Arrange in order of reactivity towards S_(N)2 reactions: (3) 1-bromobutane, 1-bromobutane,1-bromo-3-methylbutane,1-bromo-2,2-dimethly propane, 1-bromo-2-methylbutane, 1-bromo-3methylbutane.

Arrange in order of reactivity towards S_(N)2 reactions: (3) 1-bromob

1.	Arrange in order of reactivity towards S_(N)2 reactions: (3) 1-bromobutane, 1-bromobutane,1-bromo-3-methylbutane,1-bromo-2,2-dimethly propane, 1-bromo-2-methylbutane, 1-bromo-3methylbutane.
Answer» Solution :`underset(1-"bromobutane"(A)("UNBRANCHED"1^(@))"alkyl" "halide")(CH_(3)CH_(2)CH_(2)CH_(2)Br)` `underset(1-"bromo-2,2-dimethylpropane" (B)("two" CH_(3)"group at" beta-"carbon"))(CH_(3)-underset(CH_3)underset(\|)overset(CH_(3))overset(\|)(C^(beta))-overset(alpha)(C)H_(2)-Br)` `underset(1-"bromobutane"-2-"methylbutane"(C)("one" CH_(3)"group at" beta-"carbon"))(CH_(3)CH_(2)-overset(CH_3)overset(\|)(.^(beta)C)-overset(alpha)(C)H_(2)-Br)` `underset(1-"bromobutane"-3-"methylbutane"(D)("one" CH_(3)"group at" gamma-"carbon"))(CH_(3)-overset(CH_3)overset(\|)(.^(gamma)C)-overset(beta)(C)H_(2)-overset(alpha)(C)H_(2)-Br)` `S_(N)2` reactivity of `1^(@)` alkyl halides decreases as the alkyl substituents on the carbon chain get closer to the `alpha`-carbon. Thus `S_(N)2`reactitvity of unsubsituted `1^(@)` alkyl halide, A, is maximum. Reactivity of `gamma`-subsituted `1^(@)` alkyl halide, D, is somewhat lessthan that of A. The reactivity of `beta`-substituted alkyl halides B and C are less than that of D. Out of B and C, the former is less reactive beacuse its `alpha`-carbon becones more CROWDED by the presence of two methyl groups on the `beta`-carbon. Thus, `S_(N)2` reactivity decreases in the sequence: A > D > C > B.