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JIPMER 2019
Exam
JIPMER
Year
2019
Subject
Chemistry
Chapter
Haloalkanes and Haloarenes

JIPMER 2019 · Chemistry · Haloalkanes and Haloarenes

Which of the following is not correct?

Which of the following is not correct?Which of the following is not correct?

Options

  1. APhCH2Br>PhCHBrCHPh3>PhCBr(CH3)2( SN1)\mathrm{PhCH}_2 \mathrm{Br}\gt\mathrm{PhCHBrCH} \mathrm{Ph}_3\gt\mathrm{PhCBr}\left(\mathrm{CH}_3\right)_2\left(\mathrm{~S}_{\mathrm{N}} 1\right)
  2. BR−\mathrm{R}-\mathrm{I}>R−Br>R−Cl(SN2)\gt\mathrm{R}-\mathrm{Br}\gt\mathrm{R}-\mathrm{Cl}\left(\mathrm{S}_N 2\right)
  3. CCH2=CH−Cl<CH2=CH−CH2−Cl<PhCH2−Cl(S21)\begin{array}{r}\mathrm{CH}_2=\mathrm{CH}-\mathrm{Cl} \lt \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2-\mathrm{Cl} \\ \lt \mathrm{PhCH}_2-\mathrm{Cl}\left(\mathrm{S}_2 1\right)\end{array}$$
  4. DR−Cl<R−Br<R−l(SN1)\mathrm{R}-\mathrm{Cl} \lt \mathrm{R}-\mathrm{Br} \lt \mathrm{R}-\mathrm{l}\left(\mathrm{S}_{\mathrm{N}} 1\right)

Correct answer

A. PhCH2Br>PhCHBrCHPh3>PhCBr(CH3)2( SN1)\mathrm{PhCH}_2 \mathrm{Br}\gt\mathrm{PhCHBrCH} \mathrm{Ph}_3\gt\mathrm{PhCBr}\left(\mathrm{CH}_3\right)_2\left(\mathrm{~S}_{\mathrm{N}} 1\right)

Step-by-step solution

Key Idea In SN1\mathrm{S}_{\mathrm{N}} 1 reactions, carbocations are the intermediates. Greater the stability of carbocation more easily it is formed and hence, faster will be the rate of reaction. The stability of carbocation decreases in the order. 3∘3^{\circ} carbocation >2∘\gt2^{\circ} carbocation >1∘\gt1^{\circ} carbocation. Which of the following is not correct? Thus, option (a) is incorrect while trends in other options are correct.
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