A2 unit chains and rings and spectroscopy nitrogen compounds page

Chemistry Rules!^'06
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Chains and Rings and Spectroscopy - Nitrogen Compounds

Primary Amines

Formation of primary amines -

Alkyl amines
Aromatic amines

Reactions of amines

With acids
Diazotisation

Amino Acids

Amides

Nitriles

Nitrogen Compounds - Primary Amines

The only amines that need to be considered here are primary amines, i.e. amines in which the nitrogen atom has only one carbon atom bonded to it,

where R=any alkyl or arene group.

(1) Formation of Primary Amines:

(a) Formation of alkylamines -

Simple primary amines, such as ethylamine, are prepared by the reaction of a haloalkane with an excess of alcoholic ammonia :

(see the haloalkanes section in chains and rings)

(b) Formation of phenylamine -

Phenylamine is prepared not by a substitution reaction as above, but by the reduction of nitrobenzene by a refluxing mixture of tin and concentrated hydrochloric acid :

(2) Reactions of Primary amines:

(a) Reaction with mineral acids -

Primary amines react with dilute mineral acids in a similar manner to ammonia. That is, they react to form salts :

Amines react with nitric and sulphuric acids to produce the corresponding nitrates and sulphates in a similar manner.

(b) Diazotisation -

When phenylamine reacts with a freezing cold (i.e. 0 ^oC) mixture of nitrous acid, HNO₂, and hydrochloric acid, a diazonium salt is formed :

This salt can be reacted with phenols, e.g. 2-naphthol, to give intensively coloured azo-dyes :

These dyes are used for colouring clothing, etc..

(3) Basicity :

Like ammonia, primary amines act as bases, with the lone pair on the nitrogen atom accepting a proton from an acid. However, the basic strength of the amine is directly related to the groups bonded to the nitrogen atom.

Any alkyl groups attached to the nitrogen push electron density towards the lone pair of the nitrogen atom, thus increasing its reactivity towards acids. i.e. alkyl amines are stronger bases than ammonia :

Any arene groups, i.e. a benzene ring, attached to the nitrogen atom pull electron density away from the lone pair on the nitrogen atom. This has the effect of making the lone pair on the nitrogen atom less able to accept a proton so making the aromatic amine a weaker base than ammonia :

So, overall, alkyl amines are stronger bases than ammonia which is a stronger base than phenylamine :

RNH₂ > NH₃ > ArNH₂

Nitrogen Compounds - Amino Acids

The only amino acids that are of concern at A level are those with the amine group and carboxylic acid groups on adjacent carbon atoms, i.e. a-amino acids, RCH(NH₂)COOH,

where R=H or any alkyl or arene group.

The amine and carboxylic acid groups behave as bases and acids as normal :

At a compound-specific pH level both the amine group and the carboxylic acid groups are present as ions. This is known as a zwitterion :

Nitrogen Compounds - Optical Isomerism

Isomers of a molecule that result from a difference in the arrangement of groups in 3-D space, and not in the type and/or position of covalent bonds, are known as stereoisomers.

There are two types of stereoisomerism: geometrical (or cis/trans) isomerism and optical isomersim. Geometrical isomerism results from a lack of rotation around a carbon-carbon p-bond, and has already been discussed in Chains and Rings:Alkenes.

Optical isomerism occurs as a result of a molecule having a mirror image that is non-superimposable on it. In Organic Chemistry that is usually the result of a carbon atom present having four different groups attached to it. Such a carbon atom is known as a chiral carbon atom:

Even though there is no chiral carbon atom present optical isomerism is also possible in Inorganic Chemistry, for example when a hexadentate transition metal complex ion is formed from three bidentate ligands:

{see the transition elements option in trends and patterns for more details}

As shown above, all a-amino acids, with the exception of glycine - H₂NCH₂COOH, have a carbon atom bonded to four different groups.

The two optical isomers are identical in virtually all respects. They will do the same general chemical reactions and will give the same results with I.R., N.M.R. and Mass Spectroscopy. Their physical properties are the same (for example, melting point, boiling point, refractive index), with the exception of their effect on plane-polarized light.

When light that has been polarized, so that it travels in only one plane, is shone on an optical isomer and the isomer is rotated, at a particular angle the light is absorbed. The other optical isomer will absorb the light when rotated in the opposite direction.

Amino acids present in living systems all block plane-polarized light when rotated in a positive (i.e. clockwise) direction. They are called D- isomers.

When synthetic reactions are carried out that make compounds with the possibility of optical isomers, a 50:50 mixture of both isomers will be made. This is called a racemic mixture. Such a mixture will appear to have no effect on plane-polarized light, as the two isomers cancel out each others effect.

For example, when turning ethanal into 2-hydroxypropanoic acid (lactic acid) the first stage is the nucleophilic addition of HCN to ethanal.

The attack of the cyanide ion, ^-:CN, on the carbonyl group can occur either from above or beneath the molecule:

Each pathway is equally likely, so a 50:50 mixture of the two stereoisomers will be formed:

Nitrogen Compounds - Proteins

Proteins are polypeptide compounds formed by the condensation polymerisation of a-amino acids. This could be one a-amino acid or, which is more likely in nature, a mixture of amino acids :

Proteins can be broken down by the action of heat and hydrochloric acid (as they are in the stomach) to amino acids :

The amino acids can then used by the body to make the chemicals it requires.

Nitrogen Compounds - Amides

(1) Nomenclature :

The basis for an amide is the -CONH₂ group - known as an amide or acid amide because it is derived from a carboxylic acid. The number of carbon atoms present determines the prefix for the compound -

1 carbon atom - methan_______

2 carbon atoms - ethan________

3 carbon atoms - propan_______

etc.....

The suffix for all amides is simply amide tagged onto the end of the prefix directly.

Exemplar compounds -

ethanamide -	butanamide -

(2) Formation :

Amides are most usually formed by the reaction of an acid chloride (R-COCl) with ammonia (NH₃) or a primary amine (R'-NH₂).

RCOCl + NH₃

RCONH₂ + HCl

RCOCl + R'-NH₂

RCONHR' + HCl

Nitrogen Compounds - Nitriles

(1) Nomenclature :

(2) Formation :

See the haloalkanes section in the AS unit Chains and Rings.

(3) Reactions :

(i) Hydrolysis -

Nitriles are hydrolysed in the presence of dilute acid and heat to give an amide first, and then on further reaction a carboxylic acid.

RCN + H⁺ + 2H₂O

RCO₂H + NH₄⁺

The final stage of the synthetic production of 2-hydroxypropanoic acid (lactic acid) involves the hydrolysis of the cyanohydrin produced after the nucleophilic addition of HCN to ethanal (see above):