GRE Subject Test: Chemistry : Substitution Reactions

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #2 : Help With Sn2 Reactions

In reactions involving the alkylation of acetylide ions, it is preferred that the alkyl halide be primary. What is the reason for this?

Possible Answers:

The mechanism for these reactions is SN1

The reactions generally occur in two steps

The reaction involves a carbocation as intermediate

An answer cannot be determined without more information about the reaction conditions

The mechanism for these reactions is SN2

Correct answer:

The mechanism for these reactions is SN2

Explanation:

The reason that the alkyl halide is preferred to be primary is because the mechanism for these reactions is SN2. SN2 indicates a substitution reaction that takes place in one step. A primary alcohol is preferred to prevent steric congestion caused by the simultaneous binding of the nucleophile and release of the leaving group. This reaction mechanism is faster because it omits the formation of a carbocation intermediate.

In contrast, SN1 reactions take place in two steps and involve the formation of a carbocation intermediate.

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