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Arrange the compounds of each set in order of reactivity towards S_(N)2 displacement : (i) 2- Bromo-2-methylbutane , 1- Bromopentane, 2- Bromopentane (ii) 1-Bromo -3- methylbutane, 2- Bromo -2- methylbutane, 3- Bromo -2- methylbutane. (iii) 1-Bromobutane, 1- Bromo -2, 2- dimethylpropane, 1- Bromo -2- methylbutane, 1- Bromo -3- methylbutane. |
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Answer» Solution :The reactivity `S_(N)2` reactions is affected by STERIC hindrance , greaterthe steric hindrance smaller the reactivity (i) `underset("2-Bromo-2-METHYLBUTANE "(3^(@))) (CH_(3) - underset(Br) underset(|) overset(CH_(3)) overset(|) C - CH_(2) CH_(3) ) "" underset("1 - Bromopentane "(1^(@) )) (CH_(3)CH_(2)CH_(2)CH_(2)CH_(2) - Br) ""underset("2 - Bromopentane "(2^(@)))(CH_(3) - underset(Br) underset(|)CH - CH_(2)CH_(2)CH_(3))` On steric grounds, the order of reactivityin `S_(N)2`reactions follows the order : `1^(@) gt 2^(@) gt3^(@) ` . Therefore , order of reactivity of the given alkyl bromides is: 1 - Bromopentane ` gt ` 2 - Bromopentane ` gt ` 2 - Bromo -2- methylbutane . (ii) `underset("1 - Bromo -3- methylbutane "(1^(@))) (CH_(3) - overset(CH_(3))overset(3|) (CH) - overset(2)CH_(2)overset(1)CH_(2)Br ) "" underset("2 -Bromo -2- methylbutane "(3^(@))) (overset(1)CH_(3) - underset(Br) underset(|) overset(CH_(3))overset(2|) C - overset(3)CH_(2) overset(4)CH_(3)) "" underset("3-Bromo -2- methylbutane "(2^(@))) (overset(4)CH_(3) - underset(Br) underset(|) overset(3) (CH) - overset(CH_(3))overset(2|) CH - overset(1)CH_(3))` On steric grounds, the order of reactivity of alkyl halides in `S_(N)2` reactions of the given alkyl bromides is : 1 -Bromo -3- methylbutane ` gt` 2 - Bromo -3- methylbutane ` gt` 2 - Bromo -2- methylbutane . (iii)  Steric hindrance is MAXIMUM in II with three methyl group on `beta` - carbon , followed by III , with two alkyl groups on `beta` - carbon atom , followed by IV , with ONE alkyl group on `beta` - carbon atom . Thus , the reactivity of the given alkylbromides decreases in the order : 1 - Bromobutane ` gt` 1-Bromo -3- methylbutane ` gt` 1 - Bromo -2- methylbutane ` gt` 1 - Bromo -2- , 2 dimethylpropane |
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