Chemistry& 241
C lark College
Homework 4 SOLN
1. Indicate, with asterisks (*), all of the chiral centers in each of the following molecules. For each
molecule, what is the maximum number of stereoisomers that can be formed?
Remember, a chiral center is an sp3-hybridized atom (typically carbon, but not always!) with 4
unique groups attached to it.
O
O
H
*
*
*
*
H
*
*
H2N
*
HO
N
H
H
O
*
O
O
O
There are 6 chiral centers in the first molecule, so there are 26, or 64, different stereoisomers.
But only one is Progesterone
The second structure only has two stereocenters, for 22, or 4, isomers. But our bodies only
recognize one as sweet Aspartame (Nutrasweet®, Equal®)
2. For each chiral center, prioritize the groups (1 – 4) attached to each chiral center. Based on your
priorities, assign R or S configurations to the chiral center.
O
2
4
H
H3CH2C
OH
3
4
1
Cl
Br
H3CH2C
H
2
H
2
CH3 3
1
H
4
H3C
H3CH2C
3
1
OH
1
Br
H3CH2C
2
H
4
3
CH3
(R)-1-bromopropanol (R)-2-chlorobutane (R)-4-hydroxy-4-methyl-2-hexanone (S)-3-bromo-3-methylpentene
Homework 4
Fall 2008
Page 1 of 2
Chemistry& 241
C lark College
3. Indicate the absolute configuration (R or S) of all stereocenters for the following molecules.
4. Name the compounds in the previous problem.
CH3
H
Cl
Cl
S H3CH2C
Br
a)
b)
Cl
S
S
H3C
H
CH3
CH2CH3
H CH3
S-2-chlorobutane
(2S, 3S)-2-bromo-3-chlorobutane
CH3
Cl Cl
c)
d)
R
Cl
H R
H
Br R
CH2OH
R-1,1-dichloro-2-methylcyclohexane
(2R, 3R)-2-bromo-2-chlorobutanol
5. For the following molecules, use Fischer projections to draw out all stereoisomers found and give
the relationships between them.
a) 2-bromo-3-chlorobutane
H
H
CH3
Cl
Br
CH3
Cl
Br
CH3
H
H
CH3
H
Br
CH3
Cl
H
CH3
enantiomers
Cl
H
CH3
H
Br
CH3
1&3, 1&4, 2&3 and
2&4 are diastereomers
enantiomers
b) 2,3,4-pentanetriol
CH3
H
OH
H
OH
H
OH
CH3
CH3
HO
OH
H
OH
HO
H
CH3
meso
meso
CH3
HO
H
HO
H
H
OH
CH3
CH3
H
OH
H
OH
HO
H
CH3
1&2, 1&3, 1&4, 2&3
and 2&4 are
diastereomers
enantiomers
c) 1,2-dichlorocyclopentane (do not use a Fischer projection, use wedge-dash)
1&2, 1&3 are diastereomers
Cl
Cl
Cl
Cl
Cl
Cl
meso
enantiomers
6. Provide definitions for Brønsted-Lowry acids and bases, and Lewis acids and bases.
A B-L acid is a molecule or ion that donates a hydrogen ion (proton) into the solution. A B-L base is a
molecule or ion that accepts a proton from the solution.
A Lewis acid is an electron-pair acceptor (typically has positive charge and/or an empty orbital). A
Lewis base is an electron-pair donor (typically has a lone pair or a pi bond).
Homework 4
Fall 2008
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