General Organic Chemistry
15.0 Inductive effects
15.0 Inductive effects
In covalent bonds between two different atoms, the electron in the $\sigma -$ bond are not shared equally. The electrons are attracted towards the most electronegative atom.
An arrow is drawn above the line representing the covalently bonded electrons shifts towards higher electronegative atom as shown in the figure.
Electron are pulled in the direction of the arrow.
| When the atom $(X)$ is more electronegative than carbon | When the atom $(Z)$ is less electronegative than carbon |
Electrons attracted to $X$
Negative inductive effect (-I effect) | Electrons attracted to carbon
Positive inductive effect (+I effect) |
| -I groups | +I groups |
$X=Br$, $Cl$, $NO_2$, $OH$, $OR$, $SH$, $SR$, $NH_2$, $NHR$, $NR_2$, $CN$, $CO_2H$, $CHO$, $COR$ | $Z=R$ (alkyl or aryl), metals (e.g. $Li$ or $Mg$ |
| The more electronegative the atom $(X)$, the stronger the -I effect | The more electropositive the atom $(Z)$, the stronger the +I effect. |
Note:
The inductive effect of the atom rapidly diminishes as the chain length increases.
The overall polarity of a molecule is determined by the individual bond polarities, formal charges and lone pair contributions and this can be measured by the dipole moment $\mu$.
Higher the dipole moment (measured in debyes $(D)$), more polar will be the compound.
Inductive effects may be due to atoms or groups. Relative inductive effect have been measured with reference to hydrogen. The order of electron-withdrawing effect is,
The alkyl groups are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. Electron-withdrawing character is indicated by -I effect and electron-releasing character by +I effect. The effect is additive.
The greater the number of electron-withdrawing groups, the stronger is the effect.
