Organic Chemistry Aldehydes And Ketones - Nucleophilic Addition Of Phosphorus Ylides: The Wittig Rxn

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An Aldehyde or ketone is converted into an alkene by means of the

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Wittig rxn

Front 2

In the Wittig rxn, a phosphorus ylide, R2C-P(C6H5)3) adds to an aldehyde or ketone to yield ..

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a dipolar intermediate called a betaine

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The betaine intermediate is not isolated, rather, it

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spontaneously decomposes through a four membered ring to yield alkene and triphenylphosphine oxide
(Ph)3=O

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The net result of the betaine intermediate is the ...

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replacement of the carbonyl oxygen atom by the R2C= group originally bonded to phosphorus

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The phosphorus ylides necessary for Wittig rxn are easily prepared by?

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Sn2 rxn of primary(and sometimes secondary) alkyl halides with triphenylphosphine, followed by treatment with base

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Triphenylphine, (Ph)3P, is a good nucleophile in an Sn2 rxn, and yields of

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the resultant alkyltriphenylphosphonium salts are high

- Cuz of the positive charge on the phosphorus, the hydrogen on the neighboring carbon is weakly acidic and can be removed by a base such as butyllithium (BuLi) to generate the neutral ylide

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The Wittig rxn is extremely general in its applications, and a great many mono-, di-, and tri- substituted alkenes can be ...

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prepared from the appropriate combo of phosphorane and aldehyde or ketone

Front 8

The real value of the Wittig rxn is that it yields a ?

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pure alkene of known structure.

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The C=C bond of the Wittig Rxn in the product is always...

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exactly where the C=O group was in the reactant, and no alkene isomers (except E,Z isomers) are formed