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This is a snapshot from LG Wade Organic Chemistry. I have a doubt with a particular statement in the book. It says that Grignard reagents do not displace halides as easily as acetylide ions do.

But shouldn’t it be the other way round? Acetylide ions, having an sp-hybridised carbon, are much more electronegative compared to the sp³-hybridised carbon in Grignard reagents. So acetylide ions should have less tendency to displace halide ions by attacking with their lone pair compared to the alkyl group in Grignard reagent. Shouldn’t it?

Please help me sort this out! snapshot from LG Wade Organic Chemistry

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  • $\begingroup$ According to you, the sp carbanion is more stable and less reactive ? $\endgroup$ – user14857 May 15 '15 at 12:40
  • $\begingroup$ Then your statement is contradicting the book's one.I request you to elaborate. @user97554 $\endgroup$ – user14857 May 15 '15 at 12:40
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An acetylide ion proceeds in a nucleophilic substitution pathway (SN2 in the picture due to primary halide). The substitution involves the attack of a highly nucleophilic sp-carbon on a highly electrophilic primary carbon. The enthalpy of the C-C bond formation is relatively low due the pKa of the nucleophile being ~ 25. The ion is treated as a "soft" nucleophile that prefers to substitute alkyl halides instead of acting as a base and conducting an elimination reaction.

A grignard reagent is essentially a highly polarized bond between Carbon and the Bromide(Halide)/Magnesium complex. The electronegative difference between Carbon and Magnesium is 1.3 (http://www.thecatalyst.org/electabl.html <-- Source of values). The Carbon atom in a Gringnard is treated as a carbonanion. This carbonanion characterization is highly unstable in nature and treated as a "hard" nucleophile with pKa of ~ 50. The highly basic character of a Grignard reagent often results in an elimination reaction or no reaction at all. The transition state to substitute the alkyl halide is less stable than the Magnesium/Bromide(Halide) complex. This is due to a ligation formation between the solvent and the Magnesium atom. The solvent (ex. diethyl ether) shares non-bonding electrons with the cation and creates a stable electronic atmosphere with a lower energy than a possible C-C bond with the alkyl halide.

Substitution isn't preferred with a hard nucleophile and therefore, the acetylide ion is more likely to preform a nucleophilic substitution reaction with an alkyl halide. Note that the electronegative difference between Carbon and Magnesium supersedes the hybridization of the sp or "triple" bond complex. The bond is so high in energy that the nucleophilic character of the grignard far surpasses the electronegative character of the sp-hybridization of the acetylide ion.

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