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A2 Advanced Organic Chemistry - Nitrogen Compounds


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 AS Organic Chemistry)

(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

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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. α-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 :

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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 E/Z) isomerism and optical isomersim. Geometrical isomerism results from a lack of rotation around a carbon-carbon π-bond, and has already been discussed in AS Organic Chemistry : Alkenes.

Optical isomerism occurs as a result of a molecule having two mirror images that are non-superimposable on each other. 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 α-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 plane 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. {D- stands for dextrorotatory}

A molecule that blocks plane-polarised light when rotated in a negative(i.e. anticlockwise) direction is called the L- isomer. {L- stands for laevorotatory}

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:

OR
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Nitrogen Compounds - Proteins

Proteins are polypeptide compounds formed by the condensation polymerisation of α-amino acids. This could be one α-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.

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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

Amides can also be prepared by the reaction of an ester with ammonia, at room temperature over the course of five days. I performed this reaction for my PhD thesis: Liquid ammonia (50 ml) was poured into methanol (130 ml) which was cooled down in an ice/water bath, along with sodium methoxide (0.3 g, 6 mmol) and diethyl methylmalonate (50 g, 287 mmol). The mixture was left to stand, stoppered, for 5 days, then filtered. The solid was boiled in methanol, then filtered again, and dried. The yield was 29.92 g (90%) of white solid,

CH3CH2OOCCH(CH3)COOCH2CH3 + 2NH3 → H2NCOCH(CH3)CONH2 + 2CH3CH2OH

(3) Reactions :

Amides can be dehydrated to give nitriles. For example, I performed this reaction for my PhD thesis : A mixture of 2-methyl malonamide (10.32 g, 89 mmol), phosphorus pentoxide (29.74 g, 210 mmol), and sand (14 g) were shaken well together, then distilled, using a bunsen burner, under water pump pressure to give the product. The yield was 3.77 g (53%) of white solid, m.pt.=35-37 °C (literature m.pt.=34-34.5 °C).

H2NCOCH(CH3)CONH2 → NCCH(CH3)CN + 2H2O

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Nitrogen Compounds - Nitriles

(1) Nomenclature :

(2) Formation :

See the haloalkanes section in the AS Organic Chemistry unit.

Also, see the reactions of amides above.

(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):

(ii) Reduction -

Powerful reducing agents, such as lithium aluminium hydride, LiAlH4 (also known as lithium tetrahydridoaluminate), turn nitriles into primary amines,

RCN + 4[H] → RCH2NH2

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Monday 26th March, 2012

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