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Isomerism

AIPMT 2015MCQMedium📈 Topic in 12% papers+4−1
Ques. 1AIPMT 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

Answer

(1) inversion more than retention leading to partial racemisation

Solution

In an SN1 reaction the leaving group departs first, giving a planar (sp2) carbocation.

⇒ The nucleophile can attack from both faces, so both inverted and retained products form; as per NCERT, SN1 reactions of optically active halides are accompanied by racemisation.

∵ The departing halide ion stays close to the front face for a short time (ion pair) and partly shields it, attack from the back side is slightly favoured.

∴ Inversion is more than retention ⇒ partial racemisation.

Answer: (1)

Keep practising — Isomerism

Ques. 2AIPMT 2015

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

  1. 5
  2. 2
  3. 3
  4. 4
Ques. 3AIPMT 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. 4AIPMT 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. 5NEET 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. 6NEET 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
Ques. 7NEET 2013 (Karnataka)

Number of isomeric alcohols of molecular formula C6H14O which give positive iodoform test is

  1. three
  2. four
  3. five
  4. two
Ques. 8AIPMT 2012 (Prelims)

Which of the following acids does not exhibit optical isomerism?

  1. Maleic acid
  2. -amino acids
  3. Lactic acid
  4. Tartaric acid
Ques. 9AIPMT 2011 (Mains)

The IUPAC name of the following compound is [figure]

  1. trans-2-chloro-3-iodo-2-pentene
  2. cis-3-iodo-4-chloro-3-pentene
  3. trans-3-iodo-4-chloro-3-pentene
  4. cis-2-chloro-3-iodo-2-pentene
Ques. 10AIPMT 2010 (Mains)

Which of the following conformers for ethylene glycol is most stable?

  1. [figure]
  2. [figure]
  3. [figure]
  4. [figure]
Ques. 11AIPMT 2010 (Prelims)

In the following the most stable conformation of n-butane is

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

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