Grignard Reagents!

[LaFleur]

Hey guys,

I'm reviewing orgo using some books I have. I'm a little confused about grignard reagents. I know what they are used for, and I know that they're usually made with Mg, but I'm wondering what Li has to do with them.

Specifically, I was looking at a problem that asked me to synthesize 3-methylheptane (racemic) using only 1-butene. In the answer key, these were the steps: 1. HBr, 2. Li, 3. CuI, 4. 1-bromobutane (from 1-butene using HBr in the presence of peroxides).

I'm a little confused about how this works. Is there a reason that they are using Li and then CuI or would it be acceptable to just use Mg instead of Li and CuI to create a grignard reagent that would react with 1-bromobutane?

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[UndergradGuy7]

When adding to C=O groups, Mg and Li do the same thing.

CuX + Li is different though. Can only use Li I think. Li is usually more reactive than Mg and from wikipedia ... http://en.wikipedia.org/wiki/Grignard_reagent

"Another disadvantage of Grignard reagents is that they do not readily form carbon-carbon bonds by reacting with alkyl halides via an SN2 mechanism."

As for the mechanism RLi + CuX (where X=Cl, Br, I) forms R2-Cu-Li
the R2-Cu-Li then reacts with R'X to form R-R'

Usually the R'X has to be methyl or primary carbon or else you would get some elimination. Since this is an Sn2 replacement.

 

[LaFleur]

When adding to C=O groups, Mg and Li do the same thing.

CuX + Li is different though. Can only use Li I think. Li is usually more reactive than Mg and from wikipedia ... http://en.wikipedia.org/wiki/Grignard_reagent

"Another disadvantage of Grignard reagents is that they do not readily form carbon-carbon bonds by reacting with alkyl halides via an SN2 mechanism."

As for the mechanism RLi + CuX (where X=Cl, Br, I) forms R2-Cu-Li
the R2-Cu-Li then reacts with R'X to form R-R'

Usually the R'X has to be methyl or primary carbon or else you would get some elimination. Since this is an Sn2 replacement.

Thanks so much!

It makes a lot of sense now. I always thought Mg and Li could be interchangeable...hahaha maybe that's part of the reason I didn't do all that well in orgo. 🙄