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230          CHAPTER 7           
 
 
 
(d) As seen in both the SN1 and SN2 pathways, the by-product is tert-butanol. 
 
 
 
7.97. 
(a) The proposed mechanism is a concerted process that very closely resembles an E2 process. As such, we would 
expect this process to occur when the two bromine atoms are anti-periplanar to one another, as shown in the problem 
statement. If we perform the same reaction with a diastereomer of 1, we would expect a Z-alkene: 
 
 
 
But none of the Z-alkene is formed. The E isomer is obtained exclusively, which means that the preference for the E-
isomer is not dependent on the configuration of the starting dibromide. The E-alkene is obtained either from 1 or from 
a diastereomer of 1. As such, the reaction does not appear to have a requirement for anti-periplanarity. This is 
evidence against a concerted mechanism. 
 
(b) The reaction is not considered to be stereospecific, because the preference for the E isomer is not dependent on the 
configuration of the starting dibromide. The reaction is stereoselective, because one configuration of the alkene 
product (the E isomer) is favored over the other. 
 
 
7.98. 
In the first reaction, the OH group is converted into a better leaving group. Notice that the configuration of the center 
bearing the OH group does NOT change in the process (it remains a dash): 
 
 
 
Then, the second reaction employs DBU which is a strong base that generally does not function as a nucleophile. We 
therefore expect an E2 elimination process to occur. During an E2 process, the base removes a proton that is 
antiperiplanar to the leaving group. There are two  positions, each of which has one proton, but only one of these 
protons is anti-periplanar to the leaving group: 
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