car28374_ch05_196-241 08/18/04 14:17 Page 239
EQA
Learning By Modeling
5.50 Compound A (C6H14) gives three different monochlorides on photochemical chlorination. One
of these monochlorides is inert to E2 elimination. The other two monochlorides yield the same alkene
B (C6H12) on being heated with potassium tert-butoxide in tert-butyl alcohol. Identify compound A,
the three monochlorides, and alkene B.
LEARNING BY MODELING
Spartan 5.1
(a) Make a molecular model of propene, Minimize, and find the CPCOCOH unit that has
a dihedral angle of 0. This structure corresponds to the drawing on the left. What is its
energy?
O
O
O
O
H
CPC
H
H
COH and double bond eclipsed
H
O
O
H
CPC
O
¨
C
;
O
O
H
H
@] H
HOC
H
H
H
COH and double bond staggered
(b) Change this CPCOCOH angle from 0 to 180 and Minimize to make the conformation
shown in the drawing on the right. What is its energy?
(c) Based on these strain energy calculations, which is the more stable conformation of
propene?
(d) Replace these models with propene from the Database. This model is the most stable
conformation of propene as determined by quantum mechanical calculations. To which
one of the models in (a) and (b) does it correspond?
Spartan 5.2 The preferred conformation of both methyl groups of trans-2-butene is the same as that
for the methyl group of propene (preceding problem). What about cis-2-butene? It is difficult, if not
impossible, to tell by inspection whether van der Waals strain between cis methyl groups will control
the conformational preference.
O
O
One staggered; one eclipsed
O
O
O
O
H
O
Both eclipsed
H
H
H
@] H H]@
COH
HOC
O
O
H
O
O
CPC
H
H H
H
@] H
¨
C;
HOC
H
CPC
O
H
¨
C;
H
O
O
H
¨
;C
O
H H
H
CPC
H
H
Both staggered
(a) Make a model of cis-2-butene. The model presented will correspond to the structure on
the left. Minimize the structure and record its energy.
(b) Change one of the CPCOCOH dihedral angles from 0 to 180 to convert the structure
on the left to the one in the middle. Minimize and record the energy. Repeat to generate
and calculate the strain energy of the structure in the drawing on the right.
(c) According to molecular mechanics, what is the most stable conformation of cis-2-butene?
(d) Replace one or all of the structures with the model of cis-2-butene from the Database.
This structure represents the most stable conformation from quantum mechanical calculations and agrees with the experimentally determined most stable conformation. To which
one does it correspond?
Make a model of trans-2-butene and Minimize its structure.
(a) How does the strain energy of trans-2-butene compare to that of cis-2-butene? How does
the difference between the two compare with that obtained from heats of combustion
(Section 5.6)?
Spartan 5.3
239
car28374_ch05_196-241 07/08/04 10:50 Page 240
CHAPTER FIVE
Structure and Preparation of Alkenes: Elimination Reactions
(b) How do the CPCOC bond angles of cis- and trans-2-butene compare? What does the
difference between them tell you about the structural adjustment made by cis-2-butene to
reduce the van der Waals strain between its methyl groups?
Spartan 5.4 Refer to text Figure 5.2 for rotation about the double bond and the models that accompany it on Learning By Modeling.
(a) Compare the C(2)OC(3) bond distances of cis- and trans-2-butene and the transition state
for their interconversion. Suggest an explanation for any major differences.
(b) What is the calculated energy difference between cis- and trans-2-butene? How does this
compare with the experimental value of 3 kJ/mol (0.7 kcal/mol)? (1 atomic unit 627.47
kcal/mol)
(c) What is the calculated activation energy for the conversion of cis- to trans-2-butene?
Spartan 5.5 Make a model of (Z)-2,2,5,5-tetramethyl-3-hexene [cis-(CH3)3CCHPCHC(CH3)3] and
examine the bond angles and double-bond distance before and after minimizing it. How does the molecule respond to the van der Waals strain between the cis tert-butyl groups? What is the most obvious
structural change on minimization? Is there any indication of twisting of the double bond?
Spartan 5.6 Make models of cis- and trans-cyclooctene and replace them with the structures from the
Database. Minimize each. What is the difference between the two in respect to strain energy?
Spartan 5.7 Predict which one of the following has the largest dipole moment and compare your
answer with the values given in the Database.
H3C
CH3
H3C
CPC
H3C
CH3
Cl
CPC
CH3
Cl
A
CH3
Cl
CPC
H3C
Cl
B
Cl
CPC
Cl
C
Cl
Cl
D
Spartan 5.8 Predict the order of stability of the isomeric alkenes shown based on the principles developed in this chapter.
2-Methyl-1-butene
2-Methyl-2-butene
3-Methyl-1-butene
Strain energy calculations do not give the correct order in this case, but quantum mechanical calculations do. Verify this from the energies given under Properties in the Display menu for the molecules
in the Database.
Spartan 5.9 Make a molecular model of 1-bromo-2-methylpropane in its optimum conformation for
E2 elimination.
Spartan 5.10 Predict which stereoisomer of 1-bromo-3,5-dimethylcyclohexane will undergo E2
elimination at the faster rate. Recall that the E2 transition state requires an anti relationship between
bromine and the hydrogen that is removed by the base. The strain energies of the chair conformations in which Br is axial will provide an indication of which isomer can better achieve the necessary geometry.
CH3
H 3C
%
%
H 3C
Br
≥
%
Br
%
%
240
EQA
CH3
car28374_ch05_196-241 07/08/04 10:50 Page 241
EQA
Learning By Modeling
Spartan 5.11
(a) We often express Zaitsev’s rule as a preference for elimination in the direction that
gives the more highly substituted double bond. On that basis, which of the two
compounds shown would you expect to predominate in the E2 reaction shown?
and/or
E2
Br
Compound A
Compound B
(b) Zaitsev’s rule is also expressed in terms of a preference for formation of the more stable
isomer. Make molecular models of compound A and B and minimize each one. Which is
more stable (lower strain energy)? How does predicting the major product on the basis of
isomer stability correspond with your prediction from part (a)?
Comment: Only compound B is actually formed in this reaction. The behavior this exercise illustrates is one example of Bredt’s rule, which says that bridgehead double bonds
are highly strained and reluctant to form.
Spartan 5.12 Examine the animation of the carbocation rearrangement of Mechanism 5.2. Select the
indicated property from the Properties or Geometry menu and describe how the property changes
frame-by-frame.
(a) What happens to the charges on the carbons marked x, y, and z during the rearrangement?
O
I y n CH3
H3C ¨
; C OC ∞ CH3
H3C h h ;
x z H
O
H3C m y I
¨ CH3
H3C∞ C O C ;
;h h H
H3C x z
(b) What happens to the bond distances marked x, y, and z during the rearrangement?
O
O
y CH3
I
H3C ¨
; C O C;∞ CH3
H3C
x z H
n
n
H3C y I
¨ CH3
H3C∞ C O C ;
H
;
H3C x z
(c) How does the energy change during the rearrangement? What frame corresponds to the
transition state? Is the transition state closer in energy to the original carbocation or the
rearranged one?
241