XII__Amines-Final_note.pdf by bhabisya kumar parida

Unit XIII: Amines
Amines: Nomenclature, classification, structure, methods of
preparation, physical and chemical properties, uses,
identification of primary, secondary and tertiary amines.
Deleted portion
Diazonium salt: preparation,chemical reactions & importance in
synthetic organic chemistry
Amines
Amines can be considered as derivatives of ammonia, obtained by replacement
of one, two or all the three hydrogen atoms by alkyl and/or aryl groups.
Classification of Amines
Amines are classified as primary, secondary, or tertiary according as
one, two or three hydrogen atoms in the ammonia molecule have been
substituted by alkyl group

Structure of amines: Trigonal Pyramidal
The nitrogen atom in amine is sp3-hybridised. The three hybrid
orbitals are involved in bond formation and one hybrid atomic
orbital contains the lone pair of electrons, giving the pyramidal
geometry of amines.
Due to the presence of unshared pair of electrons, the angle C–N–E, (where E is
C or H) is less than 109.5°.
Nomenclature of Amines
Common naming system:
• Aliphatic amines are named by prefixing an alkyl group to a mine,
i.e., alkylamine.
For example:

• Secondary and tertiary amines, having two or more similar groups
are named by adding prefix ‘di’ or ‘tri’ before the name of alkyl group.
For example:
• Aromatic amines are named as derivatives of the parent member,
aniline (C6H5NH2).
For example:
IUPAC naming system:
• Aliphatic or aromatic amines are named by replacing ‘e’ in the end of
the parent hydrocarbon by ‘amine’. (Alkanamine). The secondary and
tertiary amines arenamed as N-substituted derivative of largest alkyl
group of primary amines, i.e.,N-Alkylalkanamines.
For example:
CH3NH2 C6H5NH2
Methanamine Benzenamine
• Amines containing more than one amino groups at different positions
in the parent chain, are named by specifying numbers to the carbon
atoms bearing –NH2 groups along with attaching a suitable prefix such as
di, tri, etc. to the amine. The ending ‘e’ of the hydrocarbon is retained.
For example:
H2N ‒ CH2 ‒ CH2 ‒ NH2
Ethane-1, 2-diamine

Nomenclature of Some Alkylamines and Arylamines
Question-1(Home work)
Write IUPAC names of the following compounds and classify them into
primary, secondary and tertiary amines.
(i) (CH3)2CHNH2 (ii) CH3(CH2)2NH2 (iii) CH3NHCH(CH3)2
(iv) (CH3)3CNH2 (v) C6H5NHCH3 (vi) (CH3CH2)2NCH3
(vii) m–BrC6H4NH2

Methods of Preparation of Amines
1. By reduction of nitro (RNO2) compounds:
Thisinvolves the reduction of –NO2 group either by catalytic hydrogenation
(H2/Pt or H2/Pd or H2/Ni or by chemical means using Fe/HCl or Sn/HCl

2. By reduction of alkanenitriles (RCN):
3. By reduction of amides (RCONH2):

4. Hofmann bromamide degradation reaction
In Hofmann degradation reaction, the amine formed has one carbon less than the
parent amide.
Question-2
Ans

5.Gabriel phthalimide synthesis:
Why cannot aromatic primary amines be prepared by Gabriel phthalimide
synthesis?
6. By ammonolysis of alkyl halides:
Limitation of ammonolysis

Physical Properties of Amines
1.Physical state odour
 The lower aliphatic amines are gases with fishy smell.
 Primary amines wit three or more carbon atoms are liquid and
higher members are all solids.
 Aniline and other arylamines are usually colourless but get
coloured on storage due to atmospheric oxidation.
2.Boiling point
Amines have a higher boiling point than the hydrocarbon of comparable
molecular mass. This is due to their ability to associate via intermolecular
hydrogen bonding.
Question-4
Why the boiling point of carboxylic acids & alcohols are higher
than the corresponding amines?
Explanation
Amines have lower boiling point than those of corresponding alcohols or
carboxylic acids.This is due to the reason that nitrogen is less
electronegative than oxygen and hence O-H bond is more poir than N-H
bond & therefore intermolecular hydrogen bonds in alcohols & carboxylic
acids are relatively more stronger than those in amines.
Boiling points order of various isomeric amines is:
1o > 2o > 3o

