Question #845d9
1 Answer
The inductive effect is the effect on electron density in one portion of a molecule caused by electron-withdrawing or electron-donating groups elsewhere in the molecule.
In a covalent bond between two atoms of unequal electronegativity, the more electronegative atom draws electron density towards itself. This causes the δ⁺ and δ⁻ charges of the bond dipole.
If the electronegative atom is joined to a chain of atoms, the positive charge is passed along the other atoms in the chain. This is the electron-withdrawing inductive effect.
The induced change in polarity is less than the original polarity, so the inductive effect rapidly dies out. It is significant only over a short distance.
In NMR spectroscopy, the inductive effect is a factor affecting proton chemical shifts.
If the electron density about a proton is low, the induced field due to electron motions will be weaker than if the electron density is high.
The shielding effect in will be smaller, and a lower external field will be needed for the rf energy to excite the nuclear spin.
The proton is deshielded, and its signal is shifted downfield.
For example, the CH₂ group in propane appears at δ 1.4.
An OH group attached to the CH₂ shifts the signal downfield to δ 3.6.
The effect decreases with distance from the OH group. For example, in butan-1-ol, the CH₂ signals are
CH₃—CH₂—CH₂—CH₂—OH
C-3 = δ1.4; C-2 = δ1.5; C-1 = δ3.6
