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प्रश्न
Arrange the compounds of the following set in order of reactivity towards SN2 displacement:
1-Bromobutane, 1-Bromo-2, 2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane
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उत्तर
The SN2 reaction involves the formation of a transition state with the carbon atom surrounded by 5 additional atoms (groups). A transition state requires minimal 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:
\[\begin{array}{cc}
\phantom{................................................................................................}\ce{CH3}\phantom{..............................}\ce{CH3}\\
\phantom{............................................................................................}|\phantom{....................................}|\\
\ce{\underset{1-Bromobutane}{CH3(CH2)CH2Br} > \ce{\underset{1-Bromo-3-methylbutane}{(CH3)2 - CH - CH2 - CH2Br} > \ce{\underset{1-Bromo-2-methylbutane}{CH3 - CH2 - CH - CH2Br} > \ce{CH3 - C - CH2Br}}}}\\
\phantom{.................................................................................................................................}|\\
\phantom{.....................................................................................................................................}\ce{\underset{1-Bromo-2, 2-dimethylpropane}{CH3}}\
\end{array}\]
Although all alkyl halides are 1°, the order of reactivity depends on the steric barrier around the carbon bearing the –Br atom. The more bulky groups around a carbon, the lower its reactivity towards SN2.
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संबंधित प्रश्न
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\ce{CH3CH2CHCH3}\\
\phantom{.....}|\\
\phantom{.......}\ce{Br}\
\end{array}\]
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| III | RX + KNO2 | \[\begin{array}{cc} \phantom{.......}\ce{O}\\ \phantom{.....}/\\ \ce{R - N}\phantom{....}\\ \phantom{.....}\backslash\backslash\\ \phantom{.......}\ce{O} \end{array}\] |
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(i) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5OH] \underset{}{(CH3)2CH - CH2OC2H5 + HBr}}\\
\end{array}\]
(ii) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5O-] \underset{}{(CH3)2CH - CH2OC2H5 + Br-}}\\
\end{array}\]
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