Topics to becovered
• Classification of Amines
• Nomenclature of Amines
• Preparation of Amines
• Physical properties of Amines
• Basicity of Amines
• Chemical properties of amines
• Reactions of arene diazonium salts
• Reaction with arenesulfonyl chloride
• Electrophilic aromatic substitution in
aromatic amines
By reduction ofamides
By Hofmann degradation
(Hofmann rearrangement /
Hofmann bromamide
degradation / Hofmann
hypobromite degradation )
14.
Gabriel phthalimide synthesis
i.Formation of potassium salt of
phthalimide from phthalimide on
reaction with alcoholic potassium
hydroxide.
ii. Formation of N-alkyl phthalimide
from the potassium salt by
reaction with alkyl halide.
iii. Alkaline hydrolysis of N-alkyl
phthalimide to form the
corresponding primary amine.
Intermolecular forces, boilingpoints and
solubility
• The N-H bond in amines is polar because
the electronegativities of Nitrogen (3.0)
and Hydrogen (2.1) are different.
• Due to the polar nature of N-H bond
primary and secondary amines have
intermolecular hydrogen bonding.
• The intermolecular hydrogen bonding is to
greater extent in primary amine than in
secondary amines, because primary
amines have two hydrogen atoms bonded
to nitrogen for hydrogen bond formation
17.
• Tertiary aminesdo not have intermolecular hydrogen bonding as there is no hydrogen
atom on nitrogen of tertiary amine.
• But due to polar N-C bonds, tertiary amines are polar molecules, and have
intermolecular dipole dipole attractive forces.
• Thus intermolecular forces of attraction are strongest in primary amines and weakest in
tertiary amines.
• The observed order of boiling points of isomeric amines is : primary amine > secondary
amine > tertiary amine.
• The lower aliphatic amines are gases with fishy odour, middle members are liquids and
higher members are solids under ordinary temperature and pressure.
• Aniline and other arylamines are usually colourless liquids but get coloured as they are
easily oxidised by air.
18.
• Due totheir ability to form hydrogen bond
with water molecule, lower aliphatic amines
are soluble in water.
• Solubility of amines decreases with increase
in molar mass of amines due to increase in
size of hydrophobic alkyl group.
• Aromatic amines and higher aliphatic
amines are insoluble in water.
• Since N-H bonds in amines are less polar than O-H bond in alcohol, water solubilities of
alcohols, amines and alkanes of comparable molar mass in water are in the decreasing
order: alcohols > amines > alkanes.
• The order of boiling points of alkanes, amines, alcohols and carboxylic acid of comparable
molar mass is as follows : Alkanes < Amines < Alcohols < Carboxylic acid.
The basic natureof amines is due to presence of a lone pair of electrons on the nitrogen
atom. In terms of Lewis theory, amines are bases because they can share a lone pair of
electrons on ‘N’ atom with an electron deficient species.
In terms of Lowry-Bronsted theory
21.
Basic strength ofaliphatic amines
Basic strength increases as we move from NH3 to R-NH2 and from R-NH2 to R2NH, but basic
strength decreases as we move from R2NH to R3N.
Greater is the stabilization of the protonated amine, that is, the conjugate acid, greater is the
basicity of the amine.
22.
a. Influence of+I effect on stabilization of conjugate acids of aliphatic amines and NH3
• An alkyl group exerts electron releasing inductive effect (+I) which stabilizes positive charge on
atom bonded to it.
• As we move from conjugate acid of ammonia (NH4
⊕) to that of tertiary amine (R3NH⊕), the
number of alkyl groups (R) bonded to Nitrogen goes on increasing steadily.
• This results in increasing stabilization of the conjugate acids and thereby an increasing order
of basic strength is expected.
24.
b. Influence ofsolvation by water on stabilization of conjugate acids of aliphatic amines and
ammonia
The solvent water stabilizes the conjugate acid by hydrogen bonding through the ‘H’ bonded to
the ‘N⊕’. The number of ‘H’ atoms bonded to the ‘N⊕’ decreaes from 4 in NH4
⊕ to 1 in
R3NH⊕. As a result NH4
⊕ is best stabilized by solvation while the stabilization by solvation is
very poor in R3NH⊕.
25.
c. Combined influenceof +I effect and solvation on stabilization if conjugate acids of
aliphatic amines decides the observed basic strength and pK value.
The net results is that as we move from NH3 to RNH2 to R2 NH, the basic strength
increases due to better stabilization of the corresponding conjugate acids. But 3° amine
is weaker base than 2° amine because the stabilization of conjugate acid of 3° amine by
solvation is very poor.
