge on the nuclear
carbon atoms are likely to be of higher energy content than it is on oxygen atoms,
canonical forms are therefore less stable and have a small contribution to the
hybrid.
(C) Effects of Substituents on Acidity
can
The ease of dissociation of the acidic compound, and the stability of the conjugate anion, are
both affected by the substituent groups in the molecules. Any groups which can stabilize the
resulting anion of the acid molecule will increase its acidity whereas any groups which
destabilized the anion will also decrease its acidity. General speaking,
Electron withdrawing groups increase the acidity : -F , -Br , -Cl , -NO2
Electron donating groups decrease the acidity: -CH3 , -C2H5
For carboxylic acids,
<1> With electron-donating substituent
O
E C OH
when dissociation.
O
E C O
electron donating substituent
The electron donating substituent pushes electrons
towards the elecron-rich –COO- group and thus destabilize
the anion.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
<2> With electron-withdrawing substituent
(E : electron withdrawing substituent)
O
O
E C O
E C OH
O-H bond weakened by
electron withdrawing effect
of E
+
5
+
H
anion stabilized by electron
withdrawing effect of E
These effect. can be seen in the following table.
Interpretation
<1> The more numerous the electron-withdrawing groups on the  carbon, the stronger is the acid.
Thus the order of acidity
ethanoic acid < chloroethanoic acid < dichloroethanoic acid <trichloroethanoic acid
<2> The more the electronegative the -substituent , the stronger is the acid. Thus the order of acidity
ethanoic acid < iodoethanoic acid < bromoethanoic acid < chloroethanoic acid < fluroethanoic
acid
<3> The shorter the carbon chain on the carboxylic acid, the stronger is the acid. Thus the order of
acidity
Propanoic acid < ethanoic acid < methanoic acid
Remark : The farther away of the electron-withdrawing substituent from the carbonyl group, the less
effect on acidity.
O
CH3CH2CH C OH
Cl
Ka: 1.4 x 10-3
O
O
CH3CH CH2C OH
Cl
8.9 x 10-5
CH2CH2CH2C OH
O
CH3CH2CH2C OH
Cl
3.0 x 10-5
1.5 x 10-5
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
IV.
1.
6
Reaction of Carboxylic Acids
Decarboxylation
This is the removal of carbon dioxide from a carboxylic acid group. The usual method is to heat
the sodium salt of the acid with soda-lime (NaOH + CaO). The product is a hydrocarbon, e.g.
CH3COO-Na+ + NaOH 
COO-Na+
+
2.
NaOH
Reduction Carboxylic acids are resistant to the ordinary reducing agents but lithium aluminium
hydride reduces them directly to primary alcohols:
LiAlH4
RCOO-Na+
/ dry ether
H3O+
The reagent is highly selective and the C=C , CC , or C6H5- in unsaturated acids are unaffected
by it.
3.
Conversion to Acid Derivatives
O
(i)
Conversion to Acid chlorides ( R C Cl )
There are three reagents (PCl5, PCl3 and SOCl2) for the preparation of acid chlorides and
all of them give high yield.
O
RCOOH
+
(1)
(ii)
(2)
RCOOH +
(3)
RCOOH
+
R C Cl +
PCl5
POCl3 +
HCl
PCl3
SOCl2
Conversion of Acid anhydrides
Carboxylic acids react with alkanoyl chlorides in the presence of pyridine to give acid
anhydrides.
O
RCOOH
+
Pyridine
R C Cl
What is the use of Pyridine?
The pyridine is used to remove the hydrogen chloride HCl formed, so as to shift the
equilibrium towards the products side.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
(iii)
7
Conversion of Acid amides
Carboxylic acids react with aqueous ammonia to form ammonium salts. By heating this
ammonium salt, primary amides will be obtained.
excess RCOOH
RCOOH
(iv)
+
NH3
RCOO-NH4+
RCONH2
+
H2O
Conversion to Ester
Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
through a condensation reaction known as esterification.
The reaction is a reversible equilibrium process and it is acid catalysed.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
V.
8
Acid Derivatives
(A)
Acid Chlorides
Acid chlorides are also called alkanoyl or acyl chlorides. They are derived from the-COOH
group of carboxylic acids by replacing the -OH group with -Cl group, e.g.
___________________
____________________
______________________
Reactions of Acid Chlorides
The halogen readily undergoes nucleophilic substitution by other nucleophiles such as OH-,
OR' , NH2- etc. The mechanism is quite similar to condensation reaction of aldehydes and
ketones. Aromatic alkanoyl chloride like benzoyl chloride is much less reactive, due to the
decrease in the nucleophilicity of the carbonyl carbon caused by resonance.
1.
Hydrolysis
The lower aliphatic alkanoyl chlorides are hydrolysed rapidly by cold water, e.g.
CH3COCl + H2O  CH3COOH + HCl
When the stopper of a bottle of ethanoyl chloride is removed, white fumes are evolved,
owing to interaction of the hydrogen chloride with moist air. Beware of damage to the
naked eyes.
Aromatic alkanoyl chlorides hydrolyse much more slowly, owing to their lower
solubility and their carbonyl carbon atom being less susceptible to nucleophilic attack.
