Question

1. In lecture you have learned that substitution and elimination are competing reaction and in order to predict the major product and predominant mechanism we turn to examine the nucleophile used in the reaction. The outcome of the reaction depends to a large extend on the nucleophilicity vs. basicity of the nucleophile used in the reaction among other conditions. What is the difference between nucleophilicity and basicity? (4 pts)

Answer 1

First of all, remember that basicity is a subset of nucleophilicity. All nucleophiles are Lewis bases; they donate a lone pair of electrons. A “base” (or, “Brønsted base”) is just the name we give to a nucleophile when it’s forming a bond to a proton (H+). To summarize, when we’re talking about basicity and nucleophilicity, we’re talking about these two types of events.

Basicity: nucleophile attacks hydrogen

Nucleophilicity: nucleophile attacks any atom other than hydrogen. Because we’re talking about organic chemistry here, for our purposes, this is going to mean “carbon” most of the time.

And then there’s nucleophilicity. How is nucleophilicity different from basicity? Well, since it’s not limited to simply forming a bond to hydrogen anymore, this leads to some extra complications. Let’s just talk about the measurement problem first.

Many reactions of nucleophiles are not reversible. A bond forms, a bond breaks, and that’s the end of the reaction. The problem with this from a measurement standpoint is that we often can’t determine an equilibrium constant for a reaction. And if we can’t do that, then we can’t develop a reactivity scale based on equilibria.

If we can’t measure equilibria, then what do we do? Well, we use the next best measurement available: to measure reaction rates.

There’s one important thing to remember with reaction rates. They don’t always reflect overall stability. There are a few more variables at play here.

Factor #1: Steric hindrance. Reactions where nucleophiles attack carbon-based electrophiles are significantly more sensitive to steric effects, because empty orbitals on carbon are not as accessible. Steric hindrance is like a fat goalie.

Factor #2: Solvents. The medium (solvent) in which a reaction takes place can greatly affect the rate of a reaction. Specifically, the solvent can greatly attenuate (reduce) the nucleophilicity of some Lewis bases through hydrogen bonding.

Hope this helps