SolutionsUnimolecular Nucleophilic Substitution
1. The rate law
In the solvolysis of tert-butyl chloride in methanol, what happens to the rate if the halide concentration is doubled? And if a stronger nucleophile is added?
v = k[(CH3)3CCl]: it doubles when the halide is doubled and does not change with the nucleophile, which is not involved in the rate-limiting step.
2. Racemization
What product is obtained in the SN1 hydrolysis of (R)-3-bromo-3-methylhexane? Is it optically active?
3-Methyl-3-hexanol, as a (nearly) racemic mixture: water attacks the planar carbocation from both faces with equal probability. It is not optically active.
3. Order by SN1
Order by SN1 rate: (CH3)3CBr, CH3CH2Br, (CH3)2CHBr and CH3Br.
(CH3)3CBr > (CH3)2CHBr > CH3CH2Br > CH3Br, the same order as the stability of the carbocations formed (tertiary > secondary > primary > methyl).
4. The solvent
Why is SN1 much faster in water than in diethyl ether?
Water, being polar and protic, stabilizes by solvation the carbocation and the anion formed, and also the ionization transition state, which already carries much charge.