4.4 Chemoselectivity

LEARNING GOALS

After   Chapter 4.4,    you will    be  able    to:

Describe    the types   of  compounds   that    are are most    likely  to  undergo SN1 and SN 2

reactions

Identify    the two reactive    centers of  a   carbonyl    containing  compound:

A key skill in recognizing which reactions will occur is recognizing the reactive regions within a
molecule. The preferential reaction of one functional group in the presence of other functional
groups is termed chemoselectivity.

REACTIVE LOCATIONS

Which site is the reactive site of a molecule depends on the type of chemistry that’s occurring. A
redox reagent, as described earlier, will tend to act on the highest-priority functional group. Thus, in
a molecule with an alcohol and a carboxylic acid, a reducing agent is more likely to act on the
carboxylic acid than on the alcohol. For a reaction involving nucleophiles and electrophiles,
reactions also tend to occur at the highest-priority functional group because it contains the most
oxidized carbon. A nucleophile is looking for a good electrophile; the more oxidized the carbon, the
more electronegative groups around it, and the larger partial positive charge it will experience.
Thus, carboxylic acids and their derivatives are the first to be targeted by a nucleophile, followed by
an aldehyde or ketone, followed by an alcohol or amine. Aldehydes are generally more reactive
toward nucleophiles than ketones because they have less steric hindrance.