Organic Chemistry : Elimination Mechanisms

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #11 : Elimination Mechanisms

Which of the following sets of conditions most strongly favors an E2 elimination reaction?

Possible Answers:

Strong base, polar protic solvent, secondary electrophile

Weak base, polar aprotic solvent, tertiary electrophile

Strong base, polar aprotic solvent, tertiary electrophile

Strong base, polar aprotic solvent, primary electrophile

Strong base, polar aprotic solvent, primary electrophile

Correct answer:

Weak base, polar aprotic solvent, tertiary electrophile

Explanation:

For E2 reactions a tertiary electrophile > secondary electrophile > primary electrophile. A polar aprotic solvent favors E2 (remember that polar protic solvents favor E1). A strong base is needed to undergo E2.

Example Question #1 : Help With E2 Reactions

A student carried out a substitution reaction in the lab using ether as a solvent. The student began with an optically pure reactant (100% (R)-configuration) and finished with an optically pure product (100% (S)-configuration).

The reaction went through which of the following mechanisms?

Possible Answers:

E1

E2

Either SN1 or SN2

SN2

SN1

Correct answer:

SN2

Explanation:

SN2 reactions result in an inversion of products, thus R-configured reactants become S-configured products and vice versa. Polar aprotic solvents such as ether favor SN2 reactions.

E1 and E2 are elimination mechanisms and the question asks for a substitution mechanism. SN1 results in racemization of products, not inversion. Thus, if the reaction had gone through SN1 we would see a mixture of R and S-configuration in the products. Additionally, polar protic solvents (not aprotic like ether) favor SN1 reactions.

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