CHM 2210 Exam 2 V2 | CHM 2210 Organic Chemistry I | Actual Q&A
with Rationale (CHM2210 Exam 2) | University of Central Florida
1. Which of the following terms describes a molecule that is non-superimposable on its mirror
image?
A. Achiral
B. Chiral
C. Meso
D. Constitutional isomer
Correct Answer: B
Explanation: A molecule is considered chiral if it lacks an internal plane of symmetry and
cannot be superimposed on its mirror image. These mirror images are known as
enantiomers and possess identical physical properties except for their interaction with
plane-polarized light. In biological systems, chirality is crucial as most enzymes are chiral
and distinguish between enantiomers.
2. How many stereocenters are present in 2,3-dibromopentane?
A. 1
B. 4
C. 3
D. 2
Correct Answer: D
Explanation: The molecule 2,3-dibromopentane contains two chiral centers at carbon 2
and carbon 3. Each of these carbons is bonded to four different groups: a hydrogen atom, a
bromine atom, and two different alkyl chains. Therefore, the total number of stereoisomers
possible for this molecule is four, following the two to the power of n rule.
3. Identify the relationship between a pair of molecules that are mirror images but are
superimposable.
A. Enantiomers
B. Diastereomers
C. Identical
D. Constitutional isomers
Correct Answer: C
,Explanation: If two mirror images can be superimposed, they represent the exact same
molecule. This usually occurs when the molecule contains an internal plane of symmetry,
making it achiral. Such molecules are often referred to as meso compounds if they contain
chiral centers.
4. Which configuration is assigned to a chiral center where the priority sequence of groups (1
to 3) is clockwise and the lowest priority group is on a wedge?
A. R
B. E
C. S
D. Z
Correct Answer: C
Explanation: According to Cahn-Ingold-Prelog rules, if the priority sequence 1-2-3 is
clockwise, the configuration is initially R. However, if the lowest priority group (4) is
pointing toward the viewer (on a wedge), the configuration must be reversed. Thus, a
clockwise path with group 4 on a wedge results in an S configuration.
5. Which of the following compounds is a meso compound?
A. (2R, 3R)-2,3-dibromobutane
B. (2S, 3S)-2,3-dibromobutane
C. (2R, 3S)-2,3-dibromobutane
D. (2R, 3S)-2,3-dibromopentane
Correct Answer: C
Explanation: A meso compound must have two or more chiral centers and an internal
plane of symmetry. In (2R, 3S)-2,3-dibromobutane, the symmetry plane cuts between C2
and C3, canceling out the optical activity. Note that (2R, 3S)-2,3-dibromopentane is not
meso because the two halves of the molecule are not identical due to the ethyl group.
6. What is the relationship between (2R, 3R)-tartaric acid and (2S, 3S)-tartaric acid?
A. Constitutional isomers
B. Diastereomers
C. Meso compounds
D. Enantiomers
Correct Answer: D
Explanation: Enantiomers occur when every chiral center in one molecule has the
opposite configuration in the other. Since both 2R and 3R are flipped to 2S and 3S, these
, two molecules are non-superimposable mirror images. They will rotate plane-polarized
light in equal but opposite directions.
7. Which physical property differs between a pair of enantiomers?
A. Boiling point
B. Density
C. Direction of optical rotation
D. Melting point
Correct Answer: C
Explanation: Enantiomers share identical physical properties such as boiling point,
density, and melting point in an achiral environment. Their only difference lies in how they
rotate plane-polarized light, with one rotating it clockwise and the other counter-clockwise.
They also react differently with other chiral molecules, such as chiral catalysts or enzymes.
8. A solution of 1.0 g of a compound in 10 mL of water in a 1.0 dm tube rotates light by +5.0
degrees. Calculate the specific rotation.
A. +0.5
B. +5.0
C. +500
D. +50
Correct Answer: D
Explanation: Specific rotation is calculated using the formula [alpha] = observed rotation /
(concentration in g/mL * path length in dm). In this case, concentration is 1.0g / 10mL = 0.1
g/mL. Thus, +5.0 / (0.1 * 1.0) equals +50.
9. Which of the following is true for an SN2 reaction?
A. It follows first-order kinetics.
B. It occurs with inversion of configuration.
C. It involves a carbocation intermediate.
D. It is favored by polar protic solvents.
Correct Answer: B
Explanation: The SN2 mechanism involves a backside attack by the nucleophile on the
electrophilic carbon atom. This concerted step forces the other three substituents to flip to
the opposite side, much like an umbrella in the wind. Consequently, if the starting material
is a single enantiomer, the product will have an inverted stereochemical configuration.
with Rationale (CHM2210 Exam 2) | University of Central Florida
1. Which of the following terms describes a molecule that is non-superimposable on its mirror
image?
