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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rakkoonn on May 28, 2007, 05:29:34 AM

Title: Mechanism of cyclohexanediol syntheis
Post by: Rakkoonn on May 28, 2007, 05:29:34 AM
What is the mechanism of this reaction?

(http://i38.photobucket.com/albums/e129/SowaSowa/Reakcja.jpg)

I think H2O2 contributes to creating some kind of peroxide but I have no idea what can the next steps look like
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: Sandmeyer on May 28, 2007, 06:04:46 AM
I usually see the hydroxilation of a double bond with OsO4 (syn addition if I remember well).

In this case, I suggest :

HCOOH + H2O2 ---> HCO3H and this peroxyacid (performic) epoxidizes the double bond to form an oxirane.

Then, acid aperture of the epoxide to give the diol...

Title: Re: Mechanism of cyclohexanediol syntheis
Post by: Rakkoonn on May 28, 2007, 08:51:12 AM
Ok, thanks. So it goes like this (skipping the obvious peroxide creation):

(http://i38.photobucket.com/albums/e129/SowaSowa/diol.gif)
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: english on May 28, 2007, 11:51:06 AM
The reagent indicated in the problem introduction is not a peracid (RCOOOH).  Make note of this.

The terminal O of the O-O bond is electrophilic.  You're attacking the wrong oxygen.


Protonating the epoxide is unreasonable for two reasons: (1) Your epoxide has sufficient strain to undergo substitution without further weakening the R-O bond, (2) the step in which you protonated your epoxide is under polar basic conditions.

Only weakly acidic species can exist under polar basic conditions (e.g. RCOOH, H2O, H2S, etc.) .  R2OH+ is too acidic for this medium.

Any aqeuous workup will be done after substitution in a separate step with a strong source of H+; or more appropriately, in situ with a source of H+.
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: kiwi on May 28, 2007, 01:10:10 PM
other than the obvious mistake of the nucleophile in ring opening (it will be water, not OH-), i can't see too much wrong with the reaction as given. Water will not be polar enough to pop open the epoxide without a little help, provided by protonating the epoxide oxygen.
Bear in mind pKa(formic acid): 3.6; pKa(performic acid): 7.1. Formic acid will be strong enough to produce small quantities of the protonated epoxide intermediate.
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: english on May 28, 2007, 02:04:16 PM
Water will not be polar enough to pop open the epoxide without a little help, provided by protonating the epoxide oxygen.

Yes but as you said, he's showing basic conditions, which are contrary to his initial conditions.

HO can be used, just not under his conditions.
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: Rakkoonn on May 29, 2007, 03:42:42 AM
So any chance of drawing a proper one? The reaction with peroxide is based on this:

(http://www.cem.msu.edu/~reusch/VirtualText/Images2/epoxide1.gif)

I found it at "Virtual Textbook of Organic Chemistry"
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: english on May 29, 2007, 08:13:07 AM
Looks OK to me.   ;)

Epoxidation is best understood as a concerted reaction as well,
Title: Re: Mechanism of cyclohexanediol syntheis
Post by: Rakkoonn on May 29, 2007, 10:09:30 AM
Thans a lot for the help to all three of you