The inductive effect is a permanent state of polarization. The electron density in a bond between two unlike atoms is not uniform. The electron density is more dense toward the more electronegative of the two atoms.
The inductive effect is a distance-dependent phenomenon.
The atom above acquires a slightly negative charge (), and the carbon atom a slightly positive charge (), which means the bond is polarized.
If the electronegative atom is connected to a chain of carbon atoms, then the positive charge is relayed to the other carbon atoms. , with its positive charge, exerts a pull on the electrons of , but the pull is weaker than it is between on . The effect rapidly dies out and is usually not significant after the carbon atom, or at most the ).
The inductive effect is permanent, but relatively weak, and can be easily overshadowed by the electronic effects discussed later.
There are two categories of inductive effects: the electron withdrawing (-I effect) and the electron releasing (+I effect). The latter is also called the electron donating effect. In the image above, is electron-withdrawing and is electron-donating.
These relative inductive effects are measured with reference to hydrogen:
-I effect: The -I effect is seen around a more electronegative atom or group, and electron density is higher there than elsewhere in the molecule. Electron-withdrawing groups include halogen, nitro (), cyano (), carboxy (), ester (), and aryloxy ().
+I effect: The +I effect is observed among the less electronegative atoms of the molecule by electron releasing(or electron donating) groups. The alkyl groups are usually considered electron releasing (or electron donating) groups.