Substituent Field-Inductive Effects


A substituent is a spectator atom that noticeably perturbs the reactivity of a functional group. For example, CF3CO2H contains a CO2H acidic group and three fluorine substituents. The latter make this compound much more acidic than an unsubstituted reference compound, CH3CO2H.

This definition of substituent implies a comparison between two compounds. There must be a reference compound that displays normal reactivity in addition to the substituted compound with its abnormal reactivity. Usually, the two compounds are related by replacing the substituent atoms with H, e.g. CF3CO2H vs. CH3CO2H, however chemists have been known to argue over what is the "correct" reference compound (and these arguments can be interesting).


Inductive effect: a series of bond polarizations

Imagine a four atom chain, Y-X1-X2-X3, in which Y is more electronegative than X. At first glance, the X-X bonds are nonpolar, so the only atoms with partial charges are Y and X1. Y does not appear to influence the other X atoms.

This may not be right, however. Suppose we develop our picture of the charge distribution in a different way. The Y-X1 bond is still polarized towards Y, and this creates a small d+ charge on X1. However, this also makes X1 a little more electronegative than X2. This difference may be enough to polarize the X1-X2 bond slightly so that X2 ends up with a charge too (it will be only dd+). By the same logic, the X2-X3 bond is slightly polarized and X3 may carry a very small ddd+ charge.

In short, the Y creates a chain of bond polarizations so that each atom in the chain is slightly electron deficient. This is called the inductive effect of Y and chemists used to put great store in it (and older chemists still do). In principle, the inductive effect of Y can enhance a compound's acidity by making an acidic H somewhere in the chain more positive than it would be otherwise. Molecular models, however, have shown that the effect of Y, while large on X1, is very modest on the other atoms in the chain, so it is unclear what the inductive effect can really accomplish.


Field effect: an ion-dipole interaction

The molecular modeling results suggest that we should return to our initial picture of the Y-X1-X2-X3 chain in which Y and X1 form a polar covalent bond and the remaining bonds are nonpolar. Notice that the polar bond corresponds to a dipole that points from X1 (d+) to Y (d-).

Now suppose a chemical reaction creates a negative charge on X3. As the charge develops on X3, an ion-dipole interaction appears between X3 and the X1-Y dipole. The interaction will be stabilizing because the the negative ion (X3) is closer to the positive end of the dipole (X1). This stabilizing interaction will make the reaction more favorable.

In this case, Y influences a remote atom by creating an electric field, so this is known as the field effect of Y. The magnitude of this effect depends on the electronegativity of Y, and the position of the Y-X1 dipole relative to the developing ion.


Inductive vs. Field

Although these effects operate by different mechanisms, they usually push things in the same direction. Both effects require a polar Y-X1 bond, and both will be larger when this bond is more polar. The inductive effect dies off when bonds intervene between Y and the reaction site, while the field effect dies off with increasing "line-of-sight" distance between the Y-X1 dipole and the reaction site. This distinction, bond distance vs. line-of-sight distance, rarely matters in practice, however. Therefore, chemists tend to refer to both effects by a single name ("the inductive effect") or by a combined name ("the field-inductive effect") and do not bother trying to distinguish between them.


Review problems

under construction