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2120 Blackboard problem set 2 2013 Answers - SN2 SN1 E1 E2 reactions(2).pdf

by

Department
Chemistry
Course Code
CHM 2120
Professor
Greg Flynn

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CHM 2120
Problem set 2 - ANSWERS
1. Use arrow notation to show the mechanisms of the following reactions. Use your mechanism to
predict the product of the reaction. Identify the nucleophile, its nucleophilic atom, the α carbon of the
electrophile and the leaving group.
Note: You can write the salt in the product either as NaCl (for example) or Na+, Cl-

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Problem set 2 - Answers
2
HO OH
Br
H3C
HO OH
NaOCH3
H2SO4
CH3OH
Br
H3C
OCH3
OCH3
H3C
HO OH2
O
O
H
e)
f)
+ H + H2O
+ Br
nucleophile
(oxygen is nucleophilic atom)
H2O = leaving group
αcarbon
nucleophile
(oxygen is nucleophilic atom)
αcarbon
Br = leaving group
Note: H2SO4is simply a source of H+
12
3
4
5
5
43
2
1
note the inversion of configuration
This notation indicates proton removal by a base
present in the solution. An example of the base
must be shown.
BB = HSO4-, H2O,
etc
The nucleophile and electrophile are on the same molecule
electrophile
For part f, E2 would be the major product.

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Problem set 2 - Answers
3
HO NH2
HO NH2Br
Br
HO N
H
HO N
H H
g) +
nucleophile
(nitrogen is nucleophilic atom) Br = leaving group
αcarbon
Note: In this reaction, 2 nucleophiles are present. N is chosen as the nucleophile since is lies furthest left
in the periodic table. It will be less electronegative than O and is therefore better able to donate electrons
Because the base is weak, it cannot deprotonate RNH2, therefore the H+is removed after the SN2
displacement takes place
electrophile
Br
H NEt3
Et3N
Et3N
2. A solution of 0.25 M cyclohexylbromide and 0.36 M CH3NH2 is created. Measurements show that the
reaction produces 3.6 X 10-7 mol L-1 of product each second.
a) Calculate the rate constant for the reaction.
Br
+NH2
N
reaction involves a good nucleophile, good LG, and secondary electrophile. Therefore it will
have an SN2 mechanism
= k
Br NH2
3.6 X 10-7mol L-1 s-1 = k (0.25 mol L-1)(0.36 mol L-1)
k = 4.00 X 10-6 L mol-1 s-1
HHH
N
NH2
R
Br
NH3
b) If the concentration of CH3NH2 is changed to 0.15 M, what is the initial rate of the reaction?
= k
Br NH2
= (4.00 X 10-6 L mol-1 s-1) (0.25 mol L-1)(0.15 mol L-1)
= 1.50 X 10-7 mol L-1 s-1
R
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