,
Problem Set 6, Solution Set
Chapter 7, Structure and Synthesis of Alkenes
Due: Monday, October 19, 2009
1. Read Degree
(Elements) of Unsaturation. How
many degrees of unsaturation are present in
C10H12BrClN2OS? Draw two
structures, one cyclic, the other acyclic, that have the
number of degrees of unsaturation you determined and that is
necessarily in agreement with the formula. 2. a) Estimate the heat of hydrogenation
of 3-ethyl-2-pentene using the heat
of formation table. Show
work. (E)-3-methyl-2-pentene:
ΔHfo
= -36.7 kcal/mol 4. Compound A
(C5H11Br) reacts readily with water to
form B, C5H12O. Exposure of
compound A to aq. NaOH gives only C (major)
and D (minor). Hydrogenation of C liberates
27.0±0.1 kcal/mol heat while D liberates 28.5
kcal/mol of heat during hydrogenation. What are the
structures A-D? Explain. 5. a) Determine the heat of hydrogenation
of cyclohexene from the heat
of formation tables. b) How does
this value compare with the heat of hydrogenation of an
unstrained cis-disubstituted double bond? c) Given the heat
of hydrogenation of cyclopentene (chapter 7) determine the
heat of formation of cyclopentene. d) BONUS: Why is the heat
of hydrogenation less for cyclopentene than that for
cyclohexene? Show all work. 6. Two stereoisomers, A and
B, absorb one equivalent of hydrogen upon catalytic
hydrogenation to form cyclooctane. Compound A, which
is capable of resolution, liberates 34.5 kcal/mol of heat
while B liberates 24.3 kcal/mol of heat. 7. Comment critically on the following
proposed synthesis of the now banned gasoline additive,
methyl tertiary-butyl ether (MTBE). If you believe the
reaction will be successful, provide the type of mechanism
that is operable and illustrate it with the curved arrow
formalism. If you feel that the reaction will not be
successful, state the expected product of the reaction and
the mechanism by which it is formed. Illustrate with curved
arrows. If no reaction takes place, state so and explain why
not.
C10H12BrClN2OS;
drop O and S,
C10H12BrClN2; drop Br and
Cl; add two H's,
C10H14N2; drop two N's; add
two C's and two H's
C12H16; compare with the most
saturated C12 alkane,
C12H26,
(26-16)/2 = 5 degrees of unsaturation.
Since the heat of
hydrogenation of all unstrained, equally substituted0 double
bonds is about the same and the compound in question does
not appear, use 2-methy-2-butene as a model. Its ΔHfo
= -10 kcal/mol. Its product of hydrogenation is
2-methylbutane: ΔHfo = -36.7
kcal/mol. The difference of -26.7 kcal/mol is the estimated
heat of hydrogenation.
b) Calculate the heat of hydrogenation of (E)-and
(Z)-3-methyl-2-pentene. Show work.
From the Table:
(Z)-3-methyl-2-pentene:
ΔHfo
= -36.7 kcal/mol
(Z)-3-methyl-2-pentene:
ΔHfo
= -36.7 kcal/mol
c) Use a diagram to illustrate that the difference in the
heat of hydrogenation of the two geometrical isomers in 2b
is equal to the difference in their heats of formation.
Which isomer is more stable based upon the heats of
formation? Why?
d) Assuming that ΔGo = ΔHo,
which of the three isomeric alkenes would dominate in an
equilibrium mixture? How much heat is liberated inthe
isomerization of the disubstituted alkene to the (E)-isomer?
Show work. Add the disubstituted alkene to your diagram in
2c and illustrate the heat of isomerization.
3. a) [2.2.2]-Bicyclooctane forms how many
monochloro constitutional isomers upon free radical
chlorination? What are their structures?
b) In what ratio are they expected to be formed? Show
work.
c) Are they optically active, racemic, or achiral?
d) How many different alkenes are formed from the monochloro
compounds upon treatment with a strong base? Explain and
illustrate.
Paul
Sabatier
1912 Co-Nobel Prize in Chemistry
Hydrogenation by Metal Catalysis
a) What are the structures of A and B ?
b) What are the heats of formation of A and B
?
c) What is the difference in strain energy between A
and B ?
d) What is the difference in the heat of combustion between
A and B?
e) Why is A capable of resolution?