Orangic Chemists in the House?

October 30, 2008 4:15 PM

Anyone who knows Organic Chemistry? Can you help me with one reaction mechanism?

So I've probably just been studying too long but I promised myself if I got through question 7 I could stop and take a break and finish studying tomorrow. Unfortunately I just can't figure out the mechanism for question 7 and it is driving me nuts. I feel like it should be obvious but apparently it isn't.

How do I make this...
C
|
C = C - C - C

into this...
C
|
C - C - C - C
| |
Cl Br

Ok, I see in the preview that it isn't obeying my spacing and I don't know how to make it to that so I'll just explain.

The top carbon should be able the third carbon in the chain. The chlorine should be under the second carbon and the bromine should be under the third carbon.

Thank you in advance!!!

posted by magnetsphere to science & nature (11 answers total)

Are you looking for a reaction mechanism or a synthetic pathway? It's hard to write a mechanism when you don't know what reagents you are using.

Here are the reactions you'll need in order. They should be in your text book. Of course, there may be more than one correct answer!

1) Allylic NBS bromination will brominate the tertiary carbon via a radical mechanism.
2) Oxymercuration will put an alcohol on the 2nd carbon (the more substituted carbon/see Markovnikov's rule).
3) Halogenate with PCl5 or thionyl chloride (SOCl2) to convert the alcohol into a chlorine.

Hope that helps. If you're looking for a mechanism, that might be tough for me to type!
posted by hooper4 at 6:01 PM on October 30, 2008

I think you've misread the (admittedly confusing) structure, hooper; there isn't a tertiary carbon, nor a Markovnikov/anti-Mar. product (both sides being equally substituted).

If this is a sophomore-type organic class, then the answer is probably something simple like Br₂ (or NBS, preferably) and a chloride source (Et₄NCl, maybe), or some set of conditions very much like that. If higher level, then you're going to have to address regioselectivity, which is pretty non-trivial here since it's a nigh-symmetric alkene.
posted by Dr.Enormous at 8:36 PM on October 30, 2008

Do you want the reaction mechanism for the transformation of 3-methyl-1-butene into 2-chloro-3-bromo-3-methylbutane? Or is it a different substitution pattern?
posted by koolkat at 8:38 PM on October 30, 2008

Unless I'm getting a different display, this is just 2-pentene as the S.M.

For mechanism, you're simply forming a bromonium ion with some source of Br⁺, which is getting opened by nucleophilic attack of Cl^-. There are any number of reagent combinations that will effect this (BrCl would be the simplest conceptually, but Br₂Cl^- and BrCl₂^- are known). Obviously this can be opened on either side, so you'll also get 2-bromo-3-chloropentane, but solving that problem is difficult.
posted by Dr.Enormous at 8:54 PM on October 30, 2008

No the OP is having issues with spacing which is why they should have used standard line notation, or used IUPAC names. If they would write it correctly it should be easy, and I am guessing that you answer would be correct.

OP please write it more clearly like in my example with an IUPAC name or line notation like the following for 3-methyl-1-butene

CH2=CHCH2(CH3)CH3
posted by koolkat at 9:06 PM on October 30, 2008

I am guessing that the OP is already asleep and I am heading to bed now as well so I can check back in the morning, but it is unlikely to be of any great help in finishing the homework tonight.
posted by koolkat at 9:07 PM on October 30, 2008

For future reference:

To make rows of text line up, use the <pre> tag:

<pre>
1 2 3 4
5 6 7 8
</pre>
posted by exphysicist345 at 9:12 PM on October 30, 2008

Oh, from his explanation below his structure, I interpreted "The top carbon should be able the third carbon in the chain" as "The top carbon should be above the third carbon in the chain", which would make the third carbon tertiary. I doubt the question is looking for BrCl as an answer... not a very common sophomore organic reagent...
posted by hooper4 at 9:14 PM on October 30, 2008

yeah, what koolkat said!
posted by hooper4 at 9:15 PM on October 30, 2008

Ah, yeah, my late night drunken typo parsing subroutine is on the fritz, apparently. In which case I concur that it does need some sort of allylic oxidation, be it bromination directly or SeO2 and then interconversion.
posted by Dr.Enormous at 9:57 PM on October 30, 2008

yes koolkat is correct with the starting and the ending materials (the transformation of 3-methyl-1-butene into 2-chloro-3-bromo-3-methylbutane). This is for OChem I so I know the answer has to be something that isn't too difficult or involved and the mechanism probably isn't more than 3ish steps.

The only types of reactions that we really talked about in depth in lecture are either through a carbo cation or through the use of peroxides/free radicals but using either of those I can't get the Bromine and Chlorine to end up in the correct spots. I'm probably just missing one little thing.

Sorry, I was sleepy and annoyed last night or I would have used IUPAC names originally.
posted by magnetsphere at 6:25 AM on October 31, 2008