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Nitrogen compounds - Amines

Introduction Amines can be classified as primary, secondary and tertiary amines according to the number of alkyl groups or aryl group groups attached to the nitrogen atom. R¢w 2 NH R¢w NH¢w R NR3 e.g. 1o amine 1o amine 2oamine 3 oamine

ethanamine (ethyl amine) 2o amine

hexane-1,6,-diamine

benzenamine (phenylamine or aniline)

N-ethylethanamine (diethyl amine) 3o amine

N-methylpropanamine (methyl propyl amine)

N-methylbenzenamine (methyl phenyl amine)

N,N-diethylethanamine

N-ethyl-N-methylpropan-1-amine

N,N-dimethylbenzenamine

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Quaternary ammonium compounds and amine salts

tetramethylammonium bromide

trimethylammonium iodide

dimethylammonium bromide

methylammonium iodide

The formation of amines 1. Formation of primary amines from nitriles and amides. Nitriles and amides can be reduced by LiAlH4 or catalytic hydrogenation to from primary amines.

e.g.

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2. Formation of primary, secondary, tertiary and quaternary ammonium compounds by alkylation. e.g.

3. Formation of phenylamine by reduction of nitrobenzene The nitro group ¢w 2 in nitrobenzene can be reduced by some reducing agent to amino NO group ¢w 2 . NH e.g.

Basic properties of amines Amines are basic since the lone pair electrons on the nitrogen atom can attract a proton from an acid.

The amines can be regenerated by adding a stronger base e.g. NaOH

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Comparison of the basic strength of ammonia, primary aliphatic amines and phenylamine. The basic strength of amines depends on the availability of the lone pair electrons on nitrogen atom. For ammonia,

Kb =

For a primary amine,

Kb = For phenylamine,

Kb =

It can be seen that the basicity is in the order of:

Explanation: For 1o aliphatic amine, the alkyl group attached to nitrogen atom is electron-donating. It increases the availability of lone pair electron on nitrogen. Hence it is most basic.

For ammonia, it does not have any electron-donating or electron-withdrawing group. Hence its basic strength is less than that of 1o aliphatic amine. For phenylamine, the lone pair electrons on nitrogen can delocalize over the benzene ring. As a result, the availability of lone pair electrons decreases and it is the lease basic.

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Reactions of amines 1. Reaction with ethanoyl chloride and benzoyl chloride

2. Reaction with nitric(III) acid (nitrous acid) This reaction is limited to primary amines only. a) Primary aliphatic amines Primary aliphatic amines react with nitric(III) acid to form a highly unstable aliphatic diazonium salt which decomposes spontaneously to give nitrogen gas, alcohol and other products.

The effervescence (formation of gas bubbles) of nitrogen after the addition of sodium nitrate(III) in hydrochloric acid solution serves as a test for the primary amines. b) Primary aromatic amines Primary aromatic amines react with nitric(III) acid to form a benzenediazonium salt which is more stable. It will not decompose if the temperature is kept below 5o C.

When the temperature is above 5o C, the diazonium ion would decompose to form phenol and nitrogen gas.

3. Coupling reaction of benzenediazonium ion react with naphthalene-2-ol Under ice cold condition, benzenediazonium ion react with an alkaline solution of naphthalene-2-ol to form a bright red crystalline precipitate (azo-compound).

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The nitrogen of the diazonium group is retained in this reaction. The formation of orange or bright red precipitate after the addition of sodium nitrate(III) in hydrochloric acid solution and then treated with naphthalene-2-ol serves as the test for primary aromatic amines. And also, the crystalline precipitate produced has a sharp melting point. This melting point determination can be used for the identificatio n of primary aromatic amine. Uses of amines 1. The azo-compounds are used as dyes in dyeing industrial. 2. Amine derivatives are used as drugs.

Amino Acids Amino acids are bifunctional compounds containing both acidic carboxyl group (¢w COOH) and basic amino group (¢w 2 ) . NH

aminoethanoic acid

2-aminopropanoic acid

£\ -amino acids are amino acids with both ¢w 2 and ¢w NH COOH attached to the same carbon atom. Stereochemistry of amino acids Many amino acids are optically active because of the presence of chiral centres. e.g. 2-aminopropanoic acid

Acid-base properties of amino acids Since amino acids contain both acidic group (¢w COOH) and basic group (¢w 2 ) , they show NH both properties of acids and bases.

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Moreover, the basic amino group and the acidic carboxyl group tend to neutralize each other. A zwitterions (or dipolar ion) is formed.

Because of the formation of zwitterions, amino acids show abnormal properties such as: 1) low volatility 2) high melting point 3) low solubility in organic solvent and high solubility in water 4) high dipole moment When an amino acid is dissolved in water, several species would exist. The relative proportions of these species will depend on the pH of the solution.

Dipeptides and polypeptides as dimmers and polymers of amino acids - Compounds formed by the linkage of two or more amino acids are called peptides. - Compounds formed by two amino acid molecules are called dipeptides. - Compounds formed by two amino acid molecules are called tripeptides. - Compounds formed by more than three amino acids are called polypeptides. - Inside the peptides, the different amino acid molecules are linked together by the unit called peptide linkage (amide group).

The peptide can be hydrolyzed to form the amino acid molecules by acid or alkali.

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