The hydrolysis is faster with alkali because - OH is a stronger nucleophile than H2O.
2.
Alcoholysis - Ester Formation
Acid halides react with alcohols and phenols to form esters, e.g.
Phenol behaves similarly as aliphatic alcohols but a base catalyst is required for it to
react with aromatic alkanoyl chlorides. The basic medium provides a more powerful
O
nucleophilic ion,
3.
(phenoxide ion)
Ammolysis – Amide formation
O
O
CH3
+
C
NH3
CH3
+
C
NH4Cl
NH2
Cl
Ethanamide
O
O
C
+ CH3NH2
Cl
C
+ HCl
NHCH3
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
4.
9
Anhydride Formation
O O
O
R
+
C
R'COONa
R
R'
O
Cl
5.
C
C
+ NaCl
Reduction to Alcohols
O
R
+
C
Pt
+
RCH2OH
H2
H2
Cl
6.
Ketone formation
Friedel-crafts acylation
O
O
R
catalyst
Cl
(B)
R
AlCl3
+
C
C
Acid Anhydride
O
Acid anhydrides are derived from carboxylic acids by replacing the -OH group in
C OH
by
O
a
R C O
group, e.g.
_______________________
______________________
Reactions of Acid Anhydrides
Reactions of acid anhydrides are quite similar to that of acid chlorides, although their
reactivity is less than that of acid chlorides.
.
1. Hydrolysis
Acid anhydrides are slowly hydrolysed by water to give carboxylic acids.
(RCO)2O+ H2O  RCOOH
2.
Ester FormationAcid anhydrides react with alcohols and phenols to give esters, heating
is usually required for the reaction.
(RCO)2O + R'OH 
RCOOR' + RCOOH
With phenols, alkaline medium is required to provide a powerful nucleophile, the
phenoxide ion.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
10
3.
Amide Formation
Acid anhydrides react with ammonia solution and amines to give amides.
(RCO)2O + 2NH3  RCONH2 + RCOO-NH4+
4.
Friedel-crafts Acylation
Acid anhydrides react with benzene in the presence of a little Lewis acid catalyst, AlCl3,
to form a ketone.
(C) ESTERS
O
Esters are derived from carboxylic acids by replacing the -OH group in
group from an alcohol, e.g.
___________________
____________________
C OH
by an -OR
____________________
Reactions of Esters
1. Hydrolysis of Ester
Acid-catalysed hydrolysis is the reverse of esterification:
O
R
O
H3O+ / Reflux
C
OR'
+ H2O
R
C
OH
+
R'OH
A large excess of water is used but since the reaction is reversible, the hydrolysis never goes
to completion.
Alkali-catalysed hydrolysis is also reversible:
O
R
O
OH- / reflux
C
OR'
+ H2O
R
C
OH
+
R'OH
However, in this case the carboxylic acid formed immediately reacts with hydroxide ion to
give the acid anion:
O
O
+
R
C
OH
OH-
R
C
O
+
H2O
The equilibrium in this step lies almost completely to the right, so that once the acid is
formed in the first reaction it is immediately removed by the second reaction, until practically
all the ester is converted into its hydrolysis product.
Alkaline hydrolysis is therefore much faster than acid catalysed hydrolysis; and it goes to
completion. The process is known as saponification 皂化反應 as soaps are made by this
reaction. The resulting carboxylate product can be re-acidified to get back the carboxylic
acid.
2.
Ammonolysis - Amide Formation
Ester reacts with ammonia or amines to form amides.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
3.
11
Reduction to Alcohols
This is a good way to prepare primary alcohols as esters are reduced by lithium aluminium
hydride (LiAlH4), e.g.,
(D) ACID AMIDES
O
C OH
and
Amides are derived from carboxylic acids by dropping the -OH group in
replacing it with a -NH, group, e.g.
___________________
_____________________
_____________________
Substitutions of the hydrogens on the nitrogen atom by alkyl groups can also occur, e.g.
________________________
__________________________
Reactions of Acid Amides
1. Hydrolysis
The amides are readily hydrolysed on refluxing with dilute acid or alkali.
O
R
O
H3O+ / Reflux
C
NH2
+
H2O
R
O
R
C
C
OH
+
NH4+
O
NH2
+
OH-
R
C
O
+
NH3
The fact that heating an amide with caustic soda liberates ammonia whilst an amine does
not react enables the two types of compound to be distinguished.
2.
Dehydration
Nitrile Formation Heating amides with a dehydrating agent such as phosphorus
pentoxide leads to formation of nitrile.
Lok Sin Tong Leung Chik Wai Memorial School
F.7 Chemistry
Carboxylic acids & their derivatives
3.
12
Hofmann Degradation
Amides react with KOH solution and Br2 in a rearrangement reaction to give primary
amines containing one carbon atom less. Thus it is useful for descending a homologous
series.
O
CH3CH2
3.
C
NH2
+
KOH
+
Br2
CH3CH2NH2
K2CO3
+
KBr
+
H2O
Reduction of Amines
Amides are reduced to primary amines by either lithium aluminium hydride or sodium
in ethanol (dissolving metal reduction).
O
R
C
LiAlH4 / dry ether
NH2