Inductive effect
source : SlideShare
Inductive effect
When a covalent bond is formed between two similar atoms, then bonded electron pair is attracted equally by both the nucleus. It means bonded electron pair occupies a central position between the two nuclei. This type of covalent bond is known as Nonpolar covalent bond.
When a covalent bond is formed between two dissimilar atoms, then bonded electron pair is not equally attracted by both the nucleus . Bonded electron pair is more attracted towards the more electronegative atom. This type of covalent bond is known as Polar covalent bond.
Ex.1-
In this ‘X’ is more electronegative than Carbon. So bonded electron pair is more attracted towards X.Hence ‘ X’ attains a partial negative charge while C-atom attains partial positive charge.
Ex.2-
If ‘ Y ‘is less electronegative than Carbon then bonded electron pair is more attracted by Carbon . Hence Carbon attains a partial negative charge & ‘Y’ partial positive charge.
‘ Inductive effect may be defined as the permanent displacement of electrons forming a covalent bond towards the more electronegative element or group.’
The inductive effect is always transmitted along a chain of carbon atoms.The inductive effect is represented by the symbol (arrow pointing towards the more electronegative element or group). This effect is negligible after C3.
source : chem-guide-blogger
The decrease in the effect is denoted by using a greater number of sign of delta.
Types of Inductive effect-
1) -I effect
2) +I effect
-I effect –
Any atom or group , which attracts electrons more strongly than hydrogen , is said to have -I effect(electron attracting or withdrawing ).
Ex-
-NO2 , -Cl , -Br , -I , -F , -COOH , -OCH3 etc.
-NO2 > -CN > -F > -Cl > -Br > -I > -CF3 > -OH > -OCH3 > -C6H5 > H
——————————–> Decreasing order of -I effect
+ I effect –
The atom or group that attracts electron less strongly than hydrogen is said to have + I effect (electron repelling or electron releasing ).
Ex-
-CH3 , -C2H5 , (CH3)2CH- ,(CH3)3 C- ,etc.
(CH3)3 C- > (CH3)2CH > -C2H5 > -CH3 > H
————–> Decreasing order of +I effect
Facts explained by Inductive effect –
1) Reactivity of Alkyl halide –
Alkyl halides are more reactive than alkanes.Due to presence of C-X bond in alkyl halide , they undergo inductive effect.
t- butyl chloride (tertiary halide ) is more reactive than methyl chloride because + I effect of three methyl groups increases the -I effect of -Cl atom by repelling electrons towards tertiary C- atom.
Thus Cl – atom in t- butyl chloride can be easily replaced by other atom.
2) Strength of Carboxylic acid –
The strength of an acid depends upon the ease with which an acid ionises to give proton.
Acidity of carboxyl group may be due to inductive effect. Thus strongly electronegative group helps in repelling proton from – OH of acid or -COOH group. This makes the acid more stronger.
Ex-
a) Halogenated fatty acids are much stronger than the parent fatty acid s. Acidity of halogenated fatty acid increases with the increase in the electronegativity of halogen .
NC- CH2COOH > F-CH2 COOH > Cl-CH2COOH > Br-CH2COOH > I-CH2COOH > CH3O- CH2COOH > HCOOH > CH3COOH > CH3CH2COOH
b) The inductive effect in di & tri halogen substituted acid is more stronger than mono halogen acid.
c) The inductive effect decreases rapidly as the group responsible for the effect moves away from the source. The strength of acid is proportionately decreased.
( source gr. is -COOH & gr. or atom responsible for inductive effect is -Cl. )
d) Alkyl groups are electron repelling group. So Formic acid is much stronger than acetic acid. Similarly propanoic acid , dimethyl acetic acid & trimethyl acetic acid are weaker acid than acetic acid.
e) Basic character of amines -The basic nature of amines is due to the presence of unshared electron pair on nitrogen atom which accepts proton. Lone pair of electrons is available for co ordination with proton, determines the relative basic strength of amines.
Due to + I effect of alkyl groups , the N- atom becomes strongly electronegative, so lone pair of electrons on N – atom in amines is more easily available than in NH3 . Hence amines are stronger base than ammonia.The relative basic character of amines is not as follows-
t- amine > sec. amine > pri. amine > NH3
Due to steric effect the correct order of decreasing basic character of amines is –
sec.amine > pri. amine > tertiary amine > ammonia
f) Aniline is weaker base than the aliphatic amines because in aniline , lone pair of electrons on nitrogen is delocalised due to resonance. So lone pair of electron is less available for protonation but in aliphatic amine ,resonance is not possible.
g) Basicity of substituted aniline is decreased when -I effect group like -NO2 is present. Due to -I effect of -NO2 group, lone pair of electron on N -atom is displaced away from it. So it is less available for protonation . So nitro anilines are weaker base than aniline.
h) Basicity of substituted aniline is increased with groups of + I effect like -OCH3. Due to +I effect of groups , electron pair on N atom easily available for protonation. So compound becomes more basic than aniline. Hence p- methoxy aniline is more stronger than aniline.