[OpaquePlanet]

I am many years out of university, but this question has been haunting me.

Back when I took Organic Chemistry, we did synthesized Benzoic Acid from Bromobenzene to show the Grignard Reaction.  Although I followed the lab instructions as well as ever, my reaction turned blood red (IIRC, this was while it was being boiled in the distillation flask, while attached to the condenser tube).  It took a long time to change back to clear, and longer than anyone else's to distill.  I don't know if that has to do with the red colour or not.

I ended up failing that class for other reasons, so when I took it again the next year I made absolutely certain I followed the instructions to a tee.  The red colour appeared again, and again I was the only one.  At this point I was ready to ask the TA to induct me into the secret order of alchemists, because surely turning Bromobenzene into Blood was a fabulous accomplishment.  None of the TAs could explain it either year, and my professor was sure I'd messed something up terribly (even though my yield was okay and my melting point spot-on).

Since this was years ago, I don't have my lab books handy, but if need be I can try to find them to get the exact steps of the Lab (reagents, etc).  Basically I just want some rational explanation for the blood-red colour in an procedure that was supposed to be colourless the whole way through.  Some kind of reassurance that I'm not crazy or possessed.

[discodermolide]

I am many years out of university, but this question has been haunting me.

Back when I took Organic Chemistry, we did synthesized Benzoic Acid from Bromobenzene to show the Grignard Reaction.  Although I followed the lab instructions as well as ever, my reaction turned blood red (IIRC, this was while it was being boiled in the distillation flask, while attached to the condenser tube).  It took a long time to change back to clear, and longer than anyone else's to distill.  I don't know if that has to do with the red colour or not.

I ended up failing that class for other reasons, so when I took it again the next year I made absolutely certain I followed the instructions to a tee.  The red colour appeared again, and again I was the only one.  At this point I was ready to ask the TA to induct me into the secret order of alchemists, because surely turning Bromobenzene into Blood was a fabulous accomplishment.  None of the TAs could explain it either year, and my professor was sure I'd messed something up terribly (even though my yield was okay and my melting point spot-on).

Since this was years ago, I don't have my lab books handy, but if need be I can try to find them to get the exact steps of the Lab (reagents, etc).  Basically I just want some rational explanation for the blood-red colour in an procedure that was supposed to be colourless the whole way through.  Some kind of reassurance that I'm not crazy or possessed.

Traces of bromine?

[orgopete]

I can only guess on this one, but given that this was a student prep, I'd have to think about what happens in these labs. I would guess that it was a contaminant from an earlier reaction. A small amount of compound can produce a lot of color. If water is the solvent of choice for cleaning glassware, I suspect a residue. 

[OpaquePlanet]

Thanks dudes/ladies.

I was wondering about trace contaminants from prior reactions.  We washed our glassware with water and rinsed with acetone.  I was always pretty fastidious about cleanup (I love the smell of acetone), but some of my classmates were not.

Wow, yes, just looked it up and bromine is just that blood-red colour.  Fabulous.

[Nosterius]

However, bromine would probably react with the magnesium metal to form MgBr2.

I think residual contamination would be a better bet. Compound such as 2,2'-bipyridine react with Grignard reagents to form a red charge transfer complex.

[discodermolide]

However, bromine would probably react with the magnesium metal to form MgBr2.

I think residual contamination would be a better bet. Compound such as 2,2'-bipyridine react with Grignard reagents to form a red charge transfer complex.

It only takes traces of bromine to produce the red coloration.

[Nosterius]

Indeed, but I have trouble imagining how a single student, using probably a shared bottle of bromobenzene could produce bromine (mechanistically speaking) in reductive conditions. I would expect the red color to disappear almost instantly.

One hypothesis for the contamination; most student clean very well their round bottomed flask... but some forget to clean adequatly the condenser. Otherwise, could Grignard reagents react with oxygen to form colored species? If the setup was not properly purged with nitrogen...

[discodermolide]

Indeed, but I have trouble imagining how a single student, using probably a shared bottle of bromobenzene could produce bromine (mechanistically speaking) in reductive conditions. I would expect the red color to disappear almost instantly.

One hypothesis for the contamination; most student clean very well their round bottomed flask... but some forget to clean adequatly the condenser. Otherwise, could Grignard reagents react with oxygen to form colored species? If the setup was not properly purged with nitrogen...

The original poster said he repeated the same experiment a year apart from the first one. It is unlikely that he would get a set of glassware with the same impurities which caused a red coloration. And he said his yield was ok and the MPt. good.
I still say the red color is due to traces of bromine.

[OpaquePlanet]

The original poster said he repeated the same experiment a year apart from the first one. It is unlikely that he would get a set of glassware with the same impurities which caused a red coloration. And he said his yield was ok and the MPt. good.
I still say the red color is due to traces of bromine.

She, not he, but thanks.     I'm inclined to think it was Bromine too, now that it's been pointed out to me.  The hue is spot on.

student clean very well their round bottomed flask... but some forget to clean adequatly the condenser.

This was the boiling flask before the condenser; I don't see how a contaminant in the condenser could affect the colour before it even gets there.

[discodermolide]

She, not he, but thanks.     I'm inclined to think it was Bromine too, now that it's been pointed out to me.  The hue is spot on.

[/quote]

I apologise to the "she" for being relegated to the ranks of the "he".