[ptb969]

Hallo,
i am studying the kinetics of the SN2 reactions of some alkyl bromide.
When we perform the reactions of alkyl bromide with neutral nucleophiles such as amines, we can follow the course of the reaction by the conductrometry method which shows the increase of the conductance of the bromide ion.
However, when we carry out the reaction with some ionic nucleophiles like the methoxide ion, it turns out problematic. The presence of the methoxide ion may interfere the determination of bromide ion quantitatively.
I am thinking of using a selective indicator which may form a colored complex with bromide ion, this way it may be possible to use UV method to follow the reaction process. However, I still have no idea what should be a proper indicator for the bromide ion.
Do you have any suggestion in this aspect? or any advice concerning the way to perform a kinetic study of the reaction in order to determine the rate of the reaction.
I look forward to discussing with you.
Sincerely,
PTB969

[HP]

Where methoxide ion come from ?

[lavoisier]

I have a couple of suggestions.

1. the old-fashioned approach, suitable for people with little resources. You could take samples of the reaction mixture at appropriate intervals, acidify with nitric acid to stop the reaction, remove the alkyl bromide by extraction (with ether?) and titrate Br- with silver nitrate.

2. use an ion-selective electrode for Br- (yes, it exists, I cheched on my old textbook of analytical chemistry!), unless it's affected by your other reagents/solvents.

[Borek]

2. use an ion-selective electrode for Br- (yes, it exists, I cheched on my old textbook of analytical chemistry!), unless it's affected by your other reagents/solvents.

These are mostly based on AgBr and their behavior in the non-water solution can be unpredictable. But you are right that they are worth of trying, as this will be very simple solution.

[ptb969]

Hi there again,
thank you very much for your concerns. Here are my replies to you:
To HP: the methoxide ion is supposed to be used as a nucleophile in the reaction with the benzhydyl bromide. We would like to determine the rate of this reaction in DMSO-MeOH.
To Lavoisier and Borek: I appreciate your suggestions very much. I have thought of the titration, this seems to be an old-fashioned  method but it might help. However, if we have to do the termination of the reaction and subsequently some extraction, I would rather use GC method to determine the concentration of the benzhydryl bromide.
I think ion-selective electrode for bromide is the best methode of choice. Nevertheless, when we consult the suppliers, they can only guarantee a appropriate performance of the electrode in aqueous
solution not in any organic solvents like DMSO.

[HP]

I am not very competent in analytical chemistry but think about possible using simple conductometry in reaction environment to determine Br- as function of  media conductivity.

[Borek]

I am not very competent in analytical chemistry but think about possible using simple conductometry in reaction environment to determine Br- as function of  media conductivity.

Can be hard if the solution contains other ions in excess.

[HP]

Aha like the mysterious methoxide one

[ptb969]

Yes, I agree with you that it would be hard because there is not only the bromide ion which is released, but also the methoxide ion which is used as an in-excess-nucleophile. The idea of using an ion-selective electrode still has no promise.
That is why I am thinking of using UV. What would you think if I would use an indicator for a base (methoxide) like phenolphtalein or bromothymol blue and follow the decay of the colored conjugate base of the indicators?
Best regards to all of you.
PTB

[Dude]

Can you monitor the disappearance of the C-Br bond by IR or the appearance of the C-N bond by IR?  Are the absorptions too weak?  There are many applications of IR in kinetic studies and the measurement time is very quick.