Section 10.5 The Mechanism of an SN 1 Reaction 37510.5 The Mechanism of an Reaction
Given our understanding of reactions, we would expect the rate of reaction of tert-
butyl bromide with water to be relatively slow because water is a poor nucleophile and
tert-butyl bromide is sterically hindered to attack by a nucleophile. It turns out, how-
ever, that the reaction is surprisingly fast. In fact, it is over one million times faster
than the reaction of methyl bromide—a compound with no steric hindrance—with
water (Table 10.4). Clearly, the reaction must be taking place by a mechanism differ-
ent from that of an reaction.
As we have seen, a study of the kinetics of a reaction is one of the first steps
undertaken when one is investigating the mechanism of a reaction. If we were to
investigate the kinetics of the reaction of tert-butyl bromide with water, we would find
that doubling the concentration of the alkyl halide doubles the rate of the reaction. We
would also find that changing the concentration of the nucleophile has no effect on the
rate of the reaction. Knowing that the rate of this nucleophilic substitution reaction
depends only on the concentration of the alkyl halide, we can write the following rate
law for the reaction:
Because the rate of the reaction depends on the concentration of only one reactant, the
reaction is a first-order reaction.
The rate law for the reaction of tert-butyl bromide with water differs from the rate
law for the reaction of methyl bromide with hydroxide ion (Section 10.2), so the two
reactions must have different mechanisms. We have seen that the reaction between
methyl bromide and hydroxide ion is an SN 2 reaction. The reaction between tert-butyl
ratek[alkyl halide]tert-butyl bromideCH 3 C + H 2 OCH 3CH 3Brtert-butyl alcoholCH 3 C + HBrCH 3CH 3OHSN 2SN 2SN 1
WHY CARBON INSTEAD
OF SILICON?There are two reasons living organisms are com-
posed primarily of carbon, oxygen, hydrogen, and nitrogen: the
fitnessof these elements for specific roles in life processes and
their availabilityin the environment. Of the two reasons, fitness
was probably more important than availability because carbon
rather than silicon became the fundamental building block of
living organisms, even though silicon is just below carbon in
the periodic table and, as the following table shows, is more
than 140 times more abundant than carbon in the Earth’s crust.Abundance (atoms 100 atoms)
Element In living organisms In Earth’s crustH 49 0.22
C 25 0.19
O25 47
N 0.3 0.1
Si 0.03 28/Why are hydrogen, carbon, oxygen, and nitrogen so fit for the
roles they play in living organisms? First and foremost, they are
among the smallest atoms that form covalent bonds, and car-
bon, oxygen, and nitrogen can also form multiple bonds. Be-
cause the atoms are small and can form multiple bonds, they
form strong bonds that give rise to stable molecules. The com-
pounds that make up living organisms must be stable and,
therefore, slow to react if the organisms are to survive.
Silicon has almost twice the diameter of carbon, so silicon
forms longer and weaker bonds. Consequently, an reaction
at silicon would occur much more rapidly than an reaction
at carbon. Moreover, silicon has another problem. The end
product of carbon metabolism is The analogous product
of silicon metabolism would be Because silicon is only
singly bonded to oxygen in silicon dioxide molecules
polymerize to form quartz (sea sand). It is hard to imagine that
life could exist, much less proliferate, were animals to exhale
sea sand instead of CO 2!SiO 2 ,SiO 2.CO 2.SN 2SN 2