Organic Chemistry Multiple Choice Questions on “Stereochemistry”.
1. Compounds which have different arrangements of atoms in space while having same atoms bonded to each other are said to have
a) position isomerism
b) functional group isomerism
c) chain isomerism
d) stereoisomerism
Answer: d
Clarification: Stereoisomer his contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.
2. Which of the following can make difference in optical isomers?
a) heat
b) temperature
c) polarized light
d) pressure
Answer: c
Clarification: An optically active substance is one which can rotate the plane of polarisation of plane polarised light. if you shine a beam of polarised monochromatic light (light of only a single frequency – in other words a single colour) through a solution of an optically active substance, when the light emerges, its plane of polarisation is found to have rotated.
3. Which of the following Fischer projections is different from the other three?
a) 1
b) 2
c) 3
d) 4
Answer: d
Clarification: When comparing Fischer projections, it is important to recognize that the horizontal bonds point out of the page, and vertical bonds point into the page. If a substituent is held in place, and the other three substituents are rotated, the new structure is the same as the original structure. Holding the phenyl substituent of 4 in place and rotating the other three clockwise gives 1, so these structures are the same.
4. Which of the following terms best describes the following pair of molecules?
a) Isomers
b) Constitutional isomers
c) Configurational isomers
d) Geometrical isomers
Answer: d
Clarification: These molecules have the same molecular formula (C7H14), making them isomers. However, the molecules differ in their spatial orientations due to a double bond. Therefore, the molecules can best be described as geometric isomers.
5. What is the relationship between the two groups in the following molecules?
a) They are equatorial to one another
b) They are axial to one another
c) They are cis to one another
d) They are trans to one another
Answer: d
Clarification: Axial and equatorial are terms that describe the orientation of a single group with respect to a chair and can therefore be ruled out. The bromine and chlorine are both oriented downward, on the same side of the chair. By definition, they are cis to one another.
6. What is the stereochemical relationship between the following two molecules?
a) Geometrical isomers
b) Enantiomers
c) Diastereomers
d) Identical
Answer: d
Clarification: Both molecules have the same molecular formula (C9H16BrCl) and the same connectivity. Each molecule also has three stereocenters, marked above with an asterisk, and does not contain a plane of symmetry. Therefore, both molecules are chiral. Using one of the methods outlined in this tutorial, it can be determined that these molecules are superimposable. Therefore, the molecules are identical.
7. Which of the following is an alkane which can exhibit optical activity?
a) Neopentane
b) Isopentane
c) 3–Methylpentane
d) 3–Methylhexane
Clarification: 3-Methyl pentane shows optical activity as shown below:
8. What is the molecular formula for the alkane of smallest molecular weight which possesses a stereogenic center?
a) C4H10
b) C5H12
c) C6H14
d) C7H16
Answer: d
Clarification: See as the compounds who shows optical isomerism have a chiral carbon so an unsymmetrical alkane can show the optical isomerism. 6 carbon and 8 carbon alkane is not the answer. So, the smallest alkane is 7 carbon alkane is 3 methyl hexane which have 3rd carbon as chiral carbon.
9. Which of the following statements most accurately describes the stereochemistry between the various cyclohexanes?
a) Cis-1,2-dichlorocyclohexane and trans-1,2-dichlorocyclohexane rotate plane-polarized light in opposite directions, and together in equal proportions form a racemic mixture
b) The diaxial and diequatorial forms of trans-1,3-dichlorohexane can be separated by their differing physical properties
c) Only cis-1,4-dichlorocyclohexane is achiral due to a plane of symmetry, and cis-1,4-dichlorocyclohexane is diastereomeric to trans-1,4-dichlorocyclohexane
d) The conformational isomers of trans-1,2-dichlorocyclohexane are enantiomers, which are not interconvertible, but resolvable
Answer: d
Clarification: In the below given diagram of trans-1,3-dichlorohexane the chlorines cannot be positioned diaxial or diequatorial but can only be axial and equatorial or vice versa. Additionally, if they were diastereomers or structural isomers, then they could be separated by physical means.
