Page 593 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

574 5.13. The addition of HCl to alkenes such as 2-methyl-1-butene and 2-methyl-2-butene
in nitromethane follow a third-order rate expression:
CHAPTER 5
2
Polar Addition Rate = k HCl alkene
and Elimination
Reactions
It has also been established that there is no incorporation of deuterium into
the reactant at 50% completion when DCl is used. Added tetraalkylammonium
chloride retards the reaction, but the corresponding perchlorate salt does not.
Propose a reaction mechanism that is consistent with these observations.
+
5.14. In the bromination of substituted styrenes, a
plot is noticeably curved. If
the extremes of the curve are taken to represent straight lines, the curve can be
resolved into two Hammett relationships with
=−2 8 for EWG substituents
and
=−4 4 for ERG substituents. The corresponding -methylstyrenes
give a similarly curved plot. The stereoselectivity of the reaction of the -
methylstyrenes is also dependent on the substituents. The reaction is stere-
ospecifically anti for strong EWGs, but is only weakly stereoselective, e.g., 63%
anti:37% syn, for methoxy. Discuss a possible mechanistic basis for the curved
Hammett plots and the relationship to the observed stereochemistry.
5.15. The second-order rate constants and solvent kinetic isotope effects for acid-
catalyzed hydration are given below for several 2-substituted 1,3-butadienes.
The products are a mixture of 1,2- and 1,4-addition. What information do these
data provide about the mechanism of the reaction?

R

+
R k 2 M −1 −1 k H /k D +
s
1 22×10 −2 1.2
c-C 3 H 5
3 19×10 −5 1.8
CH 3
Cl 2 01×10 −8 1.4
H 3 96×10 −8 1.8
C 2 H 5 O 60 -

5.16. The reaction of both E- and Z-2-butene with acetic acid to give 2-butyl acetate is
catalyzed by various strong acids. With DBr, DCl, and CH SO Hin CH CO D,
3
2
3
3
the reaction proceeds with largely (84 ± 2%) anti addition. If the reaction is
stopped short of completion, there is no incorporation of deuterium into unreacted
alkene, nor any interconversion of the E- and Z-isomers. When the catalyst is
changed to CF SO H, the recovered butene shows small amounts of 1-butene
3
3
and interconversion of the 2-butene stereoisomers. The stereoselectivity of the
reaction drops to 60–70% anti addition. How can you account for the changes
that occur when CF SO H is used as the catalyst, as compared with the other
3
3
acids?
5.17. A comparison of rate and product composition of the products from reaction
of t-butyl chloride with NaOCH in methanol and methanol-DMSO mixtures
3
has been reported. Some of the data are shown below. Interpret the changes in
rates and product composition as the amount of DMSO in the solvent mixture
is increased.