The 10 & 20 amines contain two & one H-atom attached to the N-atom
respectively.Therefore 10 amines are more associated by hydrogen bonds than
20 amines & have highest boiling point.
3.Solubility
Lower amines are soluble in water as they can form hydrogen bonds
with water, however the solubility decreases with increase in hydrophobic
alkyl group. Higher amines are essentially insoluble in water.
The solubility of amines in water is less than that of alcohols.This is because
electronegativity of nitrogen is lower than that of oxygen & therefore amines
form weaker hydrogen bonds as compared to alcohols and carboxylic
acids.For example,butan-1-amine is less soluble than butan-1-ol.
Amines are soluble in organic solvents like alcohol, ether and benzene.
Give possible explanation for each of the following:
(i) Why are amines less acidic than alcohols of comparable molecular masses?
(ii) Why do primary amines have higher boiling point than tertiary amines?
(iii)Ethylamine is soluble in water whereas aniline is not.Why?
Chemical properties of amines
Amines are quite reactive organic compounds.It is due to
(i) Large difference in electronegativity between nitrogen and hydrogen atom.
(ii) The presence of unshared pair of electrons over the nitrogen atom .

Amines behave as nucleophiles due to the presence of unshared electron pair on
N-atom
1.Basic character of Amines
Amines act as Lewis bases due to the presence of lone pair of
electrons on the nitrogen atom.
Salt formation
Amines, being basic in nature, react with acids to form salts.
Amine salts on treatment with a base like NaOH, regenerate the parent amine.
Basic character of amines can be better understood in terms of their Kb and pKb
values as explained below:

Base.
Structure-basicity relationship of amines
(i)Alkanamines versus ammonia
Alkanamines are stronger bases than ammonia
Reaction of alkanamines and ammonia in order to compare their
basicity.
1. Due to the electron releasing nature of alkyl group, it (R) pushes
electrons towards nitrogen and thus making them available for the
unshared electron pair for the proton of the acid.
2. The substituted ammonium ion formed from the amine gets
stabilised due to dispersal of the positive charge by the +I effect of
the alkyl group.
Hence, alkylamines are stronger bases than ammonia
(ii)Variation of basic strength ofin primary,secondary & tertiary
alkyl amines
The basic nature of aliphatic amines should increase with increase
in the number of alkyl groups.

The order of basicity of amines in the gaseous phase follows the
expected order: tertiary amine > secondary amine > primary amine
> NH3.
It is important to know that this order is followed only in gaseous
state. But this not same in aqueous state,
In that the substituted ammonium cations get stabilised 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:

Thus, the order of basicity of aliphatic amines should be: primary >
secondary > tertiary, which is opposite to the inductive effect based
order.
When the alkyl 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
hindrance to H-bonding.
Therefore, the change of nature of the alkyl group, e.g., from –
CH3 to –C2H5 results in change of the order of basic strength.
Thus, there is a subtle interplay of the inductive effect, solvation
effect and steric hindrance of the alkyl 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:
o (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3
o (CH3)2NH > CH3NH2 > (CH3) 3N > NH3
(iii)Arylamines versus ammonia
pKb value of aniline is quite high because in aniline or other
arylamines, the -NH2 group is attached directly to the benzene ring.
As a result the unshared electron pair on nitrogen atom will be less
available for protonation as it is in conjugation with the benzene
ring.
Aniline is resonance hybrid of 5 resonance structures.
On the other hand, anilinium ion obtained by accepting a proton can
have only two resonating structures (kekule).

