• 27.1

What are amino acids?

• Organic compounds derived from ammonium in which one or more hydrogen atoms in ammonia have been replaced by a carbon chain or ring.

• ALPHATIC AMINE

  • nitrogen atom attached to at least one straight or branched carbon chain

  • e.g. methylamine, CH3NH2

• AROMATIC AMINE

  • nitrogen atom attsched to an aromatic ring

  • e.g. phenylamine, C6H5NH2

Naming amines:

Naming a primary amine:

• add the suffix -amine to the name of the alkyl chain

• where a primary amine contains an amine group on any other carbon but carbon-1, add the prifix amino- and the number to indicate the position of the amine group

Secondary and tertiary amines:

• the prfixes di- or tri- are used to indicate the number of alkyl groups attached to the nitrogen atom

Reactions of amines:

Amines as bases:-

Why do amines behave as bases?

• The lone pair of electrons on the nitrogen atom can accept a proton

Salt formation:-

Amines are bases and so neutralise acids to make salts

e.g.:

• propylamine + hydrochloric acid —> propylammonium chloride

• ethylamine + sulfuric acid —> ethylammonium sulfate

Preparation of alphatic amines:

Formation of primary amines:-

1. The lone pair on the nitrogen allows ammonia to act as a nucleophile in a substitution reaction with a haloalkane

2. The product would be an ammonium salt

3. Aqueous alkalki is then added to generate the amine from the salt

e.g.

• CH3CH2CH2Cl + NH3 —> CH3CH2CH2NH3+Cl-

  1-chloropropane propylammonium chloride (salt)

• CH3CH2CH2NH3+Cl- + NaOH —> CH3CH2CH2NH2 + NaCl + H2O

  propylammonium chloride propylamine

Conditions:

• Ethanol is used as a solvent to prevent any substituation of the haloalkane by water to produce alcohols

• Excess ammonia is used to reduce further substitution of the amine group to form secondary and tertiary amines

Formation of secondary and tertiary amines:-

The reaction above is unsuitable for making pure primary amines, the product still contains a lone pair of electrons on the nitrogen atom, which is able to further react with a haloalkane to form a seconday amine.

e.g.

• CH3CH2CH2NH2 + CH3CH2CH2Cl —> (CH3CH2CH2)2NH2+Cl-

  propylamine dipropylammonium chloride

Teritary amines are formed by further reaction of the seconday amine. In this example, it would form tripropylammonium chloride, (CH3CH2CH2)3N

Preparation of aromatic amines

1. Phenylamine, C6H5NH2, is made by the reduction of nitobenzene

2. Nitobenzene is heated under reflux with tin and hydrochloric acid to form the ammonium salt, phenylammonium chloride

3. It is then reacted with excess sodium hydroxide to produce the aromatic amine, phenylamine

• 27.2

Amino acids:

What are they?:-

• an organic compound containing both amines, -NH2 and carboxylic acid, COOH, functional groups

• they have the general formula - RCH(NH2)COOH

Reactions of the amine group:

1. They act as bases to react with acids and make salts therefore, the amino acid can also react with acids to form salts

Reactions of the carboxylic acid group in amino acids:

1. The carboxylic acid can react with alkali’s to form salts and with alcohols to form esters if in the presence of concentrated sulfuric acid

   • e.g. aminoethanoic acid reacts with sodium hydroxide to form a sodium salt and water

Zwitterions:

Within the amino acid, the lone pair of electrons on the amine group is able to accept a proton from the carboxylic acid group to form an ion containing both a positive and negative charge — known as a zwitterion

They have no overall charge as the positive and negative charge cancel out

What is the isoeletric point?:-

• It is the pH at which the zwitterion is formed

• Each amino acid has its own unique isoeletric point

  • If an amino acid is added to a solution with a pH greater than its isoeletric point, the amino acid behaves as an acid and looses a proton — forming a negative ion

  • If an amino acid is added to a solution with a pH lower than its isoeletric point, the amino acid acids as a base and gains a proton — forming a positive ion

Amindes:

Amides are the product of reacing acyl chlorides with ammonia and amines

Stereoisomerism:

Compounds with the same structural formula but different arrangement of atoms

Optical isomerism:-

• Found in molecules that contain a chiral centre

  • A chiral centre is a carbon atom that is attached to four different atoms or groups of atoms

  • The presence of a chiral carbon atoms in a molecule leads to two non-superimposable mirror image structures — known as optical isomers

• When drawing optical isomers, they are shown in a 3D tetrahedral arrangement. One isomer being a mirror image of the other

• 27.3

Polyesters and polyamides

What is condensation polymerisation?:-

• Condensation polymerisation is the joining of monomers with loss of a small molecule, usually water or hydrogen chloride

• Two different functional groups are needed for this to happen

Polyesters

• monomers are joined together by ester linkages in a long chain to form the polymer

• they can be made from one monomer containing both a carboxylic acid and an alcohol group, or from two monomers — one containing two carboxylic acid groups and the other containing two alcohol groups

Polyamides

• polyamides are condensation polymers formed when monomers are joined together by amide linkages in a long chain to form the polymer

• they can be made from one monomer containing both a carboxylic acid (or acyl chloride) and an amine group — or two monomors containing two carboxylic acid groups (or acyl chlorides) and the other containting two amine groups

Hydrolysing condensation polymers

• Polyesters and polyamides can be hydrolysed using hot aqueous alkali (like sodium hydroxide) or by hot aqueous acid (like hydrochloric acid)

• Hydrolysing with a base is preferable over an acid as it is not a reversible reaction like hydrolysis with acid