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Part 6.
The Chemistry of Carboxylic Acids and their Derivatives
6.11
The chemistry of amides
- molecular structure, preparation, reactions, polyamides
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6.11
The chemistry of amides
- molecular structure, preparation, reactions, polyamides
Sub-index for this page
6.11.1
The structure and physical properties of amides
6.11.2
Preparation of amides
from acyl/acid chlorides or acyl/acid anhydrides
6.11.3
Reactions of amides -
hydrolysis with acid or alkali
6.11.4
Reactions of amides - reduction with lithium
tetrahydridoaluminate(III)
6.11.5
Polyamides - examples
of condensation polymers
6.11.1 The structure and physical properties of amides
e.g. primary aliphatic amides:
 or
 ethanamide,
 propanamide
 butanamide,
 pentanamide,
and the aromatic primary amide
 benzamide
Revision of
the sub-classification of amines and amides
| Functional group |
PRIMARY |
SECONDARY |
TERTIARY |
Comments |
| AMINES |
 |
 |
 |
There are
prim/sec/tert aliphatic (alkyl) or aromatic (aryl) amines. |
| Aliphatic amine examples |
ethylamine |
ethylmethylamine |
triethylamine |
Aliphatic amine examples.
The N of the amine group NOT directly attached to a
benzene ring |
| Aromatic amine examples |
phenylamine |
diphenylamine |
N,N-diethylphenylamine |
Aromatic amine examples
The N of the amine group directly attached to a
benzene ring. |
| Acyl or acid AMIDES |
 |
 |
 |
The amide group
comprises an amine group attached to the C of a C=O carbonyl group,
which gives it its own unique chemistry i.e. its neither an amine,
aldehyde or ketone! |
| Examples of amides |
ethanamide |
N-methylbenzamide |
N,N-dimethylethanamide |
Examples of amides
both aliphatic and aromatic |
Examples of
primary aliphatic amides and physical properties, plus one
aromatic primary amide
| Name |
Formula |
Mpt/oC |
Bpt/oC |
Comments including
solubility at room temperature |
| Methanamide |
HCONH2 |
3 |
dec.193 |
Very soluble in water. |
| Ethanamide |
CH3CONH2 |
82 |
221 |
Moderately soluble in
water. |
| Propanamide |
CH3CH2CONH2 |
81 |
213 |
Moderately soluble in
water. |
| Butanamide |
CH3CH2CH2CONH2 |
116 |
216 |
Moderately soluble in
water. |
| |
|
|
|
|
| Benzamide |
C6H5CONH2 |
132 |
290 |
Aromatic amide, slightly soluble in water. |
| |
|
|
|
|
| |
|
|
|
|
Notes on the primary aliphatic
amides
(i) For the size of the molecule,
in terms of numbers of electrons, they have relatively high melting
points and boiling points due to the extra contribution of the
permanent dipole permanent dipole intermolecular forces, including
hydrogen bonding
The contribution of the
'always present' instantaneous dipole - induced dipole intermolecular force
becomes more important as the hydrocarbon chain length increases.
Permanent dipole – permanent dipole
interaction between neighbouring polar molecules via the
carbonyl group
δ+C=Oδ–....δ+C=Oδ–
There are two sites on the
molecule
for hydrogen bonding
The –NH2 group i.e.
δ–N–Hδ+
llllδ–:N–Hδ+ as well as the hydrogen bonding due
to the carbonyl group - amine group attractions
δ+C=O:δ–llllδ+H–Nδ–
Total intermolecular force =
(instantaneous dipole – induced dipole) + (permanent dipole –
permanent dipole including hydrogen bonding) +
(permanent dipole – induced dipole).
The melting/boiling points
tend to be higher than the equivalent acid chloride and even the
carboxylic acid.
Ethanoyl chloride bpt 51oC,
ethanoic acid bpt 118oC and ethanamide 221oC,
though this comparison ignores differences in the number of
electrons in the molecule, which is 40, 32 and 32
respectively. You don't get hydrogen bonding in acid
chlorides but presumably the total permanent dipole -
permanent dipole intermolecular attractive forces (including
hydrogen bonding) is greater in ethanamide than in ethanoic
acid because it has a similar shape and same number of
electrons (clouds to distort).
