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Grignards don't add to alkyl halides to create linear carbon extensions because Grignards do transmetallation rather than SN2 displacement of halide.

Is this correct?

I thought transmetallation a) involved two metals and b) involved ligand exchange between the two metals … correct me if I'm wrong, but if I have a Grignard and an alkyl halide, I only have one metal.

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    $\begingroup$ I am not sure what the context of the question that your prof replied to was, but this (calling a metal/halogen exchange as transmetallation) is a pretty big mistake for en experienced chemist with one word "arghhh". I'm sure it was a slip of the moment (probably you bombarded him/her with questions -well done) $\endgroup$ – K_P Oct 26 '14 at 19:00
  • $\begingroup$ "Transmetallation" and "metal-halogen exchange" describe two different (ie not the same) and distinct (ie they don't partially overlap) chemical phenomena so you cannot use them interchangeably $\endgroup$ – K_P Oct 26 '14 at 19:14
  • $\begingroup$ Is Dr J happy for direct transcripts of your communications to be posted and scrutinized on a public forum? You don't want to burn any bridges..... $\endgroup$ – long Oct 27 '14 at 2:13
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Prof told me Grignards don't add to alkyl halides

Grignards can react with alkyl halides to form the expected coupling product. However yields are usually very low except under special conditions. Improved yields (but yields that would still be considered poor for a synthetic route) result when the halide involved is a good leaving group (I ~ Br > Cl), or when the Grignard is made from a stabilized "R" group such as allyl or benzyl.

Here is an interesting example where the reaction of an aryl Grignard reagent with an alkyl halide was examined.

"When a THF solution of p-tolylmagnesium bromide 1a and an equimolar amount of 1-iodoheptane under nitrogen was stirred for 48 h at room temperature, only a very small amount (7% by GC) of the expected coupling product, 1-p-tolylheptane, was formed."

So in principle, Grignard reagents are capable of reactions that look like $\ce{S_{N}2}$ reactions (who knows what the mechanism is when a metal is involved), the yields are just very low.

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  • $\begingroup$ How does stabilizing the R group affect the Grignard overall? $\endgroup$ – Dissenter Oct 27 '14 at 2:36
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    $\begingroup$ Allows the ionic form of the Grignard to be more significant. $\endgroup$ – ron Oct 27 '14 at 2:48
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You'd think that reaction of an alkyl grignard with a (primary) alkyl bromide/iodide gave all sorts of grignards via metal/halogen exchange. I guess that's what your professor means by transmetallation. You'd also get some elimination products. Certainly that's what would happen with alkyllithiums.

That's why for carbon-carbon bond formation reactions you'd use cuprates. Reduced basicity, increased nucleophilicity, all that.

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  • $\begingroup$ Is metal/halogen exchange transmetallation? EDIT: okay, I saw your edit. $\endgroup$ – Dissenter Oct 26 '14 at 17:19
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    $\begingroup$ I wouldn't call it that. Someone else may, though. I would use the word transmetallation only for the reaction of a lithio/magnesio-organic compound with a more electronegative metal, such as zinc, copper, or boron. $\endgroup$ – Abel Friedman Oct 26 '14 at 17:21
  • $\begingroup$ Why would there metal halogen exchange? $\endgroup$ – Dissenter Oct 26 '14 at 17:32
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    $\begingroup$ As for metal-halogen exchange, you'd think where are -ate complexes. Also, think Schlenk equilibrium. $\endgroup$ – Abel Friedman Oct 26 '14 at 17:36

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