26.
Basicity of arylamines
Arylaminesin general are weaker bases
than ammonia and aliphatic amines.
• In arylamines, the -NH2 group is attached
directly to an aromatic ring.
• The lone pair of electrons on nitrogen is
conjugated to the aromatic ring and is less
available for protonation.
• Aniline is resonance stabilized by the following
five resonance structures.
27.
Anilinium ion obtainedby accepting a proton does not have lone pair of electrons on
nitrogen. Hence it can be stabilized by only two resonance structures and therefore less
stabilized than aniline.
Laboratory test foramines
a. Test for aminesas the ‘base’ :
All amines 1°, 2° and 3° are basic compounds. Aqueous solution of water soluble amines
turns red litmus blue.
30.
Alkylation of amines: Hofmann’s exhaustive alkylation
• If excess of alkyl halide is used tetra alkyl ammonium halide is obtained as major product.
The reaction is known as exhaustive alkylation of amines.
• The tetraalkylammonium halides are called quaternary ammonium salts which are
crystalline solids.
• Primary, secondary and tertiary amines consume three, two and one moles of alkyl halide
respectively to get converted into quaternary ammonium salt.
• The reaction is carried out in presence of mild base NaHCO3, to neutralize the large
quantity of HX formed.
• If the alkyl halide is methyl iodide, the reaction is called exhaustive methylation of amines.
31.
When methylamine isheated with excess methyl iodide, it gives tetramethyl ammonium
iodide.
32.
Hofmann Elimination :
•When tetraalkylammonium halide is heated with
moist silver oxide, it gives quaternary ammonium
hydroxide which is a deliquescent crystalline
solid, and strongly basic like NaOH or KOH.
• Quaternary ammonium hydroxides on strong
heating undergo b-elimination to give an alkene,
the reaction is called Hofmann elimination.
• The least substituted alkene is obtained as
major product (in contrast to Saytzeff
elimination)
33.
Acylation of amines
•The amines contain replaceable hydrogen
atoms (positively polarized H) on the nitrogen
atom.
• These hydrogen atoms are replaced by acyl
groups such as acetyl group.
• On reaction of amines with acetyl chloride or
acetic anhydride, acetyl derivative of amine is
obtained.
• It is also called amide.
• Amide is less basic than the amine.
• Acylation is a nucleophilic substitution reaction.
34.
Carbylamine reaction
Aliphatic oraromatic primary amines on
heating with chloroform give foul (offensive)
smelling products called alkyl/aryl isocyanides
or carbylamines. This reaction is a test
for primary amines. Secondary and tertiary
amines do not give this test.
35.
Reaction with nitrousacid
• Nitrous acid is an unstable compound.
• Hence it is prepared in situ by adding aqueous sodium nitrite to hydrochloric acid already
mix with the substrate, that is amine.
a. Aliphatic primary amines on reaction with nitrous acid form aliphatic diazonium salts as
very unstable intermediates which decompose immediately by reaction with solvent water.
Corresponding alcohol is formed as the product of the reaction and nitrogen gas is
liberated. .
36.
b. Aromatic primaryamines
react with nitrous acid to form
diazonium salts which have
reasonable stability at 273 K.
Aryl diazonium salts are
resonance stabilized and useful
as versatile intermediates to
obtain a variety of products.
Reaction with fluoroboricacid :
Arene diazonium salt on reaction with fluoroboric acid gives
precipitate of diazonium fluoroborate which on heating decomposes
to yield fluoroarene. On the other hand when heated with aqueous
sodium nitrite in presence of copper it gives nitroarene.
• Arene diazoniumsalts when treated with certain
reactive aromatic compounds such as phenols or
aromatic amines, give azo compounds.
• These have extended conjugated system of
double bonds in which two aromatic rings are
joined through azo group -N=N-.
• This reaction is called azo coupling. Azo
compounds are brightly colored and are used as
dyes.
• This is an example of electrophilic aromatic
substitution reaction.
• Here the electrophiles are positively charged
diazonium ions.
• Substitution usually occurs para to the ring
activating group.
Benzenesulfonyl chloride (C6H5SO2Cl)is known as Hinsberg’s reagent.
a. Ethyl amine (primary amine) reacts with benzenesulfonyl chloride to form N-ethyl
benzenesulfonyl amide.
46.
b. Diethyl aminereacts with benzene-sulfonyl chloride to give N, N- diethyl benzene
sulfonamide.
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