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Answers to NCERT/CBSE Chemistry (Class XII)textbook
Chapter 13
AMINES
13.4
Arrange the following:
(i) In decreasing order of the pKb values:
C2H5NH2, C6H5NHCH3, (C2H5)2NH and C6H5NH2
In aromatic ammine there is delocalization of electrons of the N-atom over the
benzene ring.In aliphatic amines the +I effect of two C2H5 groups in (C2H5)2NH
is more than that of C2H5NH2. Therefore (C2H5)2NH is more basic than C2H5NH2.In
C6H5NHCH3 and C6H5NH2 , C6H5NH2 is more basic than C6H5NHCH3 due to
delocalization of electrons on the N-atom.Therfore the overall decreasing order
of the pKb values are
C6H5NH2 > C6H5NHCH3 > C2H5NH2 >(C2H5)2NH
(ii) In
increasing order of basic strength:
C6H5NH2, C6H5N(CH3)2, (C2H5)2NH and CH3NH2 .
In aromatic ammine there is delocalization of electrons of the N-atom over the
benzene ring.In aliphatic amines the +I effect of two C2H5 groups in (C2H5)2NH
is more than that of CH3NH2. Therefore (C2H5)2NH is more basic than CH3NH2.In
C6H5NHCH3 and C6H5NH2 , C6H5NH2 is more basic than C6H5NHCH3 due to
delocalization of electrons on the N-atom.So the increasing order of basic
strength of the amines are
C6H5NH2 > C6H5NHCH3 > CH3NH2 > C2H5NH2.
(iii) In increasing order of basic strength:
(a) Aniline, p-nitroaniline and p-toluidine
p-nitroaniline <
Aniline < p-toluidine
(b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2
In C6H5NH2, C6H5NHCH3 the N is directly attached to the benzene ring therefore
the lone pair of electrons of the N atom is the delocalized over the bezene
ring.There is a +I effect on C6H5CH2NH2 due to the presence of CH3 group of
C6H5NHCH3 , Therefore C6H5NH2, C6H5NHCH3 are weaker bases than C6H5NH2.From the
above explanations the increasing order of basic strength can be given as below
C6H5NH2 < C6H5NHCH3 < C6H5CH2NH2
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NCERT/CBSE CHEMISTRY CLASS 12 textbook
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(iv) In
decreasing order of basic strength in gas phase:
C2H5NH2, (C2H5)2NH, (C2H5)3N and NH3
In gas phase there is no hydrogen bonding , therefore stabalisation due to
hydrogen bonding is not there. Therfore the only effect to determine the strength
is the inductive effect.The +I effect increases with increase in the alkyl
group.Therefore the basic strength will be the highest in (C2H5)3N and least in
NH3. Therefore the decreasing order of basic strength in gas phase will be
(C2H5)3N > (C2H5)2NH > C2H5NH2 > NH3.
(v) In increasing order of boiling point:
C2H5OH, (CH3)2NH, C2H5NH2
The elcetronegativity of O is more than N. Therefore C2H5OH forms hydrogen
bonding and because of the hydrogen bonding the alcohols will have higher boiling
point than the amines with comparable molecular mass.Now between (CH3)2NH and
C2H5NH2 , C2H5NH2 has more hydrogen atom attached the th Nitrogen.Therfore the
extent of hydrogen bonding will be more in primary amines i.e C2H5NH2 as compared
to sec or ter amines.Therfore C2H5NH2 will have higher boiling point as compared
to (CH3)2NH .Therefore the increasing order of boiling point will be as given
below.
(CH3)2NH < (CH3)2NH < C2H5OH
(vi) In
increasing order of solubility in water:
C6H5NH2, (C2H5)2NH, C2H5NH2.
Solubility of the amines will decrease with increase in the molecular mass and
with decrease in the number of hydrogen atom s attached to the nitrogen. C2H5NH2
has the least molar mass and also has 2 hydrogen atoms attached to the nitrogen.
Therefore this is most soluble. C6H5NH2 has the highest molar mass . Therefore
the order in the increasing order of solubility in water is
C6H5NH2 < (C2H5)2NH < C2H5NH2
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