Aniline is more stable than anilinium ion as it has greater number of
resonating structure.
Therefore, the proton acceptability or the basic nature of aniline or
other aromatic amines would be less than that of ammonia.
(iv)Comparision of basic strength of aromatic amines or aryl
amines & aliphatic amines
Aromatic amines are weaker bases than aliphatic amlnes. Taking an
example of aniline and ethylamine it is observed that ethyl amine is more
basic than aniline. In aniline –NH2 group is directly attached to benzene
ring. Hence unshared pair of electron on nitrogen is less available for
protonation because of resonance
In the above resonating structures there is a positive charge on nitrogen
atom making the lone pair less available for protonation. Hence aniline is
less basic than ethyl amine which has no resonating structures. Less
basicity of aniline can also be explained by comparing the relative
stability of aniline and anilinium ion obtained by accepting a proton.
Greater the number of resonating structures, greater is the stability of
that species.
Aniline is resonance hybrid of five resonating structures whereas
anilinium ion has only two resonating structures.

(v) Effect of substituent on basic character of amines:
Electron releasing groups (e.g.,-CH3,-OCH3,-NH2 etc.) increase the
basic strength of aromatic amines because they tend to stabilise the
cation formed by protonation of amines.while electron withdrawing
groups (like – NO2, -X,-CN etc.) tend to decrease the basic strength
because they tend to destabilise the protonated amine..
Solution
Question-7(HW)
Question-8(HW)
Account for the following:
(i) pKb of aniline is more than that of methylamine.
(ii)aliphatic amines stronger bases than aromatic amines.

Question-9(HW)
2. Alkylation
Amines undergo alkylation on reaction with alkyl halides
All the three types of amines react with alkyl halides to form
quaternary ammonium salt as the final product provided alkyl halide
is present in excess.
Aromatic amines also undergo alkylation as given below.

3.Acylation
4.Benzoylation
5. Carbylamine reaction(only by 10 amines)
Aliphatic and aromatic primary amines on heating with chloroform
and ethanolic potassium hydroxide form isocyanides or
carbylamines which are foul smelling substances.
Secondary and tertiary amines do not show this reaction. This
reaction is known as carbylamines reaction or isocyanide test and is
used as a test or primary amines.

Question-10(HW)
Distinguish between following pair of compounds by suitable
chemical test.
(i)Methyl amine & dimethyl amine
(ii)Aniline & N-methyl aniline
Solution
(i)
(ii)
6. Reaction with nitrous acid
The stability of arenediazonium ion is explained on the basis of resonance.

7.Reaction with Hinsberg’s reagent
Benzene sulphonyl chloride (C6H5SO2Cl), which is also known
as Hinsberg’s reagent, reacts with primary and secondary amines
to form sulphonamides.
Amine is shaken with benzene sulphonyl chloride(Hinsberg
reagent,C6H5SO2Cl) in the presence of excess of NaOH or KOH
solution
Electrophilic substitution
Aniline is a resonance hybrid of the following five structures.
Benzene ring in aniline is highly activated.This is due to displacement of
lone pair of nitrogen towards the ring which results in the increase in the
electron density on the ring.

-NH2 group is ortho/para directingand a powerful activating group..It also
activates the ring.Due to resonance,the o/p-positions become point of high
electron density.Therefore an electrophile(NO2
+,Cl+,SO3) attack these
positions.
If we have to prepare monosubstituted aniline derivative, how can the activating
effect of –NH2 group be controlled ? This can be done by protecting the -NH2
group by acetylation with acetic anhydride, then carrying out the desired
substitution followed by hydrolysis of the substituted amide to the substituted
amine.
(b) Nitration

Direct nitration of aniline is not possible as it is susceptible to
oxidation, thus amino group is first protected by acetylation.
Although amino group is o– and p– directing in aromatic electrophilic
substitution reactions, aniline on nitration gives a substantial amount
of m-nitroaniline.

(c) Sulphonation: Aniline reacts with concentrated sulphuric acid to
form anilinium hydrogensulphate which on heating with sulphuric
acid at 453-473K produces p-aminobenzene sulphonic acid,
commonly known as sulphanilic acid, as the major product.
Aniline does not undergo Friedel craft’s reaction((alkylation or
acylation). Explain why?
Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation)
due to salt formation with aluminium chloride, 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.

Distinction between primary, secondary & teriary amines

Give the chemical tests to distinguish between the following pair
of compounds
(i)Ethyl amine & aniline
(ii)Aniline & benzyl amine

XII__Amines-Final_note.pdf by bhabisya kumar parida

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