[AdiDex]

My teacher told me Corey house is better than grignard reaction for preparing Alkanes (specially when you have to fuse two alkanes) .

I have doubt , Why there will be less byproducts in the case of Corey House ?? Corey house should be expensive . What will be the advantages of Corey house over grignard ??

Is there any case when grignard/Wurtz reaction is better than Corey House Synthesis ??

[AdiDex]

Even she didn't gave me any marks for my attempt , her explanation was Corey House give better yield and I mentioned you have to choose that method which gives best yield . Is she correct ??

[kriggy]

Im not sure if the grignard works this way without catalyst in any useful yield. Also, by reacting grignard with ethylbromide some of it will be converted into ethylgrignard which can than make quite a mess in the reaction. Honestly, I dont see why the Corey-House should be expensive?
For more C-C bond formation using grignard reagents, see http://www.organic-chemistry.org/namedreactions/kumada-coupling.shtm

[clarkstill]

I guess the organocuprate is less basic than the organolithium, so you get fewer by-products arising from deprotonation/E2-type chemistry?

[BRSM]

It is true that Grignards suck at S_N2 reaction with alkyl halides. I wouldn't expect this to work well, if at all using the Grignard. There are a number of explanations. I believe the Hard-Soft Acid-Base argument is the canonical explanation. Also, it's worth remembering that Grignard reagents themselves are very strong bases, and quite strong reducing agents, leading to side reactions. And R1MgX + R2X can give R2MgX and R1X where R1 and R2 stabilize anions to a similar extent, so scrambling is possible. Also, there's the whole Schlenk equilibrium thing where RMgX equilbrates to R2Mg and MgX2.

The Corey-House method is not significantly more expensive due to the low cost of the metals involved. To answer your second question: Wurtz couplings are pretty rare, but they can be useful for dimerizations of alkyl halides. The tend to suck at cross coupling of different halides for the reasons outlined above, but where the things that go wrong can be minimized you might consider them.

[orgopete]

I think if you write the mechanisms and just think about the reactions a bit, you can begin to understand why the Corey-House (and mech. similar) reactions are better. If coupling of Grignards were a problem, they would BE a coupling reaction. I view the cuprates as oxidative addition-reductive elimination reactions, so they are fundamentally different. Even thought Grignards can exchange ligands and form R2Mg compounds, these cannot do a reductive elimination.