Chemistry Reference
In-Depth Information
7.2 MARKOVNIKOV'S RULE
Diatomic halogen molecules such as bromine are not the only chemicals that can add across
double bonds. In fact, any protic acid, under the proper conditions, can undergo such reac-
tions. Specifically, as shown in Scheme 7.5, reaction of ethylene with an acid, HX, where X
is OH, CN, or any halide produces a substituted ethane.
Mechanistically, the addition of acids across double bonds is very similar to the reaction
of olefins with halogens. To understand this, it is important to recognize the electron-rich
character of double bonds described in Section 7.1. With this property of olefins in mind,
one recognizes that double bonds can become protonated under acidic conditions. As
illustrated in Scheme 7.6, protonated olefins are electronically very similar to the bromo-
nium ion shown in Scheme 7.3 and, as such, can be described with charge-delocalized
resonance structures. Furthermore, these resonance structures are identical to those con-
ceptually presented in Chapters 5 and 6 during discussions of hyperconjugation. Recall
that hyperconjugation is the effect leading to stabilization of carbocations (Chapter 5) as
well as being the driving force behind 1,2-hydride shifts (Chapter 6). Bringing these con-
cepts into the addition of protic acids to olefins, the step following protonation (illustrated in
Scheme 7.7) is no different than the second step of an S
N
1 substition reaction.
Unlike the addition of halogens across double bonds, addition of acids results in for-
mation of asymmetrical products. Specifically, a different group is added to each side
of the double bond. Thus, if this reaction is applied to asymmetrical olefins such as
propene, multiple products might be expected to form as illustrated in Scheme 7.8. In
fact, while a mixture of products is formed, there is an overwhelming presence of the
secondary substituted product compared to that with substitution at the primary pos-
ition. This preference of reaction products resulting from addition of protic acids across
double bonds is governed by Markovnikov's rule.
Scheme 7.5 Protic acids can add across double bonds.
Scheme 7.6 Double bonds can become protonated under acidic conditions.
