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← JEE Advanced 2014 · Paper 1
ChemistryInteger+3Question 34 of 60

JEE Advanced 2014 · Paper 1, Chemistry question 34

The total number(s) of stable conformers with non-zero dipole moment for the following compound is (are)

Fischer projection of 2,3-dibromo-2,3-dichlorobutane: upper carbon with Cl (top), Br (left), CH₃ (right); lower carbon with Br (left), Cl (right), CH₃ (bottom).
Answer and worked solution

Official answer: 3

Which stereoisomer is drawn? Swapping Cl and CH₃ on the upper carbon of the Fischer projection gives a structure with a horizontal mirror plane (CH₃ top and bottom, Br left and Cl right on both carbons): the meso form. A single swap inverts a centre, so the drawn compound is one of the chiral (±) isomers.

Staggered conformers. Looking along C2-C3, the three staggered conformers of the chiral isomer have no centre of symmetry: none can place every group anti to its identical partner. All three have a non-zero dipole moment. (The meso isomer would have one anti conformer with μ=0\mu=0.)

Answer: 3

Topics:ConformationsBond parameters and dipole moment
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