Isomerism Test

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Isomerism Test - 8

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Weekly Quiz Competition

Question 1
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Direction (Q. Nos. 1 - 8) This section contains 8 multiple choice questions. Each question has four
choices (a), (b), (c) and (d), out of which ONLY ONE option is correct.
Q.
Which is true regarding a plane polarised light?

A
As it propagate, both magnitude of electric vector and direction of polarisation varies

B
As it propagate, neither magnitude nor direction of electric vector varies

C
As it propagate, magnitude of electric vector varies periodically but direction of polarisation remains unchanged

D
Change in orientation of nicol prism has no effect on direction of polarisation

Solution

The correct answer is Option C.
As it propagates, both magnitude of electric vector and direction of polarization varies.
Question 2
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At a given temperature and for a given wavelength of plane polarised light, the fundamental property of an optically active substance is

A
magnitude of specific rotation

B
direction of specific rotation

C
Both magnitude and direction of specific rotation

D
Neither magnitude nor direction of specific rotation

Solution

The correct answer is Option C.
For a pure enantiomer, signs of specific rotation and its magnitude are fundamental properties as they do not depend on concentration of sample or length of sample tube.
Question 3
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What is the percentage composition of an enantiomeric mixture of 2-butanol with a specific rotation ? Specific rotation of pure enantiomer (+)-2-butanol is +13.52°.

A
75% (-)and 25% (+)

B
25% (-)and 75% (+)

C
50% of each enantiomer

D
67% (-) and 33% (+)

Solution

Optical purity = optical rotation of mixture/optical rotation of pure compound .
(mixture) [α]_D²⁵ = +6.76°
+[α]D = +13.57°
Since rotation of two enantiomers cancel each other, so rotation is due to enantiomeric excess.
Enantiomeric excess = observed specific rotation/max. Specific rotation = 6.76/13.52×100
= 50%
Therefore, %age of each enantiomer is given by,
If we have (+) (-) mixture(+) is in excess,
%(+) = ee/2 + 50%
= 75 % (s)
%(-) = 25% (s)
Question 4
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Which of the following is a comparatively significant factor affecting the magnitude of specific optical rotation?

A
Concentration of the substance of interest

B
Purity of sample

C
Experimental temperature

D
Length of sample tube

Solution

The correct answer is Option C.
The specific rotation of a molecule is the rotation in degrees observed upon passing polarized light through a path length of 1 decimetre (dm) at a concentration of 1 g/mL. Specific rotation is almost always reported along with the temperature, wavelength of light used, the solvent, and the concentration, since it is sensitive to these factors as well.
Magnitude of specific rotation depends on temperature. However, it does not depend on concentration, purity or length of sample tude.
Question 5
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What can be said with certainty if a compound has ?

A
The compound has atleast one chiral centre

B
The compound has R-configuration

C
The compound is not a meso-isomer

D
The optical purity of compound is less than 100%

Solution

The correct answer is Option C.
A meso compound is optically inactive with zero specific rotation.
Question 6
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If solution of a compound (30 g/100 mL of solution) has measured rotation of +15° in a 2 dm long sample tube, the specific rotation of this compound is

A
+50^o

B
+25^o

C
+15^o

D
+7.5^o

Solution

Where [α] = measured rotation, [α]sp = specific rotation, l = length of tube (in decimeter)
Question 7
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Enantiomers are

A
molecules that have atleast one chiral carbon

B
non-superimposable molecules

C
non-superimposable constitutional isomers

D
non-superimposable molecules that are mirror images of one another

Solution

Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are non-superimposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule. Chiral molecules with one or more stereocenters can be enantiomers.
Question 8
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Which of the following is not true of enantiomers ? They have the same

A
boiling point

B
melting point

C
Specific rotation

D
density

Solution

Specific rotation ([α]) is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sample of a compound in solution.

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Isomerism Test - 8

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Isomerism Test - 8

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Question 1

As it propagate, both magnitude of electric vector and direction of polarisation varies

As it propagate, neither magnitude nor direction of electric vector varies

As it propagate, magnitude of electric vector varies periodically but direction of polarisation remains unchanged

Change in orientation of nicol prism has no effect on direction of polarisation

Solution

Question 2

magnitude of specific rotation

direction of specific rotation

Both magnitude and direction of specific rotation

Neither magnitude nor direction of specific rotation

Solution

Question 3

75% (-)and 25% (+)

25% (-)and 75% (+)

50% of each enantiomer

67% (-) and 33% (+)

Solution

Question 4

Concentration of the substance of interest

Purity of sample

Experimental temperature

Length of sample tube

Solution

Question 5

The compound has atleast one chiral centre

The compound has R-configuration

The compound is not a meso-isomer

The optical purity of compound is less than 100%

Solution

Question 6

+50o

+25o

+15o

+7.5o

Solution

Question 7

molecules that have atleast one chiral carbon

non-superimposable molecules

non-superimposable constitutional isomers

non-superimposable molecules that are mirror images of one another

Solution

Question 8

boiling point

melting point

Specific rotation

density

Solution

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+50^o

+25^o

+15^o

+7.5^o