Lecture Summary 18 October 8, 2004 - Cook Group

Chapter 6 - Alkenes: Structure and Reactivity
Carbocation Stability
Lecture Summary 18
October 8, 2004
Hyperconjugation of adjacent sigma bonds also helps to stabilize a carbocation because the
electron density of the bond is donating into the empty orbital. Thus, similar to alkenes,
carbocations are more stable with more alkyl groups attached. A tertiary carbocation is more
stable than a secondary carbocation which is more stable than a primary carbocation.
R
C
R
R
> R C H >
R
R
C
H
H
H
C
H
C
H
3° carbocation 2° carbocation 1° carbocation
H
H
Electrophilic Addition of HX to Alkenes
Carbocation stability has a direct influence on the energy of the intermediate in the electrophilic
addition of HX to alkenes. That is because the transition state resembles the structure of the
carbocation intermediate (the Hammond Postulate). Thus, the more stable intermediate can be
formed by a lower energy pathway than the less stable intermediate.
In the case of cycloalkenes, there are cases where more than one stereoisomer may be formed.
These reactions are not selective due to the carbocation intermediate being sp 2 -hybridized and
planar. That is, the halide can form a bond from either the top or the bottom face of the ring.
H-Cl
H
Cl
H
Cl
+
H
CH 3
H 3 C CH 3
H 3 C CH 3
H 3 C
Cl
H-Cl
H
Cl
H
Cl
+
H CH 3
H 3 C CH 3
H 3 C
H 3 C CH 3
CH 3
H 3 C CH 3
H 3 C
Cl
could add from top or bottom
Carbocation Rearrangements
Evidence for carbocation intermediates in the electrophilic addition to alkenes is found in the fact
that products of rearrangements are obtained. Hydrogen (with it’s electrons) or even methyl
groups and other carbon groups can move one position over to an adjacent empty orbital. This
shifts the carbocation to the position the migrating group just left. Carbocations will rearrange IF
they form more stable carbocations in the process. For example, the 2° carbocation shown below
©2004 Gregory R. Cook page 1 Chem 341
North Dakota State University

will rearrange to the more stable 3° carbocation by shifting a hydrogen over one position. In the
reaction of 2-methyl-1-butene with HCl, a mixture of products is obtained.
H 3 C
H
C
CH 3
CH CH 2
H-Cl
protonate to form
the 2° carbocation
H 3 C
H H
C CH CH 2
CH 3
Cl
rearrange to form
more stable 3°
carbocation
H 3 C
H H
C CH CH 2
CH 3
Cl
H
Cl
H
Cl
H
H
H 3 C
C
CH 3
CH CH 2
H 3 C
C
CH 3
CH CH 2
Chapter 7 - Alkenes: Reactions and Synthesis
Preparation of Alkenes
Alkenes are most commonly prepared by elimination reactions. Note that these are the opposite of
additions reactions. For example, a halogenated alkane will undergo a dehydrohalogenation
reaction in the presence of a good base. Alcohols, in the presence of a strong acid, will also
undergo a stepwise elimination of water (dehydration) to produce an alkene.
Cl
K + OH -
+ KCl + H 2 O
H
H
OH
H 3 PO 4
O
H
-H2 O
H
H
H
H 2 PO 4
Electrophilic Addition of X 2 to Alkenes
Alkenes will react with many different electrophiles. Molecular bromine, chlorine or iodine is no
different. You can think of these reagents as equivalent to an X + and an X - . Unlike the addition of
HX, the halogens have the ability to share one of their lone pairs to form a bridged halonium
intermediate. This is more stable than a carbocation intermediate. The result of this is that one
face of a ring is blocked, so the second halide addition step can only occur trans to the first
halogen.
Br-Br
A bromonium
Br
Br
Br
Br
Bromide can only come from the
bottom and kick off the bromine on
the top. This reaction forms the
trans product selectively.
©2004 Gregory R. Cook page 2 Chem 341
North Dakota State University