The kinetics of solvolysis of such secondary alkyl chlorides can be studied at e

Question

The kinetics of solvolysis of such secondary alkyl chlorides can be studied at elevated temperatures. To prevent the evapoaration of the volatile alcohol, solvolysis reactions at elevated temperatures are usually carried out in a sealed ampoule. Such experiments have shown that the pseudo-first order rate constant k for the solvolysis of cyclohexyl chloride in 80% ethanol is 55 × 10-7 s-1 at 118 °C. In first-order reactions, the reaction rate constant and the reactant half-life are related via ? = ln2/k; thus the half-life of cyclohexyl chloride is about 35 hours under these conditions.

The rate of such solvolysis reactions depends on other substutuents in the cyclohexyl ring. Examine the structures of two analogs of cyclohexyl chloride that are shown below and choose the most appropriate statement regarding their reactivity:

Select one:

a. Analog A undergoes solvolysis slightly slower than the unsubstituted cyclohexyl chloride because the bulky phenyl group sterically prevents the backside attack of ethanol to electrophilic carbon. Analog B does not undergo any solvolysis because the backside of the electrophilic carbon is sterically not accessible when chloride is in the equatorial position.

b. Both analogs A and B undergoes solvolysis much faster than the unsubstituted cyclohexyl chloride. The rate enhancement is due to resonance stabilization of the carbocation that forms in the SN1 displacement by the nearby phenyl group.

c. Both analogs A and B undergo solvolysis much slower than the unsubstituted cyclohexyl chloride. The rate decrease is due to the inductive effect of electronegative sulfur, which would destabilize the positive charge that forms in the SN1 displacement.

d. Analog B undergoes solvolysis much faster than analog A or the unsubstituted cyclohexyl chloride. The rate enhancement is due to intramolecular catalysis by the sulfur, which will use its d-electrons to form a dative bond with chloride that is within the bonding distance only in the analog B; the resulting four-membered heterocyclic sulfonium ion is then opened up by ethanol.

e. Analog A undergoes solvolysis much faster than analog B or the unsubstituted cyclohexyl chloride. The rate enhancement is due to intramolecular catalysis by the sulfur, which will displace chloride via SN2 reaction; the resulting three-membered heterocyclic sulfonium ion is then opened up by ethanol.

Explanation / Answer

Ans A)

Analog A undergoes solvolysis slightly slower than the unsubstituted cyclohexyl chloride because the bulky phenyl group sterically prevents the backside attack of ethanol to electrophilic carbon. Analog B does not undergo any solvolysis because the backside of the electrophilic carbon is sterically not accessible when chloride is in the equatorial position.

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