CHEMISTRY 241A/3-4
EXAM 4
December 2, 2004
DOLLINGER
Name:Key
ID#:
1.
(24)
2.
(21)
3.
(12)
4.
(16)
5.
(15)
6.
(12)
B
(2)
Total
GOOD LUCK!
YOU SHOULD HAVE 6 PAGES!!!
Bonus ( 2 pts): What happens to the rate of an SN2 reaction if the
concentration of nucleophile is doubled?
(100)
1. ( 24 pts) Draw the MAJOR organic product(s) in each of the following reactions.
Indicate all stereoisomers formed where appropriate.
(a)
Br
NBS, hn
Br
CCl4
(b)
1. Mgo
Br
H
2. H2O
(c)
Br
CH2CH2CH2CH3
(CH3CH2CH2CH2)2CuLi
CH3
(d)
CH3
CH3
TsCl
CH3O–Na+
pyridine
DMSO
K
CH3Br
CH3
OCH3
CH3
OCH3
OH
(e)
DMSO
OCH3
(f,g)
OH
CH3
TsCl
CH3
pyridine
CH3
(h)
CH3
OTs
CH3
Br2
hn
CH3OH
∆
CH3
CH3
OCH3
CH3
2. ( 21 pts) (a) Give the mechanistic symbols (SN1, SN2, E1, E2) that are most
consistent with the following statements. Some of these statements are consistent with
more that one symbol! Indicate so!
_S N1, E1_
(i) Mechanism(s) that do not occur with primary alkyl halide substrates.
_S N2, E2_
(ii) Mechanism(s) involving a bimolecular rate limiting step.
___S N2__
(iii) Mechanism(s) particularly favored by a polar aprotic solvent.
___S N2__
(iv) Substitution reactions occurring by this mechanism(s) always proceed
with inversion.
(b) Provide the mechanistic symbol (SN1, SN2, E1, or E2) by which the following
reactions proceed.
(i)
CH3Br
Na+–OCH2CH3
CH3OCH2CH3
SN1
DMSO
(ii)
HBr
OH
(CH3)3CO–K+
(iii)
Br
Br
SN1
E2
3. (12 points) Indicate the appropriate products or starting materials formed from the
following reactions by writing the appropriate letter(s) next to each reaction and then
answer the subsequent question regarding that reaction.
Br
CH3
Br
CH3
CH3
CH3
Br
CH3
CH3
CH3
CH3
CH3
CH3
CH3
A
B
C
H3C
Br
H3C
CH3
CH3
Br
Br
CH3
CH3
CH3
CH3
CH3
CH3
CH3
D
E
F
(a)
CH3
CH3
R
R
CH3
NBS
B, E
hn
CH3
Two products formed
unequal
Are these products formed in equal or unequal amounts? _________________
(b)
CH3
CH3
CH3
HBr
CH3
A, B, D, E
Four products formed
A & E; B & D
Which of these products are formed in equal amounts? _________________
(c)
B, D
Two possible starting
materials
K+–OtBu
CH3
H3C
CH3
CH3
enantiomers
What is the stereochemical relationship between these
possible starting materials? _________________
4. (16 pts)
(a) Reaction of HCl with (S)-2-pentanol leads to (±)-2-chloropentane. Clearly explain
why this reaction proceeds from a non-racemic starting material to a racemic product.
OH
Cl
HCl
H
Cl
H
H
Cl–
Cl–
OH2
Intermediate is carbocation
which is planar. Hence, attack
can occur with equal probability
(rate) from either face, forming
two enantiomers in equal amounts.
H
H
(b) Assume that you have carried out a radical chlorination reaction on (R)-2chloropentane and have isolated (in low yield) 2,4-dichloropentane. Provide all
stereoisomers of 2,4-dichloropentane that you would expect to find in this sample
and in what ratio (equal or unequal amounts).
Cl
Cl
Cl2
Cl
hn
formed in
an unequal
amount
(R)-2-chloropentane
Cl
Cl
5. (15 pts) Provide the necessary reagents to carry out the following synthesis from the
indicated starting material. You can use and inorganic or organic reagents of 6 carbons or
fewer. You must provide reagents and intermediate compounds in your synthesis.
Br
KOH
???
EtOH
Br
HBr
Pr2CuLi
6. ( 12 pts) For each of the following pairs, circle which reaction you predict to be faster
AND briefly explain why using five words or less.
(i)
Cl
OH
Na+–OH
DMSO
Cl
SH
Na+–SH
–SH
better nucleophile
than –OH
DMSO
(ii)
CH3
H3C C CH2O–Na+
CH3Br
CH3
CH3
H3C C CH2OCH3
CH3
CH3
CH3CH2Br
H3C C CH2O–Na+
CH3
CH3Br less bulky
substrate than CH3CH2Br
CH3
H3C C CH2OCH2CH3
CH3
(iii)
OTs
Na+I–
Cl
Na+I–
DMSO
DMSO
Na+–OTs
I
–OTs
better
LG than Cl–
Na+Cl–
I
(iv)
OTs
–CN
OTs
–CN
CN
–OTs
CN
–OTs
less bulky
substrate