Answer:
Inductive effect: It is a
permanent effect of the molecule in which polarisation of sigma electrons takes
place. This polarity developes in a molecule as a result of electro negativity
of the bonding atoms. This effect is of following two types.
(i) \[\mathbf{-I}\]Effect:
Atoms or groups (Electron-attracting or withdrawing) in the
decreasing order is:
\[-N{{R}_{3}}>-N{{O}_{2}}>-CN>-COOH>-\overset{\overset{\text{O}}{\mathop{\text{
}\!\!|\!\!\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{C}}\,-H>\]
\[-\overset{\overset{\text{O}}{\mathop{\text{
}\!\!|\!\!\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{C}}\,-R>-F>-Cl>-Br>-I>O{{C}_{6}}{{H}_{5}}>-OR>\]\[-OH>-{{C}_{6}}{{H}_{5}}>-H\]
(ii)\[+\mathbf{I}\]
Effect: Groups (Electron-releasing or repelling)in the decreasing order is:
\[\underset{tert-Butyl}{\mathop{{{(C{{H}_{3}})}_{3}}}}\,,->\underset{Isopropyl}{\mathop{{{(C{{H}_{3}})}_{2}}CH}}\,->\underset{Ethyl}{\mathop{C{{H}_{3}}C{{H}_{2}}}}\,->\underset{Methyl}{\mathop{C{{H}_{3}}}}\,->D>H\]Electromeric
effect: It is temporary effect which appears as per the requirement of the
attacking reagent.
This effect involves
the complete transfer of shared pair of electrons to one of atoms joined by a
multiple bond at the requirement of attacking reagent. It is indicated by E and
is represented by a curved arrow showing
the shifting of electron pair. This effect is of following two types:
(a) When the
transfer of electrons takes place towards the attacking agent, the effect is
called +E effect, e.g., the addition acids to alkenes.
Since, \[C{{H}_{3}}\]
group is electron repelling, the electrons are transferred in the direction
shown.
The
attacking reagent is attached to that atom on which electrons have been transferred.
(b) When the
transfer of electrons takes place away from the attacking reagent, the effect
is called-E effect. For example the addition of cyanide ion to carbonyl
compounds.
The
attacking reagent does not get attached to that atom on which electrons have
been transferred,
(i) \[-I\]
effect as shown below:
As the
number of halogen atoms decreases, the overall\[-I\] effect decreases and the
acid strength decreases accordingly.
(ii) \[+I\]effect
as shown below:
As the number of alkyl groups increases, the \[+I\] effect
increases and the acid strength decreases accordingly.
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