Organic Chemistry : Help with Wittig Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Carbonyl Reactions

What is the final organic product of the reaction shown?

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Possible Answers:

III

II

V

I

IV

Correct answer:

III

Explanation:

First step: esterification

Second step: reduction

Third step: neutralization

Fourth step: oxidation to aldehyde

Fifth step: alkene metathesis

Example Question #1 : Reactions With Ketones And Aldehydes

All of the following are characteristics of a Wittig reaction except __________.

Possible Answers:

it results in the exclusive formation of trans double bonds

it proceeds through a phosphaoxetane intermediate

it produces a trialkylphosphine oxide or triarylphosphine oxide as a by-product

it involves the reaction of a phosphonium ylide with a carbonyl

it results in the formation of a carbon-carbon double bond

Correct answer:

it results in the exclusive formation of trans double bonds

Explanation:

The Wittig reaction involves the reaction of a phosphonium ylide (generated by treating a phosphonium salt with a strong base) with a ketone or aldehyde.

The reaction proceeds through a phosphaoxetane (4-membered ring containing both phosphorus and oxygen) intermediate to generate a new compound containing a carbon-carbon double bond, plus a phosphine oxide byproduct. It does not form trans double bonds exclusively; sometimes, a mixture of cis and trans isomers are obtained, and sometimes the cis isomer is the predominant product. 

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