Question

5) Which of the following statements correctly describe(s) E1 reactions of alkyl halides (RX)?

I.    Rate = k[base]

II. Rate = k[base][RX]

III.       Rate = k[RX]

IV.       The reactions occur in two distinct steps.

V. The reaction involves a carbocation intermediate.

Answer 1

III.       Rate = k[RX]

IV.       The reactions occur in two distinct steps.

V. The reaction involves a carbocation intermediate.

With secondary and tertiary alkyl halides, the E1 and SN1 mechanisms occur in protic solvents with weakly basic nucleophiles. The reactions occur more easily with tertiary alkyl halides if the nucleophile is not a strong base. The E1 mechanism is always in competition with SN1 because both occur under the same reaction conditions. These conditions are as follows: the alkyl halide is secondary and tertiary (especially); the solvent is protic, to stabilize the intermediate stage (consisting of the carbocation and departed leaving group); and the nucleophile is a weak base. With a strong base, remember that E2, bimolecular elimination, not E1 or SN1, is favored with both secondary and tertiary alkyl halides. E1 product is most often obtained in mixture with SN1, and with a very weak base. With a moderately vigorous nucleophile (like ethanol), SN1 will predominate.