[andy1111]

Hi All,

I'm studying for my DAT and was just wondering why Ginard Reagent can't react with primary alkyl halide. Thank you in advance

[discodermolide]

Hi All,

I'm studying for my DAT and was just wondering why Ginard Reagent can't react with primary alkyl halide. Thank you in advance

Of course it can react with primary halides, a nucleophilic displacement of the halide by the R of RMgX. To give the chain extended alkane and MgXHalide.

[andy1111]

I'm using Chad's video to study for the science portion and I could have swore that's what he said. I'm looking at my notes right now and it says that aceylide can react with primary alkyl halide, carbonyl, and epoxides but Grinard Regent can only react with carbonyl and epoxides. What you said does make sense.

Do you have any possible reason as to why he said that? Any further input would be gladly appreciate, disco.

[discodermolide]

I'm using Chad's video to study for the science portion and I could have swore that's what he said. I'm looking at my notes right now and it says that aceylide can react with primary alkyl halide, carbonyl, and epoxides but Grinard Regent can only react with carbonyl and epoxides. What you said does make sense.

Do you have any possible reason as to why he said that? Any further input would be gladly appreciate, disco.

No idea why he said that.
Have a look at the second scheme here:
http://en.wikipedia.org/wiki/Grignard_reaction

The problem with some Grignards is stopping them self coupling.

[orgopete]

I don't recall coupling being a problem. There are some that will upon preparation, but Wikipedia didn't mention it. I thought self coupling occurred in their preparation. Wikipedia mentioned some coupling reactions mediated by a transition metal, suggesting a different mechanism. They also noted benzyl and allyl may couple.

I've not tried reacting a primary halide with a Grignard, so I am only guessing at a reason. In the Wikipedia article, they list transmetalation reaction with aryl halides. A plausible mechanism for this would be attack upon the halide and thus exchanging the Grignard reagent, in this case a less basic aryl anion. If this same reaction were to take place with a primary bromide, attack may occur upon the bromide, but may not reach the critical point at which exchange may occur, unreactive. Either that or some transmetalation occurs. The benzyl and allyl examples may occur as the sp2 substitution make the carbon more reactive.

I should presume that if one wanted to perform a coupling reaction, one should react the Grignard with a copper or nickel reagent via an oxidative addition, the metal inserts into the primary halide, and then a reductive elimination would a create a carbon-carbon bond, regenerate the copper reagent, and form MgBr2 in a net coupling reaction.

[Dan]

Could it be a reference to the tendency for alkyl halides to eliminate in the presence of Grignards? i.e. E2 competes with SN2 - the problem is not that the Grignard doesn't react, but that the reaction is messy.

As Orgopete mentioned, couplings of this type are normally facilitated by a transition metal (less basic alkylating agent) - I don't think I've seen a Grignard used directly for this kind of coupling.