The general reaction for primary amines is:

(RCO)₂O  +  R'NH₂  ===> RCONHR'  +  RCOOH

where R and R' = alkyl or aryl, to give a secondary amide (N-substituted amide)

 

The general reaction for secondary amines is:

(RCO)₂O  +  R'NHR"  ===> RCONR'R"  +  RCOOH

where R, R' and R" = alkyl or aryl, to give a tertiary amide (N,N-substituted amide)

 

Don't be put off by coming across a cycloalkyl amine e.g. cyclohexylamine

Cyclohexylamine reacts with ethanoic anhydride to yield N-cyclohexylethanamide (an N-substituted secondary amide).

 

Again, as with acid chlorides, tertiary amines (NRR'R") cannot react with acid anhydrides to form amides because there is no hydrogen atom to be replaced by an acyl group.

For more on these reactions see

6.11 Amides - molecular structure, physical properties, preparations, reactions, brief mention of polyamides

7.13 Examples of aromatic compounds from the pharmaceutical industry and those found in natural products for the synthesis of Paracetamol - a secondary amide

TOP OF PAGE and sub-index

8.5.3 The hydrolysis of amides

If amides are heated (refluxed) with dilute strong acids or strong alkalis, they are hydrolysed back to the original acid or its salt.

(a) e.g. with strong acids (HCl, H₂SO₄)

RCONH₂  +  H⁺  +   H₂O  ==> RCOOH  +  NH₄⁺

the free carboxylic acid is formed and the ammonium salt of the acid.

 

(b) or with strong alkali (NaOH, KOH)

RCONH₂  +  OH^–  ==> RCOO^–  +  NH₃

the sodium salt of the carboxylic acid is formed and ammonia released.

TOP OF PAGE and sub-index

8.5.4 Synthetic polyamides e.g. Nylon formation, structure, uses and hydrolysis

Diamines and dicarboxylic acids (or a derivative like a diacid chloride) can undergo condensation polymerisation to yield polyamides e.g.

(a) n H₂N-R-NH₂  +  n HOOC-R'-COOH  ===>  -(-NH-R-NH-CO-R'-CO-)_n-  +  2n H₂O

(a) n H₂N-R-NH₂  +  n ClOC-R'-COCl  ===>  -(-NH-R-NH-CO-R'-CO-)_n-  +  2n HCl

NYLONS 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] e.g. depicted using structural formulae and skeletal formulae for a nylon-x and nylon-x,y where x and y = 6, shown as structural formula and skeletal formula

nylon-6 ,

nylon-6,6 ,

The next four diagrams depict the formation of a Nylon polymer and then specifically, the synthesis of Nylon-6,6.

The synthesis involves two different monomer molecules, both of which must have a functional group at each end capable of condensing and bonding with the functional group of the other monomer,

e.g. -NH₂ and -COCl  or -NH₂ and -COOH to form a secondary amide linkage H-N-C=O.

This is the same linkage in peptides and proteins formed naturally from the polymerisation of amino acids, structure H₂N-CHR-COOH, which have both condensing functional groups on the same molecule.

The general equation for the condensation polymerisation of a diacid dichloride (of a dicarboxylic acid) and a diamine with the formation of the amide link (H-N-C=O) at both ends of the monomer molecules and the elimination of a hydrogen chloride molecule for each bond that is formed.

 

The general equation for the condensation polymerisation of a dicarboxylic acid and a diamine with the formation of the amide link (H-N-C=O) at both ends of the monomer molecules and the elimination of a water molecule for each bond that is formed.

 

The equation for the condensation polymerisation of hexane-1,6-dioyl dichloride and 1,6-diaminohexane (1,6-hexanediamine, hexane-1,6-diamine, hexamethylenediamine) to make Nylon-66, with the formation of the polyamide peptide link (H-N-C=O) at both ends of the monomer molecules and the elimination of a hydrogen chloride molecule for each bond that is formed.

This is the equation for the laboratory demonstration of making Nylon-6,6 (See basic Nylon notes)

 

The equation for the condensation polymerisation of a hexane-1,6-dicarboxylic acid (hexanedioic acid, adipic acid) and hexane-1,6-diaminohexane (1,6-hexanediamine, hexane-1,6-diamine, hexamethylenediamine) with the formation of the amide link (H-N-C=O) at both ends of the monomer molecules and the elimination of a water molecule for each bond that is formed.

This is the equation for the actual industrial manufacture of Nylon-6,6 and n is around 20 000 repeating units.

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.

 

It is possible to make a Nylon with just one monomer, as long as the molecule has two functional groups, one at each end of the molecule that can condense together to give the polyamide bond e.g. Nylon-6 from 6-aminohexanoic acid.

