This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Polarimeter”.
1. What does a polarimeter measure?
a) Polarity of the substance
b) Angle of rotation of an optical active compound
c) Concentration of the substance
d) pH of the substance
View Answer
Explanation: A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.
2. A solution of 0.1 g/mL of a pure R enantiomer in a 1.0 dm (i.e., 10 cm) polarimeter rotates plane polarized light by +4.8°. What is the rotation observed on this solution in a 2 dm polarimeter?
a) +2.4°
b) +4.8°
c) +19°
d) +9.6°
View Answer
Explanation: The specific optical rotation of a sample, [α], is calculated using the equation [α] = α/cl, where α is the observed rotation, c is the concentration (in g/mL), and l is the pathlength (in dm). [α] is a constant for a sample.
3. Polarimeter works on the principle of which of the following?
a) polarisation of light
b) change of the electrical conductivity of solution with composition
c) change of angle of refraction with composition
d) change of electrical conductivity of solution with temperature
View Answer
Explanation: If light encounters a polarizer, only the part of the light that oscillates in the defined plane of the polarizer may pass through. That plane is called the plane of polarization. The plane of polarization is turned by optically active compounds. According to the direction in which the light is rotated, the enantiomer is referred to as dextro-rotatory or levo-rotatory.
4. What is the effect of the I on the specific optical rotation (α), if I gets double?
a) α gets double
b) α gets halved
c) α gets four times
d) α eight times
View Answer
Explanation: Specific rotation, [α], is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength, and concentration. Doubling l will double the value of α.
5. If a solution of a compound (30.0 g/100 mL of solution) has a measured rotation of +15° in a 2 dm tube, what is the specific rotation?
a) +50°
b) +25°
c) +15°
d) +7.5°
View Answer
Explanation: The specific optical rotation of a sample, [α], is calculated using the equation [α] = α/cl, where α is the observed rotation, c is the concentration (in g/mL), and l is the pathlength (in dm). [α] is a constant for a sample.
6. Which of the following groups has the highest priority according to the Cahn-Ingold-Prelog sequence rules?
a) CH3
b) CH2Cl
c) CH2OH
d) CHO
View Answer
Explanation: R/S and E/Z descriptors are assigned by using a system for ranking priority of the groups attached to each stereocenter. This procedure, often known as the sequence rules, is the heart of the CIP system. Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority.
7. What is the percent composition of a mixture of (S)-(+)-2-butanol, α= +13.52°, and (R)-(-)-2-butanol, α = -13.52°, with a specific rotation α = +6.76°?
a) 75%(R) 25%(S)
b) 25%(R) 75%(S)
c) 50%(R) 50%(S)
d) 67%(R) 33%(S)
View Answer
Explanation: The specific optical rotation of a sample, [α], is calculated using the equation [α] = α/cl, where α is the observed rotation, c is the concentration (in g/mL), and l is the pathlength (in dm). [α] is a constant for a sample.
8. What can be said with certainty if a compound has α= -9.25°?
a) The compound has the (S) configuration
b) The compound has the (R) configuration
c) The compound is not a meso form
d) The compound possesses only one stereogenic center
View Answer
Explanation: Compounds which rotate the plane of polarization of plane polarized light counterclockwise are said to be levorotary and correspond with negative values. It is optically active substance.
9. Which of these is a comparatively insignificant factor affecting the magnitude of specific optical rotation?
a) Concentration of the substance of interest
b) Purity of the sample
c) Temperature of the measurement
d) Length of the sample tube
View Answer
Explanation: 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.
10. What is the specific optical rotation of (S)-malic acid at a concentration of 5.5 g/ mL in the solvent pyridine at 20°C at a wavelength of 589 nm?
a) the specific rotation is –27°
b) the specific rotation is +27°
c) the specific rotation is –17°
d) the specific rotation is +17°
View Answer
Explanation: The specific optical rotation of a sample, [α], is calculated using the equation given below, α (is the measured rotation in degrees, l is the path length in decimeters, and ρ (Greek letter “rho”) is the concentration of the liquid in g/mL, for a sample at a temperature T (given in degrees Celsius) and wavelength λ (in nanometers).
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