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Questions
Arrange the compounds of the following set in order of reactivity towards SN2 displacement:
2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
Arrange the following compounds in increasing order of their reactivity towards SN2 displacement:
2-Bromo-2-methylbutane, 1-Bromopentane, 2–Bromopentane.
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Solution
The SN2 reaction involves the formation of a transition state with the carbon atom surrounded by 5 additional atoms (groups). A transition state requires minimum steric interactions. The most suitable substrates for SN2 reactions are 1° alkyl halides, followed by 2° and 3° alkyl halides. The order of reactivity towards SN2 is 1° > 2° > 3°> aryl halide. Based on this, the order will be
\[\begin{array}{cc}
\phantom{.................................}\ce{Br}\phantom{............................................}\ce{Br}\\
\phantom{................................}|\phantom{...............................................}|\\
\ce{\underset{1-Bromopentane}{H3C - (CH2)3 - CH2Br} > \ce{\underset{2-Bromopentane}{CH3 - CH - (CH2)2 - CH3}} > \ce{CH3 - C - CH2 - CH3}}\\
\phantom{................................................................................}|\\
\phantom{....................................................................................}\ce{\underset{2-Brmo-2-methylbutane}{CH3}}\
\end{array}\]
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| (I) |  |
| (II) |  |
| (III) |  |
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|
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Answer the following questions:
(a) Why racemisation occurs in SN1? (1)
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(i) CH3 – CH2 – I or CH3CH2 – Cl
(ii)
OR
(c) Arrange the following in the increasing order of their reactivity towards SN1 reactions: (2)
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(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3- methylbutane
