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  •  » t-butanol with NH4X to isobutylene and t-butyl halides

#1 2008-11-13 01:50:21

orgopete
Administrator

t-butanol with NH4X to isobutylene and t-butyl halides

Q: the experiment is about competing nucleophiles (chlorine and bromine ions) with different alcohols (in this particular case, t-butanol). the solvent -nucleophile mixture (containing the ammonium halides and concentrated sulfuric acid) was refluxed/heated for more than a hour and then purified. t-butanol was added to the to this mixture. the organic layer was extracted and then analyzed via gas chromatography. the link below is a picture of the gas chromatograph that was obtained in the experiment.

          http://www.curvedarrow.com/chem/tbutanol_gc1.jpg

i would appreciate it if you can explain this graph to me and its significance with which one is a better nucleophile. in the other part of this experiment, i concluded that bromine was a better nucleophile in the reaction with the ammonium halides and 1-butanol. here is the gc analysis for that reaction.

          http://www.curvedarrow.com/chem/butanol_gc1.jpg

A: No disrespect to the questioner, but this is really pushing it. I have a lot of experience and know a lot of reactions, but it can be pretty difficult to deduce these problems from this distance, what was given, real procedure, data, analysis, hints, etc. For example, in the first problem, I presume the t-butanol was added first to the mixture and then heated.

However, I was able to find this experiment described on a couple of websites. They indicate the bromide and chloride products are to form. From the questioner, I must presume that isobutylene is the first peak, t-butyl chloride and t-butyl bromide as the next peaks. The bromide is the highest boiling and therefore longest retention time.

In the lower gc, I am guessing that we are seeing the large peak for the injection followed by 1-chlorobutane and 1-bromobutane.

Let us start with the lower example, the reaction of 1-butanol. Since the alcohol is primary, this is going to be an SN2 reaction. Assuming equal amounts of bromide and chloride present in the transition state, since more bromide product is formed, bromide must be the better nucleophile.

In the upper example, the reaction of t-butanol, because the rate limiting step is loss of water, the nucleophilicity should not matter and one might have expected relatively equal amounts of chloride and bromide (reflecting equal concentrations). The tailing of the peaks on the gc's make it difficult to determine an accurate integral from these gc's.

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#2 2009-06-19 09:20:22

wattonstp
Member

Re: t-butanol with NH4X to isobutylene and t-butyl halides

We do this experiment in organic labs here at CCSU, and the results my students obtained varied wildly.  Sometimes the chloride peak would dominate, and sometimes the bromide peak - up to 80:20 ratios in either direction.  Doesn't make any sense to me, unless we have cross-contaminated reagent bottles (which I have to check).  We use t-butanol, which is tertiary, and is thus expected to go by an Sn1 mechanism, so there should be a 50:50 mixture of the two products, but we only saw that result in a minority of cases.  Very odd...I just don't get it!  Will have to go back into the lab myself and see what happens when I do it. 

Nice site, by the way... looks like it will be very helpful!  Steve

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#3 2009-06-19 10:51:14

orgopete
Administrator

Re: t-butanol with NH4X to isobutylene and t-butyl halides

I really don't know what individual students have done in the experiment. In the absence of the procedure, and what students actually did, I can only guess. Based upon the description, I can imagine the broad results by changing the procedure slightly.

Student A: Adds ammonium bromide, sulfuric acid, t-butanol, and ammonium chloride in that order and heats. Major pdt the bromide.
Student B: Adds ammonium chloride, sulfuric acid, t-butanol, and ammonium bromide in that order and heats. Major pdt the chloride.
Student C: Adds ammonium chloride, ammonium bromide,  t-butanol, and sulfuric acid, in that order and heats. Major pdt equal bromide and chloride.

I didn't look up the procedure being done. I just read the student contribution that I originally replied to. It appears to be the same procedure for the SN1 and SN2 reactions. There appears to be an implication that both reactions should occur at the same or similar rates. That is patently incorrect. The SN2 reaction should be much slower than the SN1 reaction. I believe the SN1 reaction will begin upon addition of the reagents. Therefore, I described experiments where the carbocation can form with high concentrations of a single halogen.

While contaminated reagent bottles could be possible, I am doubtful. This will depend on the number of reagent bottles available to the class. Let us assume one reagent is contaminated. All experiments from that bottle should give a single result. I have not seen the overall class results, but it sounds as though there is a wider range of student results than can reasonably be explained even by numerous examples of contaminated reagents.

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