MCAT Organic Chemistry Review 2018-2019

Solvent Effects

The solvent consideration is worth spending a bit more time on. In polar protic solvents,
nucleophilicity increases down the periodic table. In polar aprotic solvents, nucleophilicity increases
up the periodic table. Examples of both types of solvents are shown in Figure 4.3.

Figure 4.3. Examples    of  Polar   Protic  and Polar   Aprotic Solvents

The halogens are good examples of the effects of the solvent on nucleophilicity. In protic solvents,
nucleophilicity decreases in the order:

I–  >   Br– >   Cl– >   F–

This is because the protons in solution will be attracted to the nucleophile. F– is the conjugate base
of HF, a weak acid. As such, it will form bonds with the protons in solution and be less able to access
the electrophile to react. I–, on the other hand, is the conjugate base of HI, a strong acid. As such, it is
less affected by the protons in solution and can react with the electrophile.

MCAT EXPERTISE

If  a   solvent is  not given   on  Test    Day,    assume  that    the reaction    occurs  in  a   polar   solvent.    Polar

solvents—whether    protic  or  aprotic—can dissolve    nucleophiles    and assist  in  any reaction    in

which   electrons   are moved.  Organic chemistry   is  all about   moving  electrons,  so  it’s    less

common  for these   reactions   to  be  performed   in  nonpolar    solvents.

In aprotic solvents, on the other hand, nucleophilicity decreases in the order: