CHEMISTRY 246A
FINAL
Marder
December 16, 2002
Name:__________________________
ID#:____________________________
Break each problem down into parts if you can. Before starting, read through the entire exam.
Determine which questions you are best able to answer and be sure to budget time to answer
those questions first.
GOOD LUCK!
1._____
(60)
2._____
(14)
3._____
(6)
4._____
(40)
5._____
(20)
6._____
(10)
7._____
(10)
8._____
(40)
XC._____
(10)
Total______ (200 + 10XC)
___________________________
YOU SHOULD HAVE 12 PAGES!!!
1
1.
Short answer questions.
a) Underline the compound with 3 (three) (and only three!) stereocenters ( 5 points)
OH
Br
CH3
H3C
H3C
HO
CH3
CH3
O
HO
HO
OH
OH
Br
CH3
__________________
b)
Which of the following equilibria are favored to the right?: (5 points)
CH3CH 2OH
CH3CH 2–
CH3CH 2O–
CH3CH 3
CH3CH 2O–
HCl
CH3CH 2OH
Cl–
CH3CH 2S–
CH3CH 2OH
CH3CH 2SH
CH3CH 2O–
NH3
CH3CH 2O–
NH2–
CH3CH 2OH
(i)
I, II
(ii)
II, III
(iii)
c)
What are the ±3 pKa’s (±3 units) of the following molecules (1 point each)
(i)
HBr
–8
(ii)
Acetic acid
4.74
(iii)
Ammonium ion
10
(iv)
Methanol
17
(v)
Ethyne
(vi)
Ammonia
25
33
2
III, IV
(iv)
I, III
d) Underline the most stable carbocation (5 points)
CH3
CH3
CH3
CH2F
_______
e) Circle 4,5-Dimethyl-1-hexyne. (5 points)
HC CCH2CH(CH3)CH(CH3)2
CH3C CCHCH(CH3)3
CH3
f)
Indicate the formal charge of nitrogen and oxygen in the following structure. If an atom is
formally neutral, indicate a charge of zero. (4 points)
N
O
N: __+1______
O: ___–1_____
3
g)
Circle the energy- reaction coordinate diagram that represents the reaction reaction shown
below. (5 points)
SH
–
DMSO
OH
OH
E
SH
E
Reaction Coord.
Reaction Coord.
E
E
Reaction Coord.
h)
–
Reaction Coord.
Consider a compound known as ketene: CH2=C=O. (4 points)
(i) How many carbon-carbon σ-bonds does this molecule contain? ___1______
(ii) How many π-bonds does this molecule contain? ___2_______
(iii) What is the hybridization of the central carbon of ketene? _____sp_____
(iv) What is the C-C-O angle in ketene? ___180____
4
i) Which of the following compounds would react most rapidly with NaSMe in an SN2 reaction?
(5 points)
(1) 1-iodopentane.
(2) 1-bromopentane.
(3) 1-flouropentane.
(4) 1-pentanol.
(5) Neopentyl iodide.
j) Which of the following is NOT an nucleophile? (4 points)
(1) NH3
(2) CH3O–
(3) Br2
(4) All are electrophiles.
(5) None are electrophiles.
k) Identify the following as either Z or E configurations. (4 points each)
OH
O
OH
guaiol
E
nootkatone
(aroma of grapefruit)
Z
β-santalol
(sandalwood oil)
Z
5
2. Designate the mechanism (SN1, SN2, E1, E2) that is most consistent with the following
statements. Some of these statements are consistent with more that one symbol! Indicate so! (14
points)
All
(i) Alkyl bromides react faster than primary alcohols in reactions that proceed by
these mechanisms.
SN1
(ii) Benzyl halides react with sodium ethoxide in ethanol by this mechanism.
SN1, E1
(iii) These reaction mechanisms involve carbocation intermediates.
SN2, E2
(iv) ) Rearrangement reactions will not occur for reaction proceeding by these
mechanisms.
E2, E1
(v) Very bulky bases and sterically hindered sites with leaving groups attached
favor this type of reaction.
SN1, E1
(vi) These reactions are often run in protic solvents.
