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264 CHAPTER 8 
 
(k) The two groups being added across the alkene are 
OH and OH, and they must be installed via an anti 
addition. This can be achieved by treating the alkene 
with a peroxy acid, followed by aqueous acid: 
 
 
 
8.64. Let’s begin by drawing the structures of the 
alkenes under comparison: 
 
 
 
Addition of HBr to 2-methyl-2-pentene should be more 
rapid because the reaction can proceed via a tertiary 
carbocation. In contrast, addition of HBr to 4-methyl-1-
pentene proceeds via a less stable, secondary 
carbocation. 
 
8.65. When treated with molecular bromine (Br2), the 
alkene is converted to an intermediate bromonium ion, 
which is then subject to attack by a nucleophile. We 
have seen that the nucleophile can be water when the 
reaction is performed in the presence of water, so it is 
reasonable that the nucleophile can be H2S in this case. 
This should give the installation of an SH group (rather 
than an OH group) at the more substituted position: 
 
 
 
8.66. We begin by drawing all possible alkenes with the 
molecular formula C5H10 (using a methodical approach 
similar to the one described in the solution to Problem 
4.3): 
 
 
Among these isomers, only two of them will undergo 
hydroboration-oxidation to afford an alcohol with no 
chiral centers, shown here: 
 
1) BH3 THF
2) H2O2, NaOH
1) BH3 THF
2) H2O2, NaOH
OH
OH 
 
The remaining four isomers will undergo hydroboration-
oxidation to produce alcohols that do possess a chiral 
center. 
 
8.67. We expect a syn addition of D and D across the 
alkene, giving a pair of enantiomers: 
 
 
 
8.68. 
(a) First draw the starting alkene. Treating this alkene 
with HBr will result in a tertiary alkyl halide. But if 
peroxides are present, a radical process will occur, 
resulting in the formation of a secondary alkyl halide, as 
shown: 
 
 
(b) First draw the starting alkene. Treating this alkene 
with HBr will result in a tertiary alkyl halide, as shown: 
 
HBr Br
 
 
(c) First draw the starting alkene. A syn dihydroxylation 
is required in order to produce a meso diol. We have 
seen several reagents that can be used to accomplish a 
syn dihydroxylation, such as cold potassium 
permanganate: 
 
 
 
Alternatively, we could achieve the same result with 
catalytic osmium tetroxide and a suitable co-oxidant. 
 
(d) First draw the starting alkene. An anti 
dihydroxylation is required in order to prepare 
enantiomeric diols. This can be accomplished by 
converting the alkene into an epoxide, followed by acid-
catalyzed ring opening of the epoxide, as shown. 
 
 
 
 
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