Four alkyl halides: bromomethane, 2-bromobutane, 2-bromo-2-methylpropane, and 1-bromobutane, each react with water through solvolysis by an SN1 reaction in which the solvent acts as the nucleophile. In what order will these alkyl halides react to form alcohols, from fastest to slowest?A.bromomethane → 1-bromobutane → 2-bromobutane → 2-bromo-2-methylpropaneB.bromomethane → 1-bromobutane → 2-bromo-2-methylpropane → 2-bromobutaneC.2-bromo-2-methylpropane → 2-bromobutane → 1-bromobutane → bromomethaneD.2-bromobutane → 2-bromo-2-methylpropane → 1-bromobutane → bromomethane
Question
Four alkyl halides: bromomethane, 2-bromobutane, 2-bromo-2-methylpropane, and 1-bromobutane, each react with water through solvolysis by an SN1 reaction in which the solvent acts as the nucleophile. In what order will these alkyl halides react to form alcohols, from fastest to slowest?A.bromomethane → 1-bromobutane → 2-bromobutane → 2-bromo-2-methylpropaneB.bromomethane → 1-bromobutane → 2-bromo-2-methylpropane → 2-bromobutaneC.2-bromo-2-methylpropane → 2-bromobutane → 1-bromobutane → bromomethaneD.2-bromobutane → 2-bromo-2-methylpropane → 1-bromobutane → bromomethane
Solution
The order of reactivity of alkyl halides in an SN1 reaction is determined by the stability of the carbocation intermediate. The more substituted the carbocation, the more stable it is. This is because alkyl groups are electron-donating and can help to stabilize the positive charge on the carbocation.
Here are the carbocations that would be formed from each alkyl halide:
- Bromomethane: Methyl carbocation (primary)
- 1-bromobutane: Primary carbocation
- 2-bromobutane: Secondary carbocation
- 2-bromo-2-methylpropane: Tertiary carbocation
The order of carbocation stability is tertiary > secondary > primary > methyl. Therefore, the order of reactivity of these alkyl halides from fastest to slowest is:
2-bromo-2-methylpropane (tertiary) > 2-bromobutane (secondary) > 1-bromobutane (primary) > bromomethane (methyl)
So, the correct answer is C. 2-bromo-2-methylpropane → 2-bromobutane → 1-bromobutane → bromomethane.
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