Functional derivatives of carboxylic acids
Compounds such as acid chlorides, amides, esters etc., are called carboxylic acid
derivatives because they differ from a carboxylic acid only in the nature of the group or atom that has replaced the -OH group of carboxylic acid.
Group replacing - OH Name Structure Example
–Cl Acid chloride R C Cl
O CH3 C Cl
O
Acetyl chloride
–NH2 Acid amide R C NH2
O
CH3 C NH2
O
Acetamide
–OR’ ester R C OR'
O CH3 C OCH3
O
Methyl acetate
–OOCR Acid anhydride R C
O
O C
O
R CH3 C
O
O C
O
R
Acetic anhydride
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| Group replacing - OH | Name | Structure | Example |
|---|---|---|---|
| –Cl | Acid chloride | OR C Cl | OCH C ClAcetyl chloride3 |
| –NH2 | Acid amide | OR C NH2 | OCH C NHAcetamide3 2 |
| –OR' | ester | OR C OR' | OCH C OCHMethyl acetate3 3 |
| –O O CR | Acid anhydride | O OR C O C R | O OCH C O C RAcetic anhydride3 |
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Relative reactivity of Acid derivatives The reactivity of the acid derivatives follows the order
R C Cl > R C O C R > R C OR’ > R C NH2
OO OO O
The above order of reactivity can be explained in terms of i) Basicity of the leaving group ii) Resonance effect
(i) Basicity of the leaving group Weaker bases are good leaving groups. Hence acyl derivatives with weaker bases as
leaving groups (L) can easily rupture the bond and are more reactive. The correct order of the basicity of the leaving group is H2N : > : OR > RCOO : > : Cl Hence the reverse is the order of reactivity.
(ii) Resonance effect Lesser the electronegativity of the group, greater would
be the resonance stabilization as shown below. This effect makes the molecule more stable and reduces the
reactivity of the acyl compound. The order of electronegativity of the leaving groups follows the order – Cl > - OCOR > - OR > - NH2
Hence the order of reactivity of the acid derivatives with nucleophilic reagent follows the order
acid halide > acid anhydride > esters > acid amides
Nomenclature
Compound (common name, Structural formula,
IUPAC Name)
IUPAC Name
Prefix with position number
Root used Primary
suffix Secondary
Suffix
Acetyl chloride CH3 C Cl
O
Ethanoylchloride
– eth ane/ oyl chloride
Propionyl chloride C2H5 C Cl
O
Propanoylchloride
– prop ane/ oyl chloride
Benzoyl chloride C6H5 C Cl
O
Benzoylchloride
– Benz ane/ oyl chloride
R C
O
G
R C
O
G
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| C ompound(common name, Structural formula, IUPAC Name) | IUPAC Name | |||
|---|---|---|---|---|
| Prex with position number | Root used | Primar y sux | Secondar ySux | |
| Acetyl chlorideCH C ClOEthan3oylchloride | – | eth | ane/ | oylchloride |
| Propionyl chlorideC H C ClOProp2 an5 oylchloride | – | prop | ane/ | oylchloride |
| B enzoyl chlorideC H C ClOB enzo6 5 ylchloride | – | B enz | ane/ | oylchloride |
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Acetic anhydride CH3 C O C
O
CH3
O
Ethanoic anhydride
– eth ane/ oic anhydride
Propionic anhydride CH3 C O C
O
CH2
O
CH2 CH3
Propanoic anhydride
– prop ane/ oic anhydride
Benzoic anhydride C O
O
C
O
C 6 H
5 C
6 H
5
Benzoic anhydride
– Benz oic anhydride
Esters
Methyl acetate CH3 CH3C O
O
Methyl ethanoate
Methyl meth ane/ oate
Ethyl acetate CH3 C O
O
C2H5
Ethyl ethanoate
Ethyl eth ane/ oate
Phenyl acetate CH3 C O
O
C6H5
Phenyl ethanoate
Phenyl eth ane/ oate
Acid Amides
Acetamide CH3 C NH2
O
Ethanamide
– eth ane/ amide
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| Acetic anhydrideCH C O C CHEthanoic anhydride3 O O 3 | – | eth | ane/ | oic anhydride |
|---|---|---|---|---|
| Propionic anhydrideCH CH C O C CH CHPropanoic anhydride3 2 O O 2 3 | – | prop | ane/ | oic anhydride |
| B enzoic anhydrideC H C O C C HO OB enzoic anhydride6 5 6 5 | – | B enz | oic anhydride | |
| Esters | ||||
| Methyl acetateCH C O CHOMet3 hyl ethanoate 3 | Methyl | meth | ane/ | oate |
| Ethyl acetateCH C O C HOE3thyl ethanoate 2 5 | Ethyl | eth | ane/ | oate |
| Phenyl acetateCH C O C HOP3henyl ethanoat 6e 5 | Phenyl | eth | ane/ | oate |
| Acid Amides | ||||
| AcetamideCH C NHO3 2Ethanamide | – | eth | ane/ | amide |
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Propionamide C2H5 C NH2
O
Propanamide
– prop ane/ amide
Benzamide C6H5 C NH2
O
Benzamide
– benz – amide
12. 14. 2. Acid Halides:
Methods of Preparation of acid chloride:
Acid chlorides are prepared from carboxylic acid by treating it with anyone of the chlorinating agent such as SOCl2, PCl5, or PCl3
1) By reaction with thionyl Chloride (SOCl2)
C OH Cl
O O
CH3 C+ SOCl2 + HCl + SO2
Acetyl chloride
CH3
Acetic acid
This method is superior to others as the by products being gases escape leaving the acid chloride in the pure state.
