[Fmeub]

I had to prepare a "starter" Grignard reagent by adding a few ml of a mixture of 2-Phenylethyl bromide and anhydrous ether to some magnesium turnings in a testtube. The turnings then had to be grounded using a glass rod until the reaction started.

The glass rod must have been contaminated with something, because the mixture became a bright red colour. I left the testtube in a corner of the fumehood and continued with the rest of the experiment so that I wouldn't lose too much time.
A little later I saw that the colour in the testtube changed from a bright red to a clear light pink and it seemed as if a thin layer (almost like ice) had formed on the surface of the mixture in the testtube.

I am very inexperienced and this happened a few weeks ago so I can't remember much more than what I have mentioned above.

Does anyone maybe have a clue as to what could have been on the glass rod to cause this change in colour?

[russellm72]

Hi.

Could of you perhaps of had some iodine on your glass rod or more likely a dirty test tube? Normally grignards are better made under anhydrous conditions under an inert atmosphere so i am surprised your using a test tube albeit the one you are making is quite easy to generate. I am assuming this is some kind of School or University experiment when the cost of equipment dictates the experimental design.

If you have to use a test tube try oven drying it first and perhaps try the reaction under a positive flow of nitrogen e.g. easiest in your case an upturned funnel over the top. If possible you'd be better sonicating the test tube and contents as it's a good way to start off grignards but i'm concerned at the lack of control in your experiment. If you are making a decent ammount it could just take off! Normally one adds the alkyl halide initially at first to commence reaction then add the rest at a rate to cause a gentle reflux. Remember ether is very volatile and you may lose it out of your test tube.

Good luck

R.

[Fmeub]

Hello!

The mixture in the testtube was just the "starter". The apparatus was set up for  refluxing, and as soon as the reaction started in the testtube, it was supposed to be added rapidly to the rest of the turnings in a flask assembled in the reflux apparatus. I'm sorry if this is a bit confusing. The reflux apparatus was dried, but not the glass rod. I can really not remember whether the testtube was dried or not.

I made another "starter" reaction after the first one failed, (or changed colour) and the rest of the experiment worked (and I survived!).

I can understand your concern for the lack of control, but I just left out some of the details. We are a large group, and so your assumption about the cost of equipment that dictates the experimental design is correct.

Would iodine therefore be responsible for the change in colour?   

[kiwi]

No, iodine in grignards gives a cloudy green-grey colour (iodine is often used to corrode the surface of the Mg in order to activate it - just one of the methods of grignard witchcraft).
Many compounds give bright colours with organometallics, either via charge-transfer complexes (eg. organolithiums and 2,2-bipyridyl) or deprotonation to a coloured anion (eg. double deprotonation of diphenylacetic acid). So exactly what was in your tube is hard to say.