# Exam 2
Chemistry 2401 October 22, 2009
(15 ) I. Name each of the following. For D and E your name should also include an indication of the
stereochemistry.
CH3
CH3
CH2CH2CH3
A. CH3 CH CH2CH CH2CH3
4-ethyl-2-methylheptane
H
CH3
CH3
B.
H
C.
CH3
CH3
H
2,5-dimethylhexane
CH3
1,1,3-trimethylcyclopentane
H
CH3
CH CH3
CH2
H
Br
E.
D.
CH3
H
trans-1-bromo-3-methylcyclohexane
C Cl
CH2CH2C H3
S-2-chloropentane
(24 ) II. Draw structural formulas of each of the following.
A. The R enantiomer of the compound in IA.
B. The S enantiomer of the compound in IB.
C H3
C H2C H2CH3
C H3
CH
C H3C HC H2
C H2C H3
CH3
H
C. The other conformer of 1D.
CH3
D. A diastereomer of 1D
H
H
Br
H
CH3
H
Br
E. The enantiomer of the compound in IE.
C H3
F. Isopentyl bromide
H
CH3
CH3 CH CH2CH2 Br
Cl C
CH3CH2CH2
CH3
G. A Newman projection formula of the most
stable conformation resulting from rotation
around the C2-C3 bond of 2,3-dimethylpentane.
CH3 H
CH3
H
C H2C H3
CH3
CH3
H. The meso form of 3,4-dimethylhexane
H
H
CH3
CC
CH3CH2
CH2CH3
(16 ) III. Circle the correct answer..
The stretching and contraction of a rubber band is an interesting thermodynamic example. When
the rubber band is stretched it becomes warmer. From this we know that the stretching process is
(exothermic) with a value of H that is (negative) and that the attractive forces in the stretched
state are (stronger) than in the relaxed state. When the rubber band is released it spontaneously
returns to the relaxed state which means that this relaxing process is (exergonic). In order for this
to be true the value of S must be (positive) and the relaxed state is (more) disordered that the
stretched state. Now if a rubber band is suspended from a support and attached to a mass it can
be placed in a state of equilibrium between the stretched and relaxed states. At this point of
equilibrium the value of G is zero). If this partially stretched rubber band is heated it will become
(shorter).
H
(9) III. Cis-1,2-dimethylcyclopropane suffers from all three of the types of
strain that we have discussed. In each case explain the exact molecular
origin each of type of strain in this molecule.
C H3
H
C H3
H
H
A. Torsional strain Torsional strain in this molecule results from the repulsion between the
bonding electrons that hold the Hs and the CH3s to the ring. This is due to the fact that all of
these bonds are eclipsed.
B. Steric strain Since the two methyl groups are cis, they are very close and their electron clouds
repel. They cannot occupy the same space.
C. Angle strain Since the angles between the internal bonds in a cyclopropane ring are 60o and the
angles between the sp3 hybridized orbitals used to make the bonds are 109o, these orbitals are
unable to overlap end-on-end. This means that the bonds between the carbon atoms in the ring are
weaker than a normal C-C bond in an acyclic compound would be.
(4) IV. The Fischer projection formula at the right is D-ribose,
The sugar present in DNA. It contains 3 stereogenic centers.
a. How many enantiomers of D-ribose exist? __1___
b. How many diastereoisomers of ribose exist? __6___
CH O
OH
OH
OH
CH2OH
(24) V. How are the compounds in each pair related to each other? Are they identical, enantiomers,
diastereomers, constitutional isomers or none of the above?
Cl
H
CH3CH2
H
CH3
and
C CH2OH
H
CH3CH2
C CH2OH
C H3
Cl
and
CH3
Identical
H
H
O
C C OH
and
enantiomers
C H3
Cl
C C OH
enantiomers
H
H
H
CH3
H
H
and
CH3
H
H
Br
CH3
and
CH3
Cl
H
C H3
Cl
diastereomers
H3C
CH3
CH3
H
H
HO
CH3
Cl
O
HO
Br
C
C
and
Br
CH2C H3
Identical
CH2CH3
H
H
C
H3C
C
Br
H
enantiomers
CH3
H
CH3
and
and
H
diastereomers
constitutional isomers
(10) In lab in a few weeks we are going to form 2-methyl-2-butene by the dehydration of 2-methyl-2butanol. The mechanism for this reaction is shown below.
H2O
CH3
CH3
CH3
H
CH3 C CH2CH3
CH3 C CH2CH3
CH3 C CH2CH3
slow
OH
O H2
very fast
A
B
equilibrium
H
fast
CH3
CH3 C CH CH3
Draw an energy coordinate diagram below for this reaction that reflects the information provided about
the speeds of each step. Indicate the position of the two intermediates, A & B, on your diagram.
B
Energy
A
Extent of Reaction