Physical properties:
• They emit pale fumes of hydrogen chloride when exposed to air on account of their reaction with water vapour.
• They are insoluble in water but slowly begins to dissolve due to hydrolysis.
Chemical properties:
They react with weak nucleophiles such as water, alcohols, ammonia and amines to produce the corresponding acid, ester, amide or substituted amides.
1) Hydrolysis. Acyl halides undergo hydrolysis to form corresponding carboxylic acids
C OH
O O
CH3 C+ HOH + HClCH3 Cl Acetyl chloride Acetic acid
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| PropionamideC H C NHOPr2 o5panamide 2 | – | prop | ane/ | amide |
|---|---|---|---|---|
| B enzamideC H C NHO6 B5enzamide 2 | – | benz | – | amide |
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2) Reaction with Alcohols (Alcoholysis) gives esters.
C OC2H5
O O
CH3 C+ HOC2H5 + HClCH3 Cl
Acetyl chloride Ethyl alcohol Ethyl acetate
3) Reaction with Ammonia (Ammonolysis) gives acid amides.
C NH2
O O
CH3 C+ H + HClCH3 Cl
Acetyl chloride
NH2
Ammonia Acetamide
4) Reaction with 1o and 2o Amines gives N-alkyl amides.
R C NHR'
O O
C+ H + HClRCl NHR’ N-alkylamide1o amine
R C NR'2
O O
C+ H + HClRCl NR'2 N,N-dialkylamide2o amine
(5) Reduction.
(a) When reduced with hydrogen in the presence of ‘poisoned’ palladium catalyst, they form aldehydes. This reaction is called Rosenmund reduction. We have already learnt this reaction under the preparation of aldehydes
C
O O
CH3 C+ H 2 + HClCH3 Cl
Acetyl chloride
Pd - BaSO 4
H
Acetaldehyde 2[H]
(b) When reduced with LiAlH4 gives primary alcohols.
CH3 C Cl + 4(H) LiAlH4
CH3 CH2 OH + HCl
Ethyl alcohol
O
Acid anhydride
Methods of preparation
1. Heating carboxylic acid with P2O5
We have already learnt that when carboxylic acids are heated with P2O5 dehydration takes place to form acid anhydride.
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2. By reaction of acid halide with a salt of carboxylic acids.
Acid chlorides on heating with sodium salt of carboxylic acids gives corresponding anhydride.
C Cl
O
CH3 + C Na
O
CH3
C
O
CH3
C
O
CH3
O + NaCl
Sodium acetate Acetic anhydride
O
Acetylchloride
Chemical properties 1. Hydrolysis
Acid anhydride are slowly hydrolysed, by water to form corresponding carboxylic acids.
O
Acetic anhydride Acetic acid
CH3 C O C CH3 + H OH 2CH3 C OH
O O
2. Reaction with alcohol Acid anhydride reacts with alcohols to form esters.
O
Acetic anhydride
CH3 C O C CH3 + H OC2H5
O O
CH3 C OC2H5 + CH3 C
O
Ethylalcohol Ethyl acetate Acetic acid
OH
3. Reaction with ammonia
Acid anhydride reacts with ammonia to form amides.
O
Acetic anhydride Acetic acid
CH3 C O C CH3 + H NH2 CH3 C NH2 + CH3
O O
C OH
O
AcetamideAmmonia
4. Reaction with PCl5
Acid anhydride reacts with PCl5 to form acyl chlorides. O
Acetic anhydride Acetyl chloride
CH3 C O C CH3 + PCl5 2CH3 C Cl + POCl3
O O
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Esters Methods of preparation 1. Esterification
We have already learnt that treatment of alcohols with carboxylic acids in presence of mineral acid gives esters. The reaction is carried to completion by using an excess of reactant or by removing the water from the reaction mixture. 2. Alcoholysis of Acid chloride or Acid anhydrides
ii) Treatment of acid chloride or acid anhydride with alcohol also gives esters Physical Properties
Esters are colour less liquids or solids with characteristic fruity smell. Flavours of some of the esters are given below.