See a
Detailed comparative discussion of boiling points of 8 organic molecules
(ii) The lower members are quite
soluble in water, because the amide molecules can hydrogen bond with
water
via the carbonyl group or the amine group.
e.g.
δ–N–Hδ+llllδ–:O–Hδ+
and
δ+C=O:δ–llllδ+H–Oδ–
The solubility of benamide in
water is inhibited by the hydrophobic benzene ring.
Examples of
hydrogen bonding between
amide molecules in the pure liquid and the hydrogen bonding between water
molecules and dissolved amide molecules.
TOP OF PAGE
and sub-index
6.11.2
Preparation of amides from acyl/acid chlorides or acyl/acid anhydrides
The organic synthesis of amides from acid/acyl chlorides and
ammonia Because acid chlorides
react with water, the reaction is usually carried in anhydrous
conditions - all reagents and glassware should be dry.
Examples of
nucleophilic addition of ammonia to acid/acyl chlorides, subsequent elimination gives the
primary amide and hydrogen
chloride/hydrochloric acid unless excess ammonia/amine is used -
which gives the chloride salt of the base (which I have chosen to do in the
equations below - assuming excess of the ammonia/amine base).
(a) Using acyl/acid chlorides
RCOCl + R'NH2
===> RCONHR + HCl
R = alkyl or aryl, R' = H, alkyl or
aryl
(1) ethanoyl chloride
+ ammonia ==> ethanamide + hydrogen chloride
+ 2NH3 ===>
+ NH4+Cl-
This equation illustrates the
formation of the primary aliphatic amide, ethanamide
(2) pentanoyl chloride
+ ammonia ==> pentanamide + hydrogen chloride
+ 2NH3 ===>
+ NH4+Cl-
This equation illustrates the
formation of the primary aliphatic amide, pentanamide
(3) benzoyl chloride + ammonia
===> benzamide + hydrogen chloride
+ 2NH3 ===>
+ NH4+Cl-
This equation illustrates the
formation of the primary aromatic amide, benzamide
The organic synthesis of
secondary amides from acid/acyl chlorides reacting with a
primary amine, AND the
formation of tertiary amides by acid chlorides reacting with
secondary amines. (4) ethanoyl chloride +
methylamine ===> N-methylethanamide + hydrogen chloride
+
2
===>
+ CH3NH3+Cl-
This illustrates the
formation of a secondary amide, N-methylethanamide
(5) ethanoyl chloride +
phenylamine ===> N-phenylethanamide + hydrogen chloride
+
2
===>
+ C6H5NH3+Cl-
These illustrates the
formation of a secondary amide, N-phenylethanamide
(6) ethanoyl chloride +
dimethylamine ===> N,N-dimethylethanamide + hydrogen chloride
+ 2
===>
+ (CH3)2NH2+Cl-
+ 2
===>
+ (CH3)2NH2+Cl-
This illustrate the
formation of a tertiary amide, N,N-dimethylethanamide
need 4th skeletal formula!
For more see
6.7.4
Reaction of
acid/acyl chlorides with
ammonia: primary amide formation & mechanism
(b) Using acyl/acid anhydrides
(RCO)2O + R'NH2
===> RCONHR + RCOOH
R = alkyl or aryl, R' = H (ammonia), alkyl or
aryl (aliphatic or aromatic amines
Examples
(i) Formation of a primary
acid/acyl amide
propanoic anhydride +
ammonia ===> propanamide + propanoic acid
+ NH3 ===>
+
+ NH3 ===>
+
(ii)
Formation of a secondary
acid/acyl amide (an N-substituted
amide)
ethanoic anhydride +
methylamine ===> N-methylethanamide + ethanoic acid
+ CH3NH2 ===>
+
and
ethanoic anhydride +
phenylamine ===> N-phenylethanamide +
ethanoic acid
+
===>
+
For more see
6.7B.6
Reactions of acid/acyl anhydrides including those with ammonia and amines
TOP OF PAGE
and sub-index
6.11.3 Reactions of amides - hydrolysis with acid or alkali
If amides are refluxed with strong acid
(hydrochloric or sulfuric) or alkali potassium/sodium hydroxide, the product
is the original carboxylic acid or its salt.
