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Advanced A/AS Level Organic Chemistry: Amides - preparation and reactions

Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

6.11 Amides - molecular structure, preparation, reactions

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

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

INDEX of all carboxylic acids and derivatives notes

All Advanced A Level Organic Chemistry Notes

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6.11.1 The structure and physical properties of amides

diagram structure of acid amide acyl amide functional group general structural formula advanced level organic chemistry 

e.g. primary aliphatic amides:  (c) doc b or (c) doc b ethanamide, (c) doc b propanamide

(c) doc b butanamide, (c) doc b pentanamide, and the aromatic primary amide  (c) doc b 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 (c) doc b

ethylamine

(c) doc b

ethylmethylamine

(c) doc b

triethylamine

Aliphatic amine examples.

The N of the amine group NOT directly attached to a benzene ring

Aromatic amine examples (c) doc b

phenylamine

(c) doc b

diphenylamine

(c) doc b

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 (c) doc b

ethanamide

(c) doc b

N-methylbenzamide

(c) doc b

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.

explaining diagram hydrogen bonding between amide molecules and hydrogen bonding between amides and water molecules in aqueous solution

Examples of hydrogen bonding between amide molecules in the pure liquid and the hydrogen bonding between water molecules and dissolved amide molecules.


6.11.2 Preparation of amides from acyl/acid chlorides

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).

(i) ethanoyl chloride + ammonia ==> ethanamide + hydrogen chloride

(c) doc b + 2NH3  ===>  (c) doc b +  NH4+Cl-

This equation illustrates the formation of the primary aliphatic amide, ethanamide

 

(i) pentanoyl chloride + ammonia ==> pentanamide + hydrogen chloride

(c) doc b +  2NH3  ===>  (c) doc b +  NH4+Cl-

This equation illustrates the formation of the primary aliphatic amide, pentanamide

(iii) benzoyl chloride  +  ammonia  ===>  benzamide  +  hydrogen chloride

(c) doc b +  2NH3  ===>  (c) doc b +  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.

(i) ethanoyl chloride  +  methylamine  ===>  N-methylethanamide  +  hydrogen chloride

(c) doc b  +  2(c) doc b  ===>  (c) doc b  +  CH3NH3+Cl-

This illustrates the formation of a secondary amide, N-methylethanamide

 

(ii) ethanoyl chloride  +  phenylamine  ===>  N-phenylethanamide  +  hydrogen chloride

(c) doc b  +  2(c) doc b  ===>  (c) doc b  +  C6H5NH3+Cl-

These illustrates the formation of a secondary amide, N-phenylethanamide

 

(iii) ethanoyl chloride  +  dimethylamine  ===> N,N-dimethylethanamide + hydrogen chloride

(c) doc b  +  2(c) doc b  ===>  (c) doc b  +  (CH3)2NH2+Cl-

(c) doc b  +  2(c) doc b  ===>  (c) doc b  +  (CH3)2NH2+Cl-

This illustrate the formation of a tertiary amide, N,N-dimethylethanamide

 

For more see

6.7.4 Reaction of acid/acyl chlorides with ammonia: primary amide formation & mechanism

 


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

(c) doc b (aq)  +  H+(aq)  +  H2O(l)  ===>  (c) doc b (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

(c) doc b (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)  ===>  (c) doc b (aq)


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)

(c) doc b  +  4[H]  ===>  (c) doc b  +  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.

 

 


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 (c) doc b 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 the skeletal formula of Nylon-6,6

(c) doc b  or   (c) doc b

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

 

Polyamides like Nylon make strong fibres for clothing and ropes and Kevlar is fabricated into a very strong material used in body armour!

 

For some basic notes on the structure and use of polyamides see

Condensation polymers, Nylon, Terylene, comparing thermoplastics, fibres, thermosets

and High performance polymers like KEVLAR

and note that polypeptides are also polyamides


Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of all carboxylic acids and derivatives notes

 All Advanced Organic Chemistry Notes

 

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