ABC Chemistry – Best Chemistry Coaching in Dwarka, Delhi
Question 1
The two compounds given below are:
Correct Answer: (b) Identical
Explanation:
A 180° rotation about the carbonyl carbon–carbon bond converts one structure into the other. Since the two structures are interconvertible by simple rotation and no bond breaking is required, they represent the same molecule and are therefore identical, not enantiomers or diastereomers.
Explanation:
A 180° rotation about the carbonyl carbon–carbon bond converts one structure into the other. Since the two structures are interconvertible by simple rotation and no bond breaking is required, they represent the same molecule and are therefore identical, not enantiomers or diastereomers.
Correct Answer: (a) 1R,4R
Explanation:
The substituents at each chiral centre are numbered according to the CIP priority rules. The atom with the highest atomic number is assigned the highest priority. After orienting the lowest priority group away from the observer, the sequence of remaining substituents is traced. Both stereocentres show a clockwise arrangement, leading to R-configuration at positions 1 and 4.
Explanation:
The substituents at each chiral centre are numbered according to the CIP priority rules. The atom with the highest atomic number is assigned the highest priority. After orienting the lowest priority group away from the observer, the sequence of remaining substituents is traced. Both stereocentres show a clockwise arrangement, leading to R-configuration at positions 1 and 4.
Question 2
The configurations at the two stereocentres in the compound given below are:
Correct Answer: (a) 1R,4R
Explanation:
The substituents at each chiral centre are numbered according to the CIP priority rules. The atom with the highest atomic number is assigned the highest priority. After orienting the lowest priority group away from the observer, the sequence of remaining substituents is traced. Both stereocentres show a clockwise arrangement, leading to R-configuration at positions 1 and 4.
Explanation:
The substituents at each chiral centre are numbered according to the CIP priority rules. The atom with the highest atomic number is assigned the highest priority. After orienting the lowest priority group away from the observer, the sequence of remaining substituents is traced. Both stereocentres show a clockwise arrangement, leading to R-configuration at positions 1 and 4.
Question 3
In the most stable conformation of trans-1-t-butyl-3-methylcyclohexane,
the substituents at C-1 and C-3, respectively, are:
Correct Answer: (c) Equatorial and axial
Explanation:
The t-butyl group is extremely bulky and therefore strongly prefers the equatorial position to avoid 1,3-diaxial interactions. Hence, at C-1 the t-butyl group is equatorial.
Since the substituents are trans-1,3-disubstituted, when one substituent is equatorial, the other must be axial. Therefore, the methyl group at C-3 occupies the axial position.
Explanation:
The t-butyl group is extremely bulky and therefore strongly prefers the equatorial position to avoid 1,3-diaxial interactions. Hence, at C-1 the t-butyl group is equatorial.
Since the substituents are trans-1,3-disubstituted, when one substituent is equatorial, the other must be axial. Therefore, the methyl group at C-3 occupies the axial position.
Question 4
CSIR-JUNE 2011
Multiple Choice Question
Consider two redox pairs:
(1) Cr(II)/Ru(III)
(2) Cr(II)/Co(III)
The rate of acceleration in going from an outer-sphere to an inner-sphere mechanism is lower for (1) relative to (2). Its correct explanation is:
(1) Cr(II)/Ru(III)
(2) Cr(II)/Co(III)
The rate of acceleration in going from an outer-sphere to an inner-sphere mechanism is lower for (1) relative to (2). Its correct explanation is: