Effectiveness of Grignard reagent

Question

So for my experiment, I am synthesizing grignard's reagent (phenylmagnesium bromide). I am trying to change the dryness of ether and see if there is a change in products. Is there a simple method to quantify the results? I need to figure out the effectiveness of the product. If water is introduced, the reaction will probably form alkanes and magnesium hydroxide rather than phenylmagnesium bromide. It has to be a simple method since I am working in a school lab.

Answer 1

For a grignard reaction to work, the ether must be free from water. This is usually achieved via distillation over sodium/potassium using benzophenone as an indicator. As you are in a school lab, the solvent provided to you is likely already dried/dry enough for purpose.

If the solvent is a little wet, two things will happen:

Firstly, it is more difficult to get the grignard formation to 'initiate' as the water effectively quenches the reagent as it forms. However, with persistence it is usually possible to get them to initiate when making simple grignards such as PhMgBr.

The other issue however, is that as the water is quenching any grignard reagent formed until all the water is used up, you will make less reagent than you think you are (in this sense the reaction is self-drying) meaning if you do think your ether is wet, you should aim to make a greater quantity of reagent than required.

It is possible to titrate a grignard reagent to identify the actual concentration of solution made, although in an undergraduate practical it is likely that you will use the reagent in excess anyway.

In terms of identifying your product, you should carry out the reaction, work it up, purify, and use a technique such as NMR to see if your product has formed. If this is unavailable, simple IR spectroscopy can help you determine if the functional groups in your starting materials have changed (e.g. Carbonyl to alcohol).

Good luck with your reactions