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Isomerism

NEET 2013MCQMediumNot asked in last 5 yrs+4−1
Ques. 1NEET 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

Answer

(4) III > II > I

Solution

I and III are enol forms and II is the keto form (pentane-2,4-dione) of the same compound.

In I the C=C bond is isolated; it is not conjugated with C=O and there is no intramolecular H-bond, so I is the least stable form.

In III the C=C bond is conjugated with C=O, and the enolic –OH forms an intramolecular hydrogen bond with the C=O oxygen (a stable six-membered ring), which is why III is more stable than the keto form II (pentane-2,4-dione exists mainly in the enol form).

∴ Order of stability: III > II > I

Answer: (4)

Keep practising — Isomerism

Ques. 2NEET 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. 3NEET 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. 4AIPMT 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. 5AIPMT 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. 6AIPMT 2010 (Mains)

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

  1. [figure]
  2. [figure]
  3. [figure]
  4. [figure]
Ques. 7AIPMT 2010 (Prelims)

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

  1. [figure]
  2. [figure]
  3. [figure]
  4. [figure]
Ques. 8AIPMT 2009

Which of the following compounds will exhibit cis-trans (geometrical) isomerism?

  1. Butanol
  2. 2-Butyne
  3. 2-Butanol
  4. 2-Butene
Ques. 9AIPMT 2008

How many stereoisomers does this molecule have? CH3CH=CHCH2CHBrCH3

  1. 8
  2. 2
  3. 4
  4. 6
Ques. 10AIPMT 2007

CH3-CHCl-CH2-CH3 has a chiral centre. Which one of the following represents its R-configuration?

  1. [figure]
  2. [figure]
  3. [figure]
  4. [figure]
Ques. 11AIPMT 2007

If there is no rotation of plane polarised light by a compound in a specific solvent, though thought to be chiral, it may mean that

  1. the compound is certainly meso
  2. there is no compound in the solvent
  3. the compound may be a racemic mixture
  4. the compound is certainly achiral.

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