All
(vii) These reactions proceed faster when an excellent leaving group is present.
3. Choose an appropriate base for the reactions below and write it in over the arrow. (3 points
each)
Top: milder base KOH so as not to encourage elimination, bottom, strong base, needed to
deprotonate alcohol, such as NaNH2
6
4.
For the reaction shown below fill in the boxes with either the product or reagents as
required. (4 points for each answer)
CH2Br
1. Mg, ether
2.
H3O+
O
OH
O
+ enantiomer
+ enantiomer
CH3
CH3
Na
OH
OH
O
OCH3
SNa
Br
HBr
CH3
CH3I
S
O
H2, Pd
+
CHO
CHO
+ enantiomer
+ enantiomer
CH3
H
Br
Br
H3C
CH3
Br
Br2
CCl4
Br
H
H
Br
Br
H
CH2CH 3
7
CH3
H
Br
H
Br
Br
Br
H
H
CH2CH 3
5. Consider the following six pairs of structural formulas. For each pair, indicate whether the
structures represent identical molecules, constitutional isomers, enantiomers, or diastereomers.
(20 points)
Br
Br
and
CH3CH2
CH3
CH3
CH2CH 3 diastereomers
and
ldentical
OH
OH
H
CH3
H
OH
H
OH
H
OH
H
OH
H
OH
H
CH3
HO
and
CH3
H
CH3
diastereomers.
trans-1-tert-butyl-4-(2-fluoroethyl)cyclohexane and cis-4-(sec-butyl)-1-fluoro-2propylcyclopentane constitutional isomers
CH3
CH3
CH3
H
CH3
H
CH3CH2
H
and
CH2CH 3
H
CH3
CH3
8
Identical
6.
Provide a curved arrow mechanism (3 steps) for the following reaction. (10 points)
O
CH2CH 2CH CH2
OH–
(catalytic)
O
OH
CH2OH
OH
H
O
H
O
CH2CH 2CH CH2
O
CH2O
O
7.
Show a complete mechanism for the reaction shown below using the curved arrow
formalism. (10 points)
H
H
O
H
CH3
H
H
O
9
O
H
H
8. Provide a synthesis to get from the starting material on the left to the product on the right. To
receive full (or partial) credit show the product of each step of the synthesis with the reagents
required to effect each step. (10 points each)
OC2H5
(a)
Bromine, light, potassium t-butoxide, hydroboration, NaNH2, ethyl bromide
(b)
NBS, potassium t-butoxide, bromine, potassium t-butoxide, NaNH2, butyliodide, sodium, liquid
ammonia.
(c)
CH3OCH2CH2C
CH
CH3OCH2CH2CH 2CH2CN
Lindlar’s catalyst, H2, HBr, peroxide, NaCN
10
(d)
OH
SCH3 plus enantiomer
Note trans stereochemistry between methyl and SCH3!!
H2SO4 or some other path to get elimination, hydroboration, SOCl2, NaSCH3, DMF
11
Extra Credit. When benzyl bromide is added to a suspension of KF in benzene, no reaction
occurs. When 18-crown-6 is added, benzyl fluoride can be isolated in high yield. If LiF is used in
place of KF, no reaction occurs even with 18-crown-6. However, if 12-crown-4 is used, the LiF
reacts. Explain briefly. (10 points)
The KF is not soluble in benzene, the 18-crown-6 binds the potassium ion making the KF soluble
which is then highly reactive and reacts with the benzyl bromide. the 18-crown-6 is the wrong
size to bind lithium ion so it is not soluble in benzene; however, the 12-crown-4 can bind lithium
ion efficiently so the LiF, and thus the naked fluoride ion, can react with benzyl bromide.
Chart of Useful Information
Polar Aprotic Solvents
Polar Protic Solvents
Dimethyl formamide (DMF; (CH3 )2 NCHO)
Water (H2 O)
Dimethyl sulfoxide (DMSO; (CH3 )2 SO)
Methanol (MeOH; CH3 OH)
Acetonitrile (CH3 CN)
Ethanol (EtOH; CH3 CH2 OH)
Acetone (CH3 COCH3 )
Acetic Acid (CH3 CO2 H)
12