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8.
Which of the following alkyl halides would you expect to undergo S
N
1 reaction most rapidly ?
CH
3
CH
2
CH
2
CH
2
CH
2
Br
CH
3
CH
2
CH
2
CH Br
CH
3
CH
3
CH
2
C Br
CH
3
CH
3
(A)
(B)
(C)
(D) They will not undergo S
N
1 reaction
(E) They react at the same rate
The key is to know how a Sn1 reaction proceeds (mechanism). For Sn1, the leaving group departs
before bond forming happens. Therefore the intermediate is carbocation, which is the most stable on
a tertiary carbon. The answer is (C). (can you explain why and how the rate depends on the stability
of the intermediate?)
9.
For the following reaction, the overall enthalpy change is:
(A) -12 kcal/mol
(B) +12 kcal/mol
(C) -300 kcal/mol
(D) +300 kcal/mol
(E) +15 kcal/mol
The key here is to understand how the heat of the reaction is related to bond dissociation energy
(BDE) and the definition of BDE. Also it is essential to know that bond breaking requires energy.
The answer is (A)
10.
Which of the following alkyl halides would you expect to give the highest yield of substitution
product (S
N
2) with CH
3
CH
2
O
-
Na
+
?
CH
3
CH
2
CH
2
CH
2
CH
2
Br
CH
3
CH
2
CH
2
CH Br
CH
3
CH
3
CH
2
C Br
CH
3
CH
3
(A)
(B)
(C)
(D) They will give same yield of substitution products
(E) None of them gives substitution products
For SN2, the nucleophile has to attack from the back of the leaving group. Therefore the carbon
being attacked must not be sterically hindered, otherwise elimination will compete. The answer here
is (A) which would give the best yield of substitution product.
CH
3
CH
2
CH
2
Br + HBr
CH
3
CH
2
CH
3
+ Br
2