S.No Ester Flavour
1 Amyl acetate Banana
2 Ethyl butyrate Pineapple
3 Octyl acetate Orange
4 Isobutyl formate Raspberry
5 Amyl butyrate Apricot
Chemical Properties 1. Hydrolysis
We have already learnt that hydrolysis of esters gives alcohol and carboxylic acid. 2. Reaction with alcohol ( Transesterification)
Esters of an alcohol can react with another alcohol in the presence of a mineral acid to give the ester of second alcohol. The interchange of alcohol portions of the esters is termed transesterification
O
Ethyl acetate
CH3 C OC2H5 + HOC3H7
O
CH3 C
Ethyl alcohol
H+
OC3H7+ C2H5OH
Propyl acetatePropyl alcohol
The reaction is generally used for the preparation of the esters of a higher alcohol from that of a lower alcohol. 3. Reaction with ammonia (Ammonolysis)
Esters react slowly with ammonia to form amides and alcohol. O
Ethyl acetate
CH3 C OC2H5 + H
O
CH3 C
Ethyl alcohol
NH2 + C2H5OH
Acetamide
NH2
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| S.No | Ester | Flavour |
|---|---|---|
| 1 | Amyl acetate | Banana |
| 2 | Ethyl butyrate | Pineapple |
| 3 | Octyl acetate | Orange |
| 4 | Isobutyl formate | R aspberr y |
| 5 | Amyl butyrate | Apricot |
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4. Claisen Condensation Esters containing at least one ∝- hydrogen atom undergo self condensation in the presence
of a strong base such as sodium ethoxide to form b- keto ester. O
Ethyl acetate
CH3 C OC2H5 + H
O
CH3 C
Ethyl alcohol
C2H5ONa CH2
Ethyl aceto acetateEthyl acetate
CH2 C OC2H5
O
C OC2H5 + C2H5OH
O
5. Reaction with PCl5 Esters react with PCl5 to give a mixture of acyl and alkyl chloride
O
Ethyl acetate
CH3 C OC2H5 + PCl5
O
CH3 C
Ethyl chloride
Cl + C2H5Cl + POCl3
Acetyl chloride
Evaluate yourself
Why is acid anhydride preferred to acyl chloride for carrying out acylation reactions ?
Acid Amides
Acid amides are derivatives of carboxylic acid in which the – OH part of carboxyl group has been replaced by – NH2 group. The general formula of amides are given as follows.
R C NH2
O Now, we shall focus our attention mainly on the study of chemistry of acetamide.
Methods of Preparation
1. Ammonolysis of acid derivatives
Acid amides are prepared by the action of ammonia with acid chlorides or acid anhydrides. O
Acetyl chloride
CH3 C Cl + H
O
CH3 C NH2 + HCl
Acetamide
NH2
O
Acetic anhydride Acetamide
CH3 C O C CH3 + H CH3 C NH2 + CH3
O O
NH2 C OH
O
2) Heating ammonium carboxylates Ammonium salts of carboxylic acids (ammonium carboxylates) on heating, lose a
molecule of water to form amides. O
Ammonium acetate
CH3 C O - NH+ 4
O
CH3 C NH2 + H2O
Acetamide
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3) Partial hydrolysis of alkyl cyanides (Nitriles)
Partial hydrolysis of alkyl cyanides with cold con HCl gives amides
Methyl cyanide
CH3 C CH3 C NH2
Acetamide
N Conc HCl
H2O / OH O
Chemical Properties
1. Amphoteric character
Amides behave both as weak acid as well as weak base and thus show amphoteric character. This can be proved by the following reactions.
Acetamide (as base) reacts with hydrochloric acid to form salt O
Acetamide
CH3 C
O
CH3 C NH3 Cl
Acetamide hydrochloride
NH2 + HCl +
Acetamide (as acid) reacts with sodium to form sodium salt and hydrogen gas is liberated. O
Acetamide
2CH3 C
O
2CH3 C NHNa + H2
Sodium acetamide
NH2 + 2Na
2) Hydrolysis
Amides can be hydrolysed in acid or in alkaline solution on prolonged heating O
Acetamide
CH3 C
O
CH3 C OH + NH4Cl
Acetic acid
NH2 + H2O dil HCl
O
CH3 C
O
CH3 C ONa +
Sodium acetate
NH2 NaOH
Acetamide
NH 3
3) Dehydration
Amides on heating with strong dehydrating agents like P2O5 get dehydrated to form cyanides.
Acetamide
O
CH3 C CH3 C N + H2O
Methyl cyanide (aceto nitrile)
NH2
P2O5
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4) Hoff mann’s degradation Amides reacts with bromine in the presence of caustic alkali to form a primary amine
carrying one carbon less than the parent amide. O
Acetamide
CH3 C CH3NH2 + K2CO3 + 2KBr + 2H2O
Methyl amine
NH2 + Br2 + 4 KOH
5) Reduction Amides on reduction with LiAlH4or Sodium and ethyl alcohol to form corresponding amines.
O
Acetamide
CH3 C
Ethyl amine
NH2 + 4 (H) CH3 CH2 NH2 + H2O LiAIH4