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Arrange the compounds of each set in order of reactivity towards S_(N)2 desplacement: (i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane. (ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-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 in `S_(N)2` reactions depends upon steric hindrance , more the steric hindrance slower the reaction. (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(|)(C)H-CH_(2)CH_(2)CH_(3))` Since due to steric reasons, the order of reactivity in `S_(N)2` reactions follows the order: `1^(@) gt 2^(@) gt 3^(@)`, therefore, order of reactivity of the given alkyl bromides is: 1-Bromopentanegt2-Bromopentanegt2-Bromo-2-methylbutane. (ii) `underset("1-Bromo-3-methylbutane "(1^(@)))(CH_(3)-overset(CH_(3))overset(|)(.^(3)C)H-overset(2)(C)H_(2)overset(1)(C)H_(2)Br)""underset("2-Bromo-2-methylbutane "(3^(@)))(overset(1)(C)H_(3)-underset(Br)underset(|)overset(CH_(3))overset(|)(.^(2)C)-overset(3)(C)H_(2)overset(4)(C)H_(3))""underset("2-Bromo-3-methylbutane "(2^(@)))(overset(1)(C)H_(3)-underset(Br)underset(|)overset(2)(C)H-overset(CH_(3))overset(|)(.^(3)C)H-overset(4)(C)H_(3))` Since due to steric reasons, the order of reactvity of alkyl halides in `S_(N)2` reactions follows the order: `1^(@)gt2^(@)gt3^(@)`, therefore, the order of reactivity of the given alkyl bromides is: 1-Bromo-3-methylbutane `(1^(@))gt2-`Bromo-3-methylbutane `(2^(@))gt`2-Bromo-2-methylbutane `(3^(@))` (iii) `underset("1-Bromobutane "(1^(@)" with no branching"))(CH_(3)CH_(2)CH_(2)CH_(2)Br)""underset("1-Bromo-2,2-dimethylpropane "(1^(@)" with two "beta-"METHYL groups"))(CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(.^(beta)C)-overset(alpha)(C)H_(2)Br)""underset("1-Bromo-2-methylbutane "(1^(@)" with one "beta-"ethyl GROUP"))(CH_(3)CH_(2)-overset(CH_(3))overset(|)(.^(beta)C)H-overset(alpha)(C)H_(2)Br)""underset("1-Bromo-3-methylbutane "(1^(@)" with one methyl group "gamma-"position"))(CH_(3)-overset(CH_(3))overset(|)(.^(gamma)C)H-overset(beta)(C)H_(2)-overset(alpha)(C)H_(2)-Br)` Since in case of `1^(@)` alkyl halides, steric hindrance increases in the order: n-alyl halides, alkyl halide with a substituent at any position other than the `beta`-position, one substituent at the `beta`-position, two SUBSTITUENTS at the `beta`-position, therefore, the reactivity decreases in the same order. THUS, the reactivity of the given akyl bromides decreases in the order: 1_bromobutanegt1-Bromo-3-methylbutanegt1-Bromo-2-methylbutanegt1-Bromo-2,2-dimethylpropane. |
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