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Isomerism

NEET 2016 (Phase 1)MCQMediumNot asked in last 5 yrs+4−1
Ques. 1NEET 2016 (Phase 1)

Which of the following biphenyls is optically active ?

  1. [figure]
  2. [figure]
  3. [figure]
  4. [figure]

Answer

(4) [figure]

Solution

A substituted biphenyl shows optical activity (atropisomerism) when:

(i) bulky groups at the ortho positions restrict free rotation about the central C–C bond, so the two rings stay non-planar, and

(ii) each ring carries two different ortho groups, so the molecule has no plane of symmetry.

(1) Only one ortho group (I) on each ring ⇒ rotation is not sufficiently hindered and a symmetric conformation is accessible ⇒ inactive.

(2) Second ring is unsubstituted, and the first ring has identical ortho groups (CH3, CH3) ⇒ plane of symmetry ⇒ inactive.

(3) Only meta substituents ⇒ free rotation ⇒ inactive.

(4) Each ring has two different bulky ortho groups (Br and I) ⇒ rotation is restricted and the molecule is non-superimposable on its mirror image ⇒ optically active.

Answer: (4)

Keep practising — Isomerism

Ques. 2NEET 2016 (Phase 1)

The correct statement regarding the comparison of staggered and eclipsed conformations of ethane, is

  1. the eclipsed conformation of ethane is more stable than staggered conformation even though the eclipsed conformation has torsional strain
  2. the staggered conformation of ethane is more stable than eclipsed conformation, because staggered conformation has no torsional strain
  3. the staggered conformation of ethane is less stable than eclipsed conformation, because staggered conformation has torsional strain
  4. the eclipsed conformation of ethane is more stable than staggered conformation, because eclipsed conformation has no torsional strain.
Ques. 3NEET 2016 (Phase 1)

The correct statement regarding a carbonyl compound with a hydrogen atom on its alpha-carbon, is

  1. a carbonyl compound with a hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol and this process is known as carbonylation
  2. a carbonyl compound with a hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol and this process is known as keto-enol tautomerism
  3. a carbonyl compound with a hydrogen atom on its alpha-carbon never equilibrates with its corresponding enol
  4. a carbonyl compound with a hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol and this process is known as aldehyde-ketone equilibration.
Ques. 4NEET 2016 (Phase 2)

Which among the given molecules can exhibit tautomerism? [figure]

  1. III only
  2. Both I and III
  3. Both I and II
  4. Both II and III
Ques. 5AIPMT 2015

The possible stereo-structures of CH3CHOHCOOH, which are optically active, are called

  1. atropisomers
  2. enantiomers
  3. mesomers
  4. diastereomers
Ques. 6AIPMT 2015

In an SN1 reaction on chiral centers, there is

  1. inversion more than retention leading to partial racemisation
  2. 100% retention
  3. 100% inversion
  4. 100% racemisation
Ques. 7AIPMT 2015

The number of structural isomers possible from the molecular formula C3H9N is

  1. 5
  2. 2
  3. 3
  4. 4
Ques. 8AIPMT 2015 (Cancelled)

Given : [figure] Which of the given compounds can exhibit tautomerism?

  1. II and III
  2. I, II and III
  3. I and II
  4. I and III
Ques. 9AIPMT 2014

Which of the following compounds will undergo racemisation when solution of KOH hydrolyses? [figure]

  1. (i) and (ii)
  2. (ii) and (iv)
  3. (iv)
  4. (i) and (iv)
Ques. 10NEET 2013

The order of stability of the following tautomeric compounds is [figure]

  1. II > I > III
  2. II > III > I
  3. I > II > III
  4. III > II > I
Ques. 11NEET 2013 (Karnataka)

Given: [figure] I and II are

  1. identical
  2. a pair of conformers
  3. a pair of geometrical isomers
  4. a pair of optical isomers

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