CHEM 2600 Assignment #2
Spring 2008
Solutions (Dr. Susan Lait)
Question 1 (4 marks)
You wish to prepare the alkene below from an alcohol. Which alcohol would be the best choice, and
why?
OH
H2SO4
heat
An alkene can be made by elimination of H2O via an E1 reaction. Since this requires formation of a
relatively stable carbocation, it would be better to use the 3˚ alcohol (which gives a 3˚ carbocation) than
the 1˚ alcohol (which gives a 1˚ carbocation):
H
OH
H C
CH3
OH
C
NOT
C
H
H
CH3
C
+
+
C
H
C
+
H2O
H
+
H2O
Also, the 3˚ carbocation is resonance-stabilized by both benzene rings while the 1˚ has no resonance
stabilization. When arguing resonance stabilization, you must always show the resonance structures.
Question 2 (6 marks)
You wish to prepare the compound in the box below from compounds containing no more than 5
carbon atoms each. Propose a series of reactions which would allow you to accomplish this task.
H
+
CN
NC
H3C
CN
H
CN
2. H2O2, NaOH
3. H2O
H3C
A
B
1. 9-BBN (or BH3), THF
C
CN
HO
H
H
H3C
CN
D
A Diels-Alder reaction between isoprene (A) and dienophile B gives substituted cyclohexene C
with the correct regiochemistry (1,4-adduct instead of 1,3 adduct). Hydroboration of C allows
cis- anti-Markovnikov addition of water across the alkene to give substituted cyclohexane D as a
mixture of enantiomers.
CHEM 2600 Assignment #2
Spring 2008
Solutions (Dr. Susan Lait)
Question 3 (10 marks)
For each reaction, draw the major organic product(s) and provide the mechanism.
(a)
OH
H+
+
solvent
O
O
O
(H+ is often CH3C6H4SO3H (i.e. TsOH); solvent is often CH2Cl2)
..
OH
..
H
H+
+
H
..
O
..
..+
..
O
H + ..
O
..
O
..
H
..
OH
..
H
H
+
H
O
..
..
O
..
..
O
..
H
+O
+
Acid-catalyzed addition to an alkene
..
H
(b)
OH
Br
+
NaH
O
solvent
(Solvent is often THF; NaH is a source of H-; usually mix NaH and alcohol then add PhCH2Br)
..
O
..
H
+
.. .O
.. .
.. H-
+
..
.
Br
.. .
SN2 reaction
..
O
..
.. .+ .. Br
.. .
H2
CHEM 2600 Assignment #2
Spring 2008
Solutions (Dr. Susan Lait)
Question 4 (15 marks)
The compound shown below is dissolved in ethanol then chlorine gas is bubbled through the solution.
Use HyperChem to help you determine what the major product(s) will be.
Cl2
CH3CH2OH
O
(a)
?
Draw the chloronium ion and both of the carbocation resonance structures. Label the two
carbocations A and B so that you can refer to them in later parts of this question.
You don’t need Hyperchem for this part of the question.
..
.
.. Cl
..
H
C
+
CH3
+..
..Cl
H
C
C
CH3
O
..
CH3
C
C
CH3
C
+
CH3
CH3
O
O
A
(b)
.
Cl
...
H
B
Perform the calculation as described above and report the energy of the ion in kcal/mol.
-1807.2976 kcal/mol
(c)
Draw the structure of the chloronium ion predicted by Hyperchem. Measure both C-Cl bond
lengths and label them on your drawing. (see next page for “measuring bond lengths”)
2.56 Å ..+..
Cl
H
1.81 Å
C
C
CH
CH3 3
1.81 Å
2.56 Å
O
(d)
According to Hyperchem, which of the two carbocation resonance structures is contributing
more significantly to the structure of the chloronium ion, A or B? (see part (a))
A (as I’ve drawn the carbocations; it’s the resonance structure with the 2˚ carbocation that’s
resonance-stabilized)
CHEM 2600 Assignment #2
Spring 2008
Solutions (Dr. Susan Lait)
(e)
Explain why your answer to part (d) could reasonably be the more stable of the two possible
carbocations.
Carbocation B is 3˚ while carbocation A is 2˚. This would imply that B is the more stable
carbocation; however, carbocation A is resonance stabilized by the furan ring while there is no
possible resonance stabilization for carbocation B. Delocalization of the positive charge onto
the carbon and oxygen atoms of the furan ring makes carbocation A the more stable of the two.
H
..
.
.. Cl
..
C
+
CH3
H
..
.
.. Cl
..
C
C
CH3
H
..
.
.. Cl
..
C
C
CH3
C
CH3
CH3
CH3
+
..O..
(f)
H
H
..O..
+O
..
H
Choose Calculate… then Orbitals… and look at the LUMO for the chloronium ion. At which
of the two carbon atoms bonded to chlorine will a nucleophile attack?
We are know that reaction will occur
at one of the two carbon atoms which
bear positive charges in a resonance
structure (i.e. the two bonded to Cl).
Of these two carbon atoms, one has a
two large lobes on it in the LUMO;
the other has no lobes on it in the
LUMO. Since lobes in the LUMO
represent electron-deficient sites
where electron density can be
donated, we can conclude that a
nucleophile will attack at the carbon
atom next to the furan ring (not the
one with two methyl groups). This is
consistent with our observation that
this is the more stable carbocation
and our knowledge that a nucleophile
will attack a chloronium ion at the
more stable carbocation.
not here
Nucleophile attacks here
(g)
Draw the major organic product of the reaction equation above.
H
CH3
CH3CH2O
Cl
CH3
O
CH3CH2OH is more plentiful than Cl-, so it is the nucleophile.
Regiochemistry is Markovnikov.
CHEM 2600 Assignment #2
Spring 2008
Solutions (Dr. Susan Lait)
Question 5 (5 marks)
When looking at cycloaddition reactions, we focused our attention on the Diels-Alder reaction;
however, there is actually a wide variety of different [4+2] cycloaddition reactions. Another class of
[4+2] cycloaddition is the 1,3-dipolar cycloaddition. In this type of reaction, a 4-electron 3-atom π
system (a “1,3-dipole”) takes the place of the diene.
While I was working on my PhD, I used one such reaction to turn a relatively simple molecule into a
complex-looking tricyclic compound:
O N
C
+
N
O-
(a)
On the reaction above, circle the atoms that make up the 1,3-dipole.
(b)
Propose a mechanism for this one-step reaction. Hint: You may find it helpful to redraw the
reactant in a slightly more product-resembling geometry. Also, consider which end of the 1,3dipole will be more nucleophilic and which will be more electrophilic.
- . ..
...
O
C
(c)
+
N
=
.. ..
..O N
+
N
C
O-
How many different stereoisomers can reasonably be produced from this reaction? Draw each
possible stereoisomer, clearly showing the relative stereochemistry. Hint: Your model kit is
your friend!
O N
O N
H
H
This molecule has two chiral centers, so there are 22 = 4 possible stereoisomers; however, two
are so strained that they don’t actually form/exist. It is impossible to make two 5-membered
rings that are trans-fused:
H
H
is not
is allowed
H
Total (40 marks)
H