4.2: Induction

We’ve seen how the difference in electronegativity of two bonded atoms affects how the electrons are distributed in a bond. But electronegativity differences can have further reaching effects than just atoms that are directly bonded to each other. We call these long-distance electronegativity effects induction. For example, consider fluoroethane versus bromoethane, below.

Notice how replacing fluorine with bromine has an effect on the electron density around the hydrogen atoms that are two and three bonds away from the halogen atom. In the molecule on the left, the more electronegative fluorine atom has a higher electron density (more red) than the bromine on the right. The result is that atoms near the fluorine have lower electron density (more blue) than the atoms near the bromine. The diagrams above demonstrate that inductive effects are stronger with more electronegative atoms, because the nearby atoms are affected more by fluorine (more blue) than bromine (more green). We can also see that induction effects are stronger at shorter distances: the hydrogen atoms two bonds away from the fluorine are very blue, while the hydrogens three bonds away are less blue (though still more blue than the hydrogens three bonds away from bromine).

Let’s look at why these long-range electronegativity effects happen using fluoromethane as an example. Fluorine has an electronegativity of 3.98, while carbon has an electronegativity of 2.55; the 1.43 electronegativity difference classifies this as a polar covalent bond with more electron density on the fluorine and less electron density on the carbon. The carbon atom is also bonded to three hydrogen atoms (electronegativity 2.20). Carbon replaces some of the electron density that it lost to fluorine by pulling electron density towards itself from the hydrogen atoms. This transfers some of the positive charge to the hydrogen atoms. The net result is that the presence of a highly electronegative atom like fluorine leads to a partial positive charge on the hydrogen atoms, two bonds away.

Like electronegativity, induction effects can help us predict the relative stabilities of charged molecules. Consider bromoethanoate, below. Bromine (2.95) is more electronegative than carbon (2.55), so draws electron density towards itself from carbon (represented by the blue arrow). The carbon partially compensates by drawing electron density from the negatively charged oxygen, albeit a bit less strongly, as represented by the thinner blue arrow. The result is that the negative charge is partially spread across the molecule, rather than being completely isolated on the oxygen atom.

If we replaced bromine with a more electronegative atom, such as fluorine (3.98), then this spreading out of the negative charge is even more pronounced, as represented by the thicker blue arrow. The fluorine pulls more electron density from the carbon, which means the carbon draws more electron density from the oxygen. Electrons (negative charges) repel each other, so the more we spread out electron density, the more stable a molecule will be. Thus, the anion on the right is more stable than the anion on the left.

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