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CBSE Class 12 Chemistry 2017 Delhi Set 1 Solved Paper

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Question : 16 of 26
Marks: +1, -0
Following compounds are given to you :
2-Bromopentane, 2-Bromo-2-methylbutane, 1-Bromopentane
(i) Write the compound which is most reactive towards SN2\mathrm{S}_{\mathrm{N}}2 reaction.
(ii) Write the compound which is optically active.
(iii) Write the compound which is most reactive towards β\beta-elimination reaction.
Solution:  
(a) H3C−CH2−CH2−CH2−CH2−Br1-Bromopentane\underset{\text{1-Bromopentane}}{\mathrm{H}_3\mathrm{C}-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{Br}}
(b) H3C−CH2−CH2−CH∣Br−CH32-Bromopentane\underset{\text{2-Bromopentane}}{\mathrm{H}_3\mathrm{C}-\mathrm{CH}_2-\mathrm{CH}_2-\overset{\mathrm{Br}}{\overset{\vert}{\mathrm{CH}}}-\mathrm{CH}_3}
(c) H3C−CH2−C∣Br∣CH3−CH32-Bromo-2-methyl butane\underset{\text{2-Bromo-2-methyl butane}}{\mathrm{H}_3\mathrm{C}-\mathrm{CH}_2-\underset{\mathrm{CH}_3}{\underset{\vert}{\overset{\mathrm{Br}}{\overset{\vert}{\mathrm{C}}}}}-\mathrm{CH}_3}
(i) These figures show that (a) contains the least steric hindrance so towards the SN2\mathrm{S}_{\mathrm{N}}2 reaction, 1-bromopentane will be the most reactive.
(ii) Figure b of 2-bromopentane has chiral carbon in it. So, it is optically active.
(iii) Towards the β\beta-elimination, 2-bromo-2methylbutane will be the most reactive as it will form most stable alkene (on account of the highest number of α\alpha-hydrogens.)
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