[mykosfemme]

Hello all! So I will be doing a presentation about Grignard reagent mechanism, and I have a few questions for discussion if anyone would like to reply.

Why is anhydrous diethyl ether a better solvent better than THF?

What drives the reaction - is it just the presence of the Br group and the partial charges on the C-Br bond that cause it to react or is it something else, like added energy to the system?

Also,when using a Grignard reagent on this molecule:



Why would it attack the aldehyde over the carbon attached to the bromine? Is it because it is less stable? Is there another place that perhaps the Grignard could have attacked other than the carbon in the aldehyde?

How would solvents other than a nonpolar, aprotic solvent affect this?

Any discussion is welcome. I am looking to learn as much as I can about the Grignard reagent. Thanks!

[kriggy]

1) Is diethylether  realy better solvent than THF? I think it depends on exact compounds you are working with.

2) Its nucleophilic addition to the aldehyde.

3) look at the mechanism, how electrophilic is the bromine carbon compared to aldehyde?

4) Ethers are used for a reason. How does grignard react with acidic protons? (hint: its very strong base with pKa over 40)

I suggest reading http://en.wikipedia.org/wiki/Grignard_reaction

[Babcock_Hall]

It's a forum policy that you must show your attempt first.

[pgk]

Attention: Do not learn it, in a wrong way.
THF is a better solvent for Grignard reactions. It has better solvent properties, a higher boiling point and it is more polar. Therefore, the reaction is accelerated by thermodynamic control and by polarity, given that polar reactions (like Grignard Reactions) are accelerated in polar solvents.
On the other hand, diethyl ether is more practical to use. It can easily be super-dehydrated by simple addition of sodium chips or sodium wire (contrary to THF that demands reflux over benzophenone and sodium, followed by distillation). It is not miscible with water that permits immediate purification by extraction (with negligible product loss) and it easily removed in the rotary evaporator, as having a lower boiling point.
But of course, all above finally depend on the exact compounds that you are working with.

[orgopete]

Re: ether v THF
Everything is relative. THF can become deprotonated to give ethylene and the enolate of acetaldehyde with butyllithium at room temperature. Grignards, less so, but.... THF has also been used to isolate R2Mg compounds (presumably because MgBr2 precipitates?). Peroxide formation? 

Re: addition v insertion
Interesting question. This may be a lot more difficult to KNOW. I seem to recall iodo-esters reacting by insertion as a faster reaction, or something like that. Since halogens are known to react as electron acceptors, e.g. Br2 + alkenes or R-I + I(-)  RH + I2, one might guess a similar reaction can occur with Grignard formation or a transmetalation. If that were the case, the difference in reactivity may be greatly influenced by the R-group. Even if a Grignard were to add/donate electrons to the Br, if no further reaction took place, it would be a "no reaction" reaction. That is, donation of electrons to Br2 can give an R-Br-Br](-) intermediate, but will readily lose Br(-). That is much less likely with an alkyl bromide.

Since I like to argue that reactions generally follow similar pathways, I like to seek a similar pathway to ones we know. We know Br2 reacts with Br(-) to give Br3(-). Then transmetalation reactions may occur by an initial addition to the halogen. This may be how arylhalides are transmetalated. I'm not saying an addition/complexation must occur, but that a series of reactions is known that makes the question a reasonable one to think about.

[pgk]

Peroxide formation is higher in THF, due to higher content of oxygen per mass compared to diethyl ether. However, THF peroxides are less volatile ( b.p. ≈ 62 oC  at 2 torr) than diethyl ether peroxides { b.p. ≈ 40 oC  at 2 torr), but still being considerable.