CHEM/ENCH 212
Tutorial 4
2020
Part 1 Multiple Choice Questions: Read each question carefully and choose the best answer
1) Hyperconjugation involves
a)
b)
c)
d)
O
an adjacent C-H σ*-orbital accepting electron density from a developing p-orbital
lone pairs on an adjacent atom donating into an empty p-orbital
stabilization of an empty p-orbital by an adjacent filled σ-bonding orbital
stabilization of empty p-orbital by an adjacent π-orbital
2) According to molecular orbital theory, why does SN1 provide a 50/50 mixture of stereochemical isomers?
a)
b)
c)
d)
e)
because the LUMO of the carbocation has equal sized lobes on both sides of the molecule
because of electron repulsion keeping all substituents as far apart as possible
because of steric relief when the molecule goes from the ground state to the transition state
because the HOMO of the carbocation has equal sized lobes on both sides of the molecule
because the bigger lobe of the σ* orbital is opposite the leaving group
3) What is the LUMO orbital on the allylic bromide that interacts with cyanide in the following SN2
reaction?
a)
b)
c)
d)
e)
C-Br s
C-Br s*
C-C p
C-C p*
empty p
4) Predict the order of reactivity (from slowest to fastest) for these carbocations in a solvolysis reaction with
water
A
a)
b)
c)
d)
e)
O
B
C
D
A<B<D<C
B<C<A<D
C<B<A<D
D<A<B<C
B<A<D<C
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CHEM/ENCH 212
Tutorial 4
2020
5) What is the reason for the difference in reactivity in the SN2 reaction between the following two alkyl
halides?
O
a) The lone pairs on oxygen weaken the C-Br bond though nb® s* interactions
b) The lone pairs on oxygen weaken the C-Br bond though nb® s interactions
c) Oxygen helps delocalize electron density in the SN2 transition state through a p orbital ® p*
interaction
d) Oxygen helps delocalize charge in the SN2 transition state through a p ® empty p orbital interaction.
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CHEM/ENCH 212
Tutorial 4
2020
6. Draw the product structure(s) for the following reactions. Draw the expected stereochemistry of the
product where appropriate.
(a)
SH
qF
O
(b)
t
fairy
pipa
m
(Exact same number of Electrophile
and Nucleophile molecules present)
(c)
HO
T
fairy
H Cl
HC N
cey
a
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CHEM/ENCH 212
(d)
Tutorial 4
O
alia
NaCN
DMF
HH
(e)
Br
2020
n
MeOH
Ngai ink
one
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CHEM/ENCH 212
Tutorial 4
2020
7. Draw a reaction mechanism for the formation of compound Y via an SN1. Use accurate curly arrows to
show electron flow and draw the structures of all reactive intermediates. You do not need to show transition
state structures.
H2O
(large excess)
Cl
OH
+
H3O
+ Cl
Y
a
D
resonance friarwation
Ceo
2
pieces
7,1
8. Draw 2 resonance structures that help explain why the SN1 reaction mechanism is possible and then
explain why product Y is only formed as a single regioisomer
I
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CHEM/ENCH 212
Tutorial 4
2020
Periodic table and pKa values to follow
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CHEM/ENCH 212
Tutorial 4
2020
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CHEM/ENCH 212
Tutorial 4
2020
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