[jena]

Hi,

My questions are:

Does cyclohexene with NBS, light, and tetrachloride produce anion or a cation?
My answer: It produces anion because of the NBS

Does 3-bromocyclohexene with water and heat produce anion or a cation?
My answer: It produces a cation because the of th water.  

Does 3-bromocyclohexene with magnesium and water produce anion, cation, or radical?
My answer: It produces a radical.

Are these correct?

Thank You

[movies]

When a reaction involves light it is usually indicative of a particular type of mechanism.  Do you know what kind?

In the second question if you produce a cation you also have to produce an anion.  It's a law of physics.  The question doesn't seem to make sense.

[jena]

1. For the the first question, I don't know if this helps any but the reaction produces 3- bromocyclohexene. Anyways getting back to the point,  I was looking over my book and it referred to this type of reaction as a radical allyic halogenation. So could it possibly be a radical.

2. For the second question, would have to add the water where the bromine is and when i add the heat wouldn't it formulate a anion intermediate. I'm not sure.

Are these correct assumptions? Also is question three alright or should I look over it again?

[arnyk]

For the first one, when you see light it usually means UV light which is a key catalyst for substitution reactions creating akyl halides.

As for the second I think it may be referring to the hydrocarbon itself whether it becomes a cation or anion.

[Donaldson Tan]

light + NBS = radical mechanism

NBS is a good initiator for free radical mechanism.

[movies]

Yes, radical for the first one, almost certainly.

I am still a little confused by the second one, as I said you can't get a cation without having an anion somewhere, so it depends on specifically what the question refers to as a "cation" or "anion."

The third one looks fine.  Oxidative addition of Mg into C-Br bonds is thought to go through a radical mechanism.

[arnyk]

For the second one, if they are asking whether the hydrocarbon itself becomes a cation or an anion, I would say anion.  Most likely it would lose an H+ in the process.

Of course if it loses the Br- then it'd be a cation...lol one of those is bound to be right.  

[movies]

Which is more stable, an allylic cation with a bromide counter ion or an allylic anion with an H₃O⁺ counter ion?

[arnyk]

As an educated guess, I would go with the hydronium counter as the more stable?  Guess I'm about to learn something new.  

[movies]

I think it's probably the other way abound.  The pKa difference between a carbanion (pKa ~40) and hydronium (pKa ~ -2) is pretty enormous.  So it'd be very tough to do.

[arnyk]

So both could and would occur, but the equilibrium would shift towards the more stable condition?

[movies]

Well, think of the other way, with water attacking the allylic bromide via an S_N2 or S_N2' attack mechanism.  The immediate product would be a protonated allylic alcohol.  The pKa's for a protonated alcohol and HBr are -2 and -7, respectively.  So in that case, the immediate product at least makes sense in terms of what is protonated and what is not.

You can draw both mechanisms, and maybe both occur to some extent, but the carbanion mechanism would have a titanic activation energy because you would have such a poor equilibrium with a pKa difference of ~42 units in the wrong direction.