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AMINES

Amines are derivatives of ammonia (NH3), obtained by replacement of 1, 2 or all the 3 hydrogen atoms by alkyl

and/or aryl groups. In nature amines are present in - proteins, vitamins, alkaloids and hormones.

Synthetic amines are present in polymers, dyestuffs and drugs.

Name of some compounds Nature Function/Use

Adrenaline secondary amine increase blood pressure

Ephedrine secondary amine increase blood pressure

Novocain synthetic amino compound an anaesthetic in dentistry

Benadryl Compound having tertiary amino group antihistaminic drug

Cationic Detergent Quaternary ammonium salts surfactants

Diazonium salts Have diazo group the preparation of a variety of aromatic

compounds including dyes

STRUCTURE OF

FUNCTIONAL GROUP:

CLASSIFICATION:

Nomenclature

PREPARATION OF AMINES: [GHAR]

1.Gabriel phthalimide synthesis

2. Hoffmann bromamide degradation reaction- an

amide (-CO-NH2) on reaction with bromine in an

aqueous or ethanolic solution of NaOH gives a 10 amine

with one carbon less than the amide.

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3.Ammonolysis of alkyl halides: the process of

cleavage of the C–X bond by ammonia molecule is

known as ammonolysis.

The free amine can be obtained from the

ammonium salt by treatment with a strong base:

The order of reactivity of halides with amines is

RI > RBr >RCl

4.REDUCTION:by LiAlH4,Na+C2H5OH,H2/Ni

a. Reduction of nitriles:

b.Reduction of amides:

c. Reduction of nitro compounds:

Physical Properties:

1. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.

2. Solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl

part.

3. 1º & 2º amines form intermolecular H- bonding. This H-Bonding is more in 1º amines than in 2º amines as

there are 2 hydrogen atoms available for H-bond formation in it. 3º amines do not form H- bond. Therefore, the

B.Pt. of isomeric amines follows the order: 1º> 2º > 3º

Basic Nature of amines:

The reaction of amines with

mineral acids to form

ammonium salts shows basic

nature of amines. Amines

have an unshared pair of

a. Alkanamines (R-NH2) versus ammonia(NH3): Due to the electron releasing nature of alkyl group(+I effect), it pushes electrons towards N

and thus makes the unshared electron pair more available for sharing

with the proton of the acid. Moreover, the substituted ammonium ion

formed from the amine gets stabilized due to dispersal of the positive

charge by the +I effect of the –R group. Hence, alkyl amines are

stronger bases than ammonia.

b. Basicity of amines in the gaseous phase: 3º amine > 2º amine >1º amine > NH3.

c. Basicity of amines in the aqueous phase, the substituted ammonium cations get stabilized not only by electron releasing effect of the alkyl

group (+I) but also by solvation with water molecules. The greater the

size of the ion, lesser will be the solvation and the less stabilised is the

ion. The order of stability of ions are as follows:



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electrons on N- atom due to

which they act as Lewis

base.

Larger the value of Kb or

smaller the value of pKb,

stronger is the base.

Structure-basicity

relationship of amines:

Basic character of an

amine depends upon

i. the ease of formation of the cation by accepting

a proton from the acid.

ii. Stability of the cation. The more stable the

cation relative to the

amine, more basic is

the amine.

When the -R group is small, like –CH3 group, there is no steric

hindrance to H-bonding. In case the alkyl group is bigger than -CH3

group, there will be steric hinderance to H-bonding. Therefore, the

change of nature of the alkyl group from -CH3 to –C2H5 results in

change of the order of basic strength. The combination of inductive

effect, solvation effect and steric hinderance of the –R group which

decides the basic strength of alkyl amines in the aqueous state. The

order of basic strength in case of methyl substituted amines and ethyl

substituted amines in aqueous solution is as follows:

d. Aryl amines are weaker base than ammonia: in aniline or other

arylamines, the -NH2 group is attached directly to the benzene ring.

The unshared electron pair on nitrogen atom is in conjugation with the

benzene ring and thus making it less available for protonation.

5 canonical structures ,it is more stable

only 2 canonical structures ,it is less stable

Hence, the proton acceptability or the basic nature of aniline or other aromatic

amines is less than that of ammonia.

e. Basicity of substituted aniline- electron releasing groups like –OCH3, –CH3 increase basic strength whereas electron withdrawing groups like

–NO2, –SO3H, –COOH, –X decrease it.