A. Achiral
B. Chiral
C. Meso
D. Constitutional isomer
Correct Answer: B
Explanation: A molecule is considered chiral if it lacks an internal plane of symmetry and
cannot be superimposed on its mirror image. These mirror images are known as
enantiomers and possess identical physical properties except for their interaction with
plane-polarized light. In biological systems, chirality is crucial as most enzymes are chiral
and distinguish between enantiomers.
2. How many stereocenters are present in 2,3-dibromopentane?
A. 1
B. 4
C. 3
D. 2
Correct Answer: D
Explanation: The molecule 2,3-dibromopentane contains two chiral centers at carbon 2
and carbon 3. Each of these carbons is bonded to four different groups: a hydrogen atom, a
bromine atom, and two different alkyl chains. Therefore, the total number of stereoisomers
possible for this molecule is four, following the two to the power of n rule.
3. Identify the relationship between a pair of molecules that are mirror images but are
superimposable.
A. Enantiomers
B. Diastereomers
C. Identical
D. Constitutional isomers
Correct Answer: C
,Explanation: If two mirror images can be superimposed, they represent the exact same
molecule. This usually occurs when the molecule contains an internal plane of symmetry,
making it achiral. Such molecules are often referred to as meso compounds if they contain
chiral centers.
4. Which configuration is assigned to a chiral center where the priority sequence of groups (1
to 3) is clockwise and the lowest priority group is on a wedge?
A. R
B. E
C. S
D. Z
Correct Answer: C
Explanation: According to Cahn-Ingold-Prelog rules, if the priority sequence 1-2-3 is
clockwise, the configuration is initially R. However, if the lowest priority group (4) is
pointing toward the viewer (on a wedge), the configuration must be reversed. Thus, a
clockwise path with group 4 on a wedge results in an S configuration.
5. Which of the following compounds is a meso compound?
A. (2R, 3R)-2,3-dibromobutane
B. (2S, 3S)-2,3-dibromobutane
C. (2R, 3S)-2,3-dibromobutane
D. (2R, 3S)-2,3-dibromopentane
Correct Answer: C
Explanation: A meso compound must have two or more chiral centers and an internal
plane of symmetry. In (2R, 3S)-2,3-dibromobutane, the symmetry plane cuts between C2
and C3, canceling out the optical activity. Note that (2R, 3S)-2,3-dibromopentane is not
meso because the two halves of the molecule are not identical due to the ethyl group.
6. What is the relationship between (2R, 3R)-tartaric acid and (2S, 3S)-tartaric acid?
A. Constitutional isomers
B. Diastereomers
C. Meso compounds
D. Enantiomers
Correct Answer: D
Explanation: Enantiomers occur when every chiral center in one molecule has the
opposite configuration in the other. Since both 2R and 3R are flipped to 2S and 3S, these
, two molecules are non-superimposable mirror images. They will rotate plane-polarized
light in equal but opposite directions.
7. Which physical property differs between a pair of enantiomers?
A. Boiling point
B. Density
C. Direction of optical rotation
D. Melting point
Correct Answer: C
Explanation: Enantiomers share identical physical properties such as boiling point,
density, and melting point in an achiral environment. Their only difference lies in how they
rotate plane-polarized light, with one rotating it clockwise and the other counter-clockwise.
They also react differently with other chiral molecules, such as chiral catalysts or enzymes.
8. A solution of 1.0 g of a compound in 10 mL of water in a 1.0 dm tube rotates light by +5.0
degrees. Calculate the specific rotation.
A. +0.5
B. +5.0
C. +500
D. +50
Correct Answer: D
Explanation: Specific rotation is calculated using the formula [alpha] = observed rotation /
(concentration in g/mL * path length in dm). In this case, concentration is 1.0g / 10mL = 0.1
g/mL. Thus, +5.0 / (0.1 * 1.0) equals +50.
9. Which of the following is true for an SN2 reaction?
A. It follows first-order kinetics.
B. It occurs with inversion of configuration.
C. It involves a carbocation intermediate.
D. It is favored by polar protic solvents.
Correct Answer: B
Explanation: The SN2 mechanism involves a backside attack by the nucleophile on the
electrophilic carbon atom. This concerted step forces the other three substituents to flip to
the opposite side, much like an umbrella in the wind. Consequently, if the starting material
is a single enantiomer, the product will have an inverted stereochemical configuration.
