Organic Chemistry question on Grignard reactions

[simms]

Benzene is often produced as a side product during Grignard rxns using phenylmagnesium bromide. How can its formation be explained? What about a balanced equation?

I have no idea on how to answer this question.. ideas?

Oh, is it because the Grignard reagent could react with unreacted bromobenzene, which favours the formation of this at high temps? Then it creates biphenyl which can dissociate into 2 benzenes?

That's my best guess.

 

[ravana]

What class is this for?

 

[simms]

This is for organic labs, part of organic chemistry, 2nd year, chemical engineering.

 

[JCase]

Benzene (or whatever R group is attached to the Mg) will be formed when water is present, which is why you must keep a gringard DRY. Rather than proceeding through attack of your carbonyl, which is the desired reaction, the benzene ring will attack any water molecules around, grabbing a proton and kicking off the MgBr. Here's a quick schematic for this side reaction: R-MgBr + H-OH --> R-H + HO-MgBr. PM me if you don't understand this completely...I'm a Chem Bio major at Cal

 

[Tiamat]

Oh god, i hated all 2 semesters of Organic chemstry. Pchem is slightly better, but isnt that great either...

 

[simms]

Originally posted by: JCase
Benzene (or whatever R group is attached to the Mg) will be formed when water is present, which is why you must keep a gringard DRY. Rather than proceeding through attack of your carbonyl, which is the desired reaction, the benzene ring will attack any water molecules around, grabbing a proton and kicking off the MgBr. Here's a quick schematic for this side reaction: R-MgBr + H-OH --> R-H + HO-MgBr. PM me if you don't understand this completely...I'm a Chem Bio major at Cal

Perfect, that's great. That's a characteristic for Grignards too. 🙂 Thanks!

 

[JCase]

Originally posted by: Tiamat
Oh god, i hated all 2 semesters of Organic chemstry. Pchem is slightly better, but isnt that great either...

Pchem better? You've got to be kidding me. I'm in second semester pchem (thermo and kinetics, first was quantum), and I hate it with a passion. We sit there trying to follow ridiculous derivations, meanwhile neglecting 90% of the actual interactions taking place. It really seems like a joke to me, especially compared to the usefulness and practicality of O Chem. But hey, there's always some ppl who prefer pchem I guess...they're usually physicists trapped in the chemistry department 🙂

 

[jst0ney]

ahhhhhhhhhhh bringing back OChem nightmares!!!!

 

[Siva]

Originally posted by: JCase
Benzene (or whatever R group is attached to the Mg) will be formed when water is present, which is why you must keep a gringard DRY. Rather than proceeding through attack of your carbonyl, which is the desired reaction, the benzene ring will attack any water molecules around, grabbing a proton and kicking off the MgBr. Here's a quick schematic for this side reaction: R-MgBr + H-OH --> R-H + HO-MgBr. PM me if you don't understand this completely...I'm a Chem Bio major at Cal

perfect answer

If you ever have to prepare/use a grignard reagent in lab, you'll see that its absolute crucial to use dried glass for the reactions and anhydrous ether as a solvent. The H in water will instantly replace the grignard reagent if they mix.