Chem 220
Fall 2016
Coastline College
Dupon
Homework Set 5 Solutions
(Distributed 9/29/16; Due on 10/6/16)
Read Chapter 5 in Wade and complete the listed problems from MasteringChemistry,
Homework #5, as well as the following problems:
A. Identify the chiral centers in each of the following molecules and provide the
number of possible stereoisomers for each one:
CH2OH
H
N
H
OH
H
HO
H
Cl
*
*
OH
*
N
*
CH2OH
*
3
2 = 8 stereoisomers
Xylitol
22 = 4 stereoisomers
O
O
NH
OH
*
*
*
*
OCH3
Ritalin
O
22 = 4 stereoisomers
B. Consider the molecule 2,3-dichloropentane, shown:
streptimidone
22 = 4 stereoisomers
N
H
Cl
*
*
(i) How many chiral centers does it have? Mark them with an
Cl
asterisk. 2 chiral centers;
(ii) How many stereoisomers of 2,3-dichloropentane are possible?
4 possible stereoisomers
(iii) Draw all diastereomers as Fischer projections and label each chiral
centerwith the appropriate R or S designation.
O
CH3
CH3
CH3
R
S
H
Cl
Cl
Cl
Cl
R
H
S
CH2CH3
S
H
H
H
Cl
R
S
CH2CH3
I
CH3
Cl
Cl
H
H
H
R
Cl
CH2CH3
II
CH2CH3
III
IV
(iv) Which isomers are enantiomers? I and II; III and IV
C. 65.3 g of camphor are dissolved 100 mL methanol and the optical rotation of the
solution is found to be –2.88°. What is the specific rotation for camphor if l = 100 mm?
[α] =
𝒂(𝒐𝒃𝒔𝒆𝒓𝒗𝒆𝒅)
𝒄𝒙𝒍
=
–𝟐.𝟖𝟖°
𝟔𝟓.𝟑𝒈
(𝟏𝒅𝒎)
𝟏𝟎𝟎𝒎𝑳
= –4.41°
D. Identify the stereochemical relationship between each pair of compounds and
specify the configurations at each chiral center as R or S:
S
R
H
CH2CH3
S
H3CH2C
OH
R
CH3
(i)
H
H3C
enantiomers
HO
S
Br
(ii)
OH
R
Br
Br
Br
enantiomers
CH3
R
HO
H
H
OH
H
OH H
S
(iii) H3C
OH
enantiomers
Cl
Cl
R
R
R
S
CH3
(v)
diastereomers
S
S
diastereomers
H3C
(iv)
R
OH
S