What is the inductive effect?

Answer 1

The inductive effect is a core concept of organic chemistry, typically covered early during the lecture series. Inductive effects can impact the pK_a of a molecule along with its nucleophilicity. We can often compare the relative acidity of molecules with similar structures and different inductive effects. Inductive effects typically arise due to electronegativity differences between constituents or functional groups within a molecule. This difference in electronegativity is transmitted through the sigma bond network of a molecule and will polarize the molecule by increasing/decreasing the electron density at specific points within the molecule.

Examining the structure of 3,3-dimethylbutanoic acid, we find that the tertbutyl alkyl group on this carboxylic acid has a positive inductive effect (denoted as a +I effect) and will increase the electron density at the carboxylic acid functional group. The blue and red arrows shown on figures 1 and 2 are the conventional way in organic chemistry to indicate an inductive effect is occurring, with the arrow pointing in the direction of increased electron density.

Figure 1. 3,3-dimethylbutanoic acid, with the red arrow indicating +I effect

When comparing figures 1 and 2, we can definitively say that 1,1,1-trifluoropropionic acid will have a lower pKa (more acidic) than 3,3-dimethylbutanoic acid. This is because of the opposing inductive effects and their impact on the acidic hydrogen of carboxylic acids. For 1,1,1-trifluoropropionic acid, the negative inductive effect (denoted as -I) of the three fluorine constituents decrease the electron density of the O-H bond, weakening the bond and allowing for the release of this acidic hydrogen. In contrast, the +I effect of the tertbutyl group of the figure 1 molecule will increase the electron density of the O-H bond and strengthen it.

Figure 2. 1,1,1-trifluoropropionic acid, with the blue arrow indicating a -I effect