 

A brief note on the manufacturing, structure and uses of Nylon

In the manufacturing process the strength of Nylon fibres are increased by a technique called cold-drawing.

The threads are tensioned to encourage the polymer molecules to line up and therefore increasing the surface-surface contact and increasing the intermolecular bonding forces of attraction.

(i) Increase in instantaneous dipole - induced dipole forces, AND

(ii) allows increased hydrogen bonding (llll) between the C=O and the NH groups of adjacent polymer molecules that have become more closely aligned with each other i.e. >C=O^δ-llll^δ+H-N<.

The resulting strong fibres are used in stockings and fabrics for carpets.

Nylon is a tough strong material that doesn't melt until ~250^oC.

Nylon is actually strong enough to be used to make mechanical parts for machines including bearings and rollers.

Nylon has a high electrical resistance and is used to make safe switches operating electrical circuits.

The hydrolysis of Nylon - hydrolysis of a polyamide

In the equations I've omitted state symbols for simplicity. Technically Nylon is (s), H₂O(l) and the rest are (aq).

(a) A reminder of the formation of Nylon-66 - the opposite of hydrolysis

The condensation polymerisation of hexane-1,6-diamine and hexane-1,6-dioic acid

n H₂N-(CH₂)₆-NH₂  +  n HOOC-(CH₂)₄-COOH 

===>  -(-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n-  +  2n H₂O

(b) Carboxylic acid amides like ethanamide are very slowly hydrolysed by water.

They are readily hydrolysed by refluxing with hydrochloric acid or sodium hydroxide.

However, In the case of polyamides, there is no reaction with water.

It would not be very good if nylon clothing slowly reacted with sweat or rain!!!!

(c) Polyamides are slowly hydrolysed by refluxing with sodium hydroxide solution e.g. for Nylon-66

  -(-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n-  +  2n NaOH 

===> n H₂N-(CH₂)₆-NH₂  +  n Na^+-OOC-(CH₂)₄-COO^-Na⁺

 and

-(-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n-  +  2n OH^- 

===> n H₂N-(CH₂)₆-NH₂  +  n ^-OOC-(CH₂)₄-COO^-

The two initial products are the free amine and the salt, disodium hexane-1,6-dioate.

You should be able to detect the fishy odour of the amine

After separation of the amine, the dicarboxylic acid can be freed by adding dilute hydrochloric acid.

Na^+-OOC-(CH₂)₄-COO^-Na⁺  +  2HCl  ===>  HOOC-(CH₂)₄-COOH  +  2NaCl

^-OOC-(CH₂)₄-COO^-  +  2H⁺  ===>  HOOC-(CH₂)₄-COOH

If you are not interested in the amine, there is no need to separate it out, just add excess hydrochloric acid and the dicarboxylic acid should precipitate out.

(c) Polyamides are hydrolysed faster by refluxing with hydrochloric acid solution e.g. for Nylon-66

-(-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n- + 2n HCl  + 2H₂O 

===> n Cl^-+H₃N-(CH₂)₆-NH₃⁺Cl^- + n HOOC-(CH₂)₄-COOH

and

-(-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-)_n-  +  2n H⁺  +  2H₂O 

===> n ⁺H₃N-(CH₂)₆-NH₃⁺  +  HOOC-(CH₂)₄-COOH

The two initial products are the free dicarboxylic acid and the chloride salt of the diamine.

If hydrochloric, nitric or sulfuric acid are spilled on nylon clothing, holes are likely to appear.

(e) A note on plastic containers used to store chemicals

Aqueous solutions of acids or alkalis can be safely stored in poly(ethene) or poly(propene) containers.  The C-H and C-C bonds are strong and non-polar so these polymers are not susceptible to chemical attack from hydrogen (H⁺) or hydroxide (OH-) ions.

However, the H-N-C=O bonds are polar and much more susceptible to chemical attack e.g. by protonation by the hydrogen ion from an acid or nucleophilic attack from a hydroxide ion.

To avoid over repetition of notes PLEASE note where to read more on the details of POLYAMIDES ...

Basic notes on Condensation polymers including school demonstration of making Nylon

The properties and uses of Nylon

Basic notes on High performance polymers like KEVLAR

An introduction to the structure, properties and uses of Kevlar - more advanced notes.

7.12 The structure, properties and uses of polyamides including more advanced notes on Nomex & Kevlar

Includes hydrogen bonding diagrams.

For addition polymerisation (polymers from alkene monomers) see

for basic notes: Addition polymers - plastics, poly(ethene),  PVC etc., uses, problems, recycling

and includes a comparison of addition and condensation polymerisation,

and for more advanced notes see Advanced Organic Chemistry Notes section 2.8:

Polymerisation of alkenes - addition polymers - structure, properties and uses of poly(alkenes)