13
Inductive Effect
ELECTRON DISPLACEMEN T EFFECTS IN COVALEN T BON DS
ri,, Electron shifting in a covalent bond· is known as electron displacement effects.
r:r Electron displacement effects are of mainly 3-types :
(i) Inductive effect
(ii) Resonance
(iii) Hyperconjugation
IN DUCTIVE EFFECT
Inductive effect is the polarization of shared _pair (covalently bonded pair) of electrons towards more
electronegative atoms. Let us consider a molecule.
888+ 88+ 8+ 8-
C-+--C-+-- C--+--c
r:r A permanent effect
r:r The electrons never leave their original atomic orbital.
r:r Operates through s bonds
r:r Polarisation of electrons is always in single direction.
r:r Its magnitude (i.e., electron withdrawing or donating power) decreases with increase in distance.
r:r There occurs partial movement of shared pair of electrons.
r:r This is a very weak effect.
r:qo 'This effect is additive in nature, i.e. two ele'ctronegative atoms exert greater induction effect than one atom
of greater electronegativity from the same position.
142 I Advance Theory in ORGAN IC CHEMISTRY
I d · . • · to 2 types.
n uctive effect showing groups can be d1v1ded in t
Inductive effec
-I )
+I
withdrawing group
(electron
(electron donating group)
INDUCTIVE EFFECTS
1nductive Effect (I Effect of Groups)
El I) For examP 1e,
ectron donating groups show positive inductive effect(+ ·
-T>-D>-H
-N-R>-0
-Se> - §
-C(CH 3h > -CH(CH 3)2 >- CH2-CH3 >- CH3
- i -
-O >-C-0
Electron withdrawing groups show negative inductive effect (-1). For example,
+ +
-OR2 >-NR3 -F >-OR >-NR2 >-CR3
+ + + + 0 0
- NCH3 >- P- CH3 >-As-CH3 >-Sb-CH3 II II
I I I I -S- R >- S-R >-S-R
CH3 CH3 CH3 CH3 II
0
-SIR3 >-GeR3
+ +
-OR2 >-0R - C •NH >-CH= NH2 >-CH -NH,
2
+ +
-SR2 >-SR - NH3>-NH2
-C=N >-CH=NH2 >-CH2-NH2 -OCH3>-S-CH3>-Se-CH3
-CH=O >-CH=NH >-CH=CH2 -F>-ct-Br>-J
-C=CN >-CH=CH2 -CH=o>-cH
2-0-CH3
-o>-CH=CH2
APPLICATION OF IN DUCTIVE EFFECT
Carbocatlons, carbanions and carbon radicals :
Carbocations, which include carbenium and carbonium ions co t .
.. I h d , n ain a po .ti
ions have three bonds to the pos1t1ve y c arge carbon (e.g. Me c + }. s, ve charge on carbon. Ca~
3
Inductive Effect I 143
r:r carbenillm ions (R ·c+) . · •• d b
. 3 are generally planar and contain an empty p1orbital. They are stabihse 1 Y
eIectron- donat mg grou 5 (R .15
generally more effectiv~ th + I and/or +M), which delocalise the positive charge; +M groups are
_. _ an+Igroups.
(a) Stability of Alkyl Carbocatlon : _ . · _ _
Stability of alkyl carbocat· , • ,) I < r, ,
ion oc + I effect
lf'the + I effect Will j . · ·11
1
stabilised c a b . ncrease then magnitude of positive charg_e on carbon will decrease so it WI
C ocat1on. ,,
..... ··"'
Stability increases
(b) Stability of Alkyl Carbanion : ar 1
1, ( V
Stability of alkyl carbanion oc• _ 1 effect
71
For examples: 1 ,';~V"•'v-v /' C ,- 1 ') - ' . r
e . ,
8
CH;>.C H~·= c R2 >-CH 3_:,tt=t =--cH 3 > CH 3 ...:::... ~= ct=r;
I
·· ·CH -
o.. ") :'1 'Ill'"':.,~: : ,1 .,.__ • ,,_ a "_, ..1.: r, ,.
(c)·1Stabi~ity '·
.,_.,,of Alkyl and Viqyl .Car,bocation:
1
,-.;.. .; '
., ,,,. ...,
7_n ~
.
i @ ";\ (±) •· - , -
CH
3
2-hybrnica,t,on '' ..1,;
> CHz=r:s;Ji~ridcart>on -~,,
· sp2-hybridised carbon is less EN sp-hybrid carbon is more electronegative
1• more stable - less stable
(d) Stability of-/t;lkyl, Viny{ a~d'Acetyienic Carbani~~s ·: •
·,_ 1 e f..__,r 6 I\ e
·CH:i,- tH2 . -CH2=tH -- , _ '> CH=C
sp3-hybrid carbon sp~-hygrid•carbon
t
Sp-hybrid carbon
(Least stable) (more stable) (most stable)
ORDER OP STABILITY
The order of stability of carbocations, carbanions and car~on ra~icals bearing electron donating (+ I) alkyl groups,
R, is as follows. (Free radicals will be formed by homolyt1c fission) _
Tertiary Secondary Primary Methyi
three t ,l groups_,, two + groups !
one + I groups no + I groups
R R R H
Cation ll@ ll@ ll@ I@
C > ~C"-'. > C > C
Stability R>V ~R R1/ ')( H H/ '--H H/ '--H
R
Anion
R
lie lie < <
.. , < x}c,
Stability
R>VC~R R H
R
R ll •
.Radical ll. > ~c > >
Stability R 1/ '--H
R>Vq~R
1144 I Advance Theory in ORGAN IC CBEMJSTRY whereas carbocatlons can b
-M groups), e
Carbanions can be stabllised by electron-withdrawing groups (-I. ·
stabilised by electron-donating groups (+i +M groups).
Solved Exam le
Which of the side chain attached to benzene ring have + I effect
's~ ,
0~ /OH
CH 3
(1
CH
I
O (iv)
~o (iii)
(i)v (ii)
~
Sol. 2 +I effect Due to E.N. difference.
E.N. order: c 5 P > c 5 P > c 5P due to% s-character.
2 3
Solved Example
'X' = Number of compound having -I group directly attached to benzene.
~COOH
~S03H ~C='N · '~# '
Sol.
Find the valu~ of 'X' ?
@f-1
Sox=?
Inductive Effect I 145
01·11111 11 i.11 ;til ~1 :11.i_ ...........1La11..1u1.11..J
WORKSHEET
1. Identify + I or - I effect whi h 18
.
c present by circled group ?
(a)~ (b) (c) (d)
(e) (f) (g) (h) @-CH=CH2
(i) (§)-C=CH (j) H2C=GH...:....@(k) ~CH=CH 2 · (I)
(m) @-~Hz (n) @-CH 2 -~H 2 (o) @-CH 2 -~H2 {p)
(q) (s) (t)
I '
Answers
~o. h : ---.&.;.....i.:~i.a....- - - " '
1. (:a) -"- I -
1
(h) +I
(e) -I ' , (f) ~IJr
('I
(i) +i U) -I (k) +I (I) -I
(o) -I (p) -I
(m) +I (n) -I
(q) _ 1 (r) None (s) -I (t) None
(Note: Inductive effect does not operate in alkanes)
000
