Engineering Chemistry Multiple Choice Questions on “Stereoselectivity and Stereospecificity”.
1. The reaction which gives exclusively one of several possible isomeric products is called ____________
a) Dynamicselective
b) Stereoselective
c) Regioselective
d) Stereospecific
Answer: c
Clarification: The reaction which gives exclusively one of several possible isomeric products is called regioselective reaction. Regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions.
2. The reaction that yields predominantly one enantiomer of a possible pair is called ____________
a) Dynamicselective
b) Stereoselective
c) Regioselective
d) Stereospecific
Answer: b
Clarification: The reaction that yields predominantly one enantiomer of a possible pair is called stereoselective reaction. Stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereo isomers during the non-stereo specific creation of a new stereo center or during the non-stereo specific transformation of a pre-existing one.
3. Stereoselectivity can be exhibited in various degrees.
a) True
b) False
Answer: a
Clarification: Stereoselectivity can be exhibited in various degrees like completely stereoselective, highly stereoselective, moderately stereoselective and so on.
4. The reaction in which stereochemically different molecules react differently is called ____________
a) Dynamicselective
b) Stereoselective
c) Regioselective
d) Stereospecific
Answer: d
Clarification: The reaction in which stereochemically different molecules react differently is called stereospecific reaction. Stereospecificity is the property of a reaction mechanism that leads to different stereo isomeric reaction products from different stereo isomeric reactants, or which operates on only one of the stereo isomers.
5. Which of the following is an example of regioselective reaction?
a) Debromination of 2,3- dibromobutane
b) 2-bromo octane with sodium hydroxide
c) Addition of HI to propylene
d) Debromination of meso dibromobutane
Answer: c
Clarification: Addition of HI to propylene is an example of regioselective reaction. Regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions.
6. Reaction of (±) 2,3- dibromobutane is slower than meso in which the two methyl groups are ____________
a) Skew
b) Anti
c) Gauche
d) Partially eclipsed
Answer: d
Clarification: Reaction of (±) 2,3- dibromobutane is slower than meso due to higher transition state in which the two methyl groups are partially eclipsed.
7. Dynamic stereochemistry helps in correlating conformation and reactivity and in stereoselective synthesis.
a) True
b) False
Answer: a
Clarification: Dynamic stereochemistry helps in correlating conformation and reactivity and in stereoselective synthesis. Stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereo isomers during the non-stereo specific creation of a new stereo center
8. R – C = C – R reacts in the presence of lindlar catalyst to give a syn product. This is an example of ____________
a) Dynamicselective
b) Stereoselective
c) Regioselective
d) Stereospecific
Answer: b
Clarification: The above mentioned reaction is an example of stereoselective reaction. Stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereo isomers during the non-stereo specific creation of a new stereo center
9. Under SN2 conditions, on reaction with NaOH, (R)-2-bromo octane gives ____________
a) (S)-2-octanol
b) (R)-2-octanol
c) Trans-2-butene
d) Cis-2-butene
Answer: a
Clarification: Under SN2 conditions, on reaction with NaOH, (R)-2-bromo octane gives (S)-2- octanol. In SN2 mechanism, one bond is broken and one bond is formed synchronously.
10. Under SN2 conditions, on reaction with NaOH, (S)-2-bromo octane gives (R)-2-octanol. This is an example of ____________
a) Dynamicselective
b) Stereoselective
c) Regioselective
d) Stereospecific
Answer: d
Clarification: Under SN2 conditions, on reaction with NaOH, (S)-2-bromo octane gives (R)-2-octanol. This is an example of stereospecific reaction. In SN2 mechanism, one bond is broken and one bond is formed synchronously.
11. Optical purity is defined as the ____________
a) Ratio of rotation of pure enantiomer to rotation of mixture
b) Ratio of rotation of mixture to rotation of pure enantiomer
c) Product of rotation of mixture and rotation of pure enantiomer
d) Sum of rotation of mixture and rotation of pure enantiomer
Answer: b
Clarification: Optical purity is defined as the ratio of rotation of mixture to rotation of pure enantiomer. It is expressed as a percentage. If the sample only rotates plane-polarized light half as much as expected, the optical purity is 50%.
12. The formula of enantiomeric excess is ____________
a) [(d+l)/(d-l)]*100
b) [(d)/(d+l)]*100
c) [(d-l)/(d)]*100
d) [(d-l)/(d+l)]*100
Answer: c
Clarification: The formula of enantiomeric excess is [(d-l)/(d+l)]*100. It gives the percentage of excess of an enantiomer.