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CHEMICAL REACTIONS OF AMINES

REACTION DESCRIPTION & REACTION COMMENT/NOTE

1.Alkylation R-X + NH3 RNH + R2NH + R3N + [R4N]+Cl- [R4N]+Cl- on reaction with

NaOH regenerate amine

2.Acylation Reaction of aliphatic and aromatic 1º and 2º amines with

acid chlorides, anhydrides and esters by nucleophilic

substitution reaction to give amide.

1.The reaction is carried out in

the presence of a base stronger

than the amine, like pyridine,

which removes HCl so formed

and shifts the equilibrium to the

right hand side.

2. Amines also react with

benzoyl chloride (C6H5COCl).

This reaction is known as

benzoylation.

3. What do you think is the

product of the reaction of amines

with carboxylic acids ?

Carbylamine

reaction/

isocyanide test

Aliphatic and aromatic 1ºamines on heating with chloroform

and ethanolic KOH form isocyanides or carbylamines which

have foul smell.

2º and 3ºamines do not show this

reaction. This reaction is used –

as a test for 1º amines & a

method for preparation of

isocyanide.

Reaction with

nitrous acid

a.1º aliphatic amines react with nitrous acid to form aliphatic

diazonium salts which are unstable & decompose to liberate

N2 gas and alcohols.

b. 1ºAromatic amines react with HNO2 at low temperatures

(0-5ºC) to form diazonium salts.

This reaction is used to –

distinguish between 1º aliphatic

& 1º aromatic amines

Reaction with

arylsulphonylchl

oride

(C6H5SO2Cl)

a. reaction with 1º amine

b. reaction with 1º amine

N, N-diethylbenzene sulphonamide does not contain any H-

atom attached to H- atom, so it is not acidic and hence

insoluble in alkali.

c. 3º amine does not react with benzenesulphonylchloride

a. This reaction is used to –

i.distinguish 1º,2º,3º amines &

2.separate 1º,2º,3º amines

ii. Now a days

benzenesulphonyl chloride is

replaced by p -toluenesulphonyl

chloride.

Electrophilic

substitution

(a) Bromination: i. –NH2 group is ortho and para directing and a powerful



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&

(b)Nitration:

(c)Sulphonation:

activating group.

ii.Aniline does not undergo

Friedel-Crafts alkylation and

acetylation due to salt formation

with AlCl3, the Lewis acid,

which is used as a catalyst. Due

to this, nitrogen of aniline

acquires positive charge and

hence acts as a strong

deactivating group for further

reaction.

i. Activating effect of –NHCOCH3 group is

less than that of –NH2

group because the

lone pair of electrons

on nitrogen is less

available for donation

to benzene ring by

resonance.

DIAZONIUM

SALTS

DESCRIPTION COMMENT/ NOTE

general formula= R N2 +X-

where R = an aryl

group and

X- may be Cl– Br-

HSO4- , BF4- , etc.

Preparation: by the reaction of aniline with nitrous

acid (HNO2) at 273-278K. HNO2 is produced in the

reaction mixture by the reaction of NaNO2 with

HCl. The reaction is known

as diazotisation.

1º aliphatic amines form highly

unstable alkyldiazonium salts.

1º aromatic amines form

arenediazonium salts which are

stable at low temperatures (273-

278 K) due to resonance.

Due to its instability, the

diazonium salt is not generally

stored and is used immediately

after its preparation.

Named by suffixing

diazonium to the name

of the parent

hydrocarbon from

which they are formed,

followed by the name

of anion

Reactions involving displacement of nitrogen:

1. Replacement by halide or cyanide ion:

Sandmeyer

reaction

Gatterman

reaction.

Used in synthesis and conversions.

Example: C6H5N2+Cl-

Benzenediazonium

chloride

2. Replacement by iodide ion:



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C6H5N2+ HSO4-

Benzenediazonium

hydrogensulphate

3. Replacement by fluoride ion:

Diazonium salts are very good

intermediates for the introduction

of –F, –Cl, –Br, –I, –CN, –OH, –

NO2 groups into the aromatic ring.

Aryl fluorides and iodides cannot

be prepared by direct

halogenation. The cyano group

cannot be introduced by

nucleophilic substitution of

chlorine in chlorobenzene but

cyanobenzene can be easily

obtained from diazonium salt.

4. Replacement by H:

5. Replacement by hydroxyl group:

6. Replacement by –NO2 group:

7.Coupling reactions:

These compounds are often

coloured and are used as dyes.



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Downloaded from 1 6...where R = an aryl group and X--may be Cl–Br -HSO 4, BF 4-, etc. Preparation: by the reaction of aniline with nitrous acid (HNO 2) at 273-278K. HNO 2 is produced