(a) Acid
Hydrolysis of amides
e.g. the formation of the free
weaker carboxylic acid
RCONH2(aq) + HCl(aq) + H2O(l)
===> RCOOH(aq) + NH4Cl(aq)
and the more correct ionic
equation
RCONH2(aq)
+ H+(aq) + H2O(l)
===> RCOOH(aq) + NH4+(aq)
e.g. propanamide hydrolysed to propanoic acid
(aq)
+ H+(aq) + H2O(l)
===>
(aq) + NH4+(aq)
(b) Alkaline hydrolysis of amides
e.g. the formation of the salt of the
parent carboxylic acid.
RCONH2(aq)
+ NaOH(aq) ===> RCOONa(aq)
+ NH3(aq/g)
and the more correct ionic equation
RCONH2(aq) + OH-(aq) ===> RCOO-(aq)
+ NH3(aq/l)
e.g. hydrolysis of benzamide to benzoic acid
(aq) + OH-(aq)
===>
 (aq)
+ NH3(aq/l)
The carboxylic acid is freed
by adding a stronger mineral acid
RCOO-(aq)
+ H+(aq) ===> RCOO-(aq)
e.g.
(aq)
+ H+(aq) ===>
(aq)
TOP OF PAGE
and sub-index
6.11.4 Reactions of amides - reduction with lithium
tetrahydridoaluminate(III)
Primary amides are reduced by lithium
tetrahydridoaluminate(III) to a primary aliphatic
amine
RCONH2 + 4[H]
== LiAlH4 ==> RCH2NH2 + H2O
The reaction must be carried at room temperature in a
dry solvent like ethoxyethane ('ether') because it reacts with water
(and ethanol too).
e.g. benzoic acid is reduced to
benzylamine (a primary aliphatic amine, not an aromatic amine)
+ 4[H] ===>
 + H2O
This is an example of the reduction of a polar C=O pi
bond via the hydride ion (H-) acting as a nucleophile
generated by the tetrahydridoaluminate(III) ion, which does not reduce
the non-polar pi bond C=C you find in alkenes.
TOP OF PAGE
and sub-index
6.11.5 Polyamides - examples of condensation polymers
Polyamides are secondary amides formed in a condensation
reaction between a dicarboxylic acid and an diamine.
To effect a condensation polymerisation. a small
molecule must be eliminated between the monomer molecules.
This is usually water or hydrogen chloride to make
the secondary polyamide polymer e.g. for water
-COOH
+ H2N- ==> -CO-NH- + H2O
is the basic condensation process.
Nylons are polyamides are formed by condensing together
a dicarboxylic acid and a diamine (nylon-x,y) OR polymerising an amino
carboxylic acid (nylon-y). [x = length carbon atoms in amine, y = length of
carbon atoms in carboxylic acid]
The formation and molecular structure of nylon-6,6
from 1,6-hexanediamine and hexanedioic acid.
(old names hexamethylenediamine and adipic
acid, and the amine can also be named as 1,6-diaminohexane.
(This polymer is called Nylon-6,6 because each monomer
has a chain of 6 carbon atoms).
Simplified equation with the elimination of water
between the joined ends of the monomer molecules.
n HOOC-(CH2)4COOH
+ n H2N-(CH2)6-NH2
==> -(-OC(CH2)4-CO-NH-(CH2)6-NH-)n-
+
2n H2O
The structural formula and skeletal formula of
Nylon-6,6
or
n = very large number
You can also make the same Nylon by using the diacid
chloride of hexanedioic acid, so the simplified equation is ..
n ClOC-(CH2)4COCl
+ n H2N-(CH2)6-NH2
==> -(-OC(CH2)4-CO-NH-(CH2)6-NH-)n-
+
2n HCl
Two equations for the formation of Nylon 66 are repeated in
structural formula style and with the repeating unit shown.
You need to be able to work out the structural formula
of the original monomers i.e. the original aliphatic dicarboxylic acid
or its dichloride and the aliphatic diamine.
You also need to be able to point out the link formed on
condensation i.e. the HN-CO- linkage.
Polyamides like Nylon make strong fibres
for clothing and ropes and Kevlar is fabricated into a very strong material
used in body armour!
See Organic chemistry Part 8.5 for more on
Nylon and
its hydrolysis with acid or alkali
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Polymerisation of alkenes to addition polymers - structure, properties, uses of
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chemistry - a mention of
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The structure, properties and uses of
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