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Ylides are prepared from primary and secondary alkyl halides with triphenylophosine followed by a base. Suggest...

Ylides are prepared from primary and secondary alkyl halides with triphenylophosine followed by a base. Suggest two reasons why ylides cannot be formed from tertiary alkyl halides

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Expert Solution

A ylide is a 1,2-dipolar compound containing a formally negatively chaggred atom directly attached to a positively charged heteroatom in which both the atoms complete their octet. e.g.Phosphonium ylides

Phosphonium ylide is prepared by the reaction of alkyl halide with triphenylophosine to form alkyltriphenylphosphonium salt, which then treated with a strong base like BuLi to form phosphonium ylide. This reaction is a substitution reaction that proeeds via SN2 mechanism. The complete reaction is

Since this is a SN2 reaaction only primary and secondary alkyl halides give good yield of the product and hence we cannot prepare it from tertiary alkyl halides.

Also due to the steric hindrance of the bulky tertiary alkyl halide, it is unable to perform SN2 mechanism efficiently and hence unable to form phosphonium ylide.

queen_honey_blossom answered 2 years ago
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