SN2 Substitution Reactions - Arrow Pushing in Organic Chemistry - page 69

Figure 4.2 Chloromethane bears a partial negative charge on the electronegative chlorine atom and a
partial positive charge on the carbon atom.
Figure 4.3 The carbon-chlorine bond in chloromethane is polarized.
Since nucleophiles, by definition, are attracted to positively charged centers, we must
consider how such centers become attractive to nucleophiles. This effect relates to the
electronegativity of the atoms attached to the center. Since an electronegative atom
retains a partial negative charge, the electronegative atom pulls electron density from the
atom on which it resides (usually carbon), leaving it with a partial positive charge. This
effect is illustrated in Figure 4.2 where an electronegative chlorine atom is attached to
carbon and inducing a partial positive charge.
When a bond joins an atom bearing a partial positive charge to an atom bearing a partial
negative charge, the bond is said to be polarized. Much in the same way a magnet possesses
a positive pole and a negative pole, the respective ends of a polarized bond are positively
and negatively charged. When referring to polarity and polarized bonds, the direction of
polarity is, by convention, from positive to negative. As shown in Figure 4.3, this is
commonly illustrated using a special arrow with a þ at the positive end and the tip pointing
in the direction of the negative end.
Because nucleophiles are attracted to sites of positive or partial positive charges, under-
standing the direction of polarity associated with a given bond serves three purposes. First,
the site to which a given nucleophile is attracted is readily identified. Second, the spatial
direction or trajectory of the reaction is readily identified. Lastly, the leaving group is
readily identified. These are all graphically summarized in Scheme 4.5 using a different
rendering of the S N 2 reaction illustrated in Scheme 4.3.
In Chapter 3, the relationship between nucleophiles and bases as influenced by steric
bulk was addressed. What was not addressed is the complementary issue surrounding
the accessibility of electrophilic (positively charged or partially positively charged) sites
to nucleophiles. In fact, in the same way that nucleophilicity decreases with increasing
steric bulk around the nucleophilic atom, the ability of a nucleophile to react with an elec-
trophile also decreases with increasing steric bulk around the site of potential S N 2 reactions.
This effect is illustrated in Scheme 4.6 using the reaction introduced in Scheme 4.3. As
illustrated, successive introduction of methyl groups adjacent to the S N 2 reaction site
results in decreased reaction rates.