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CHAPTER 8 265 
 
8.69. 
(a) Begin by drawing the starting alkyl halide. This 
tertiary alkyl halide can be converted into a primary 
alkyl halide via a two-step process (elimination followed 
by addition). In each case, we must carefully consider 
the regiochemical outcome. During the elimination 
process, there is only one regiochemical outcome, so any 
strong base will work (even if it is sterically hindered, 
although that is not necessary). In the addition process, 
we want to install Br at the less-substituted position, so 
we will need an anti-Markovnikov addition of HBr 
(using peroxides): 
 
 
 
(b) Begin by drawing the starting alkyl halide. This 
secondary alkyl halide can be converted into a primary 
alkyl halide via a two-step process (elimination followed 
by addition). In each case, we must carefully consider 
the regiochemical outcome. During the elimination 
process, there is only one regiochemical outcome, so any 
strong base will work (even if it is sterically hindered). 
In fact, in this case, there is a distinct advantage to using 
a sterically hindered base. Specifically, it will suppress 
the competing SN2 process (the substrate is secondary, so 
SN2 should be a minor product, unless a sterically 
hindered base is used). During the addition process, we 
want to install Br at the less-substituted position, so we 
will need an anti-Markovnikov addition of HBr (using 
peroxides): 
 
 
 
8.70. 
(a) Begin by drawing the starting alkene. This 
trisubstituted alkene can be converted into a 
monosubstituted alkene via a two-step process (addition, 
followed by elimination). We must be careful to control 
the regiochemical outcome of each step of the process. 
During the addition reaction, we want to install Br at the 
less-substituted position, so we treat the alkene with HBr 
in the presence of peroxides. Then, the elimination 
reaction must be performed in a way that gives the less-
substituted alkene, so we must use a strong, sterically 
hindered base (such as tert-butoxide). 
 
 
 
(b) Begin by drawing the starting alkene. This 
disubstituted alkene can be converted into a 
tetrasubstituted alkene via a two-step process (addition, 
followed by elimination). We must be careful to control 
the regiochemical outcome of each step of the process. 
During the addition reaction, we want to install Br at the 
more-substituted position, so we treat the alkene with 
HBr (without peroxides). Then, the elimination process 
must be performed in a way that gives the more-
substituted alkene, so we must use a strong base that is 
not sterically hindered, such as methoxide (hydroxide or 
ethoxide can also be used). 
 
 
 
8.71. Since this reaction proceeds through an ionic 
mechanism, we expect the mechanism to be comprised 
of two steps: 1) proton transfer, followed by 2) 
nucleophilic attack. In the first step, a proton is 
transferred from HCl to the alkene, which requires two 
curved arrows, as shown below. There are two possible 
regiochemical outcomes for the protonation step, and we 
might have expected formation of a tertiary carbocation. 
However, in this particular case, the other regiochemical 
outcome is favored because it involves formation of a 
resonance-stabilized cation. As a result of resonance 
stabilization, this cation is even more stable than a 
tertiary carbocation, and the reaction proceeds via the 
more stable intermediate. This cation is then captured by 
a chloride ion in the second step of the mechanism, 
which requires two curved arrows, as shown: 
 
 
 
8.72. Protonation of the alkene requires two curved 
arrows, as shown in the first step of the following 
mechanism. This leads to the more stable, secondary 
carbocation (rather than a primary carbocation). This 
secondary carbocation then undergoes a rearrangement, 
in which one of the carbon atoms of the ring migrates (as 
described in the problem statement). This is represented 
with one curved arrow that shows the formation of a 
more stable, tertiary carbocation. In the final step of the 
mechanism (nucleophilic attack), the carbocation is 
captured by a bromide ion. This step requires one 
curved arrow, going from the nucleophile (bromide) to 
the electrophile (the carbocation), as shown: 
 
 
 
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