[ptb969]

Hallo,
I have been working with benzhydryl bromide for quite a while.
Since the study depends strongly on the purity of the compound (you may have read some of my posts concerning the study of the kinetics of the reactions of benzhydryl bromide), I am looking for a way to purify benzhydryl bromide.
It is usually contaminated with benzhydrol which has similar b.p therefore the distillation turns out problematic.
I am thinking of recrystalization but I am not quite sure whether it helps because benzhydryl bromide has a low m.p and it is well soluble in aprotic solvent. Using protic solvents may not be a good choice since benzhydryl bromide solvolises quite fast in these media.
Anyone has a better idea? I would appreciate very much your suggestion.
Thank you.
PTB

[russellm72]

Hi,

What kind of quantity do you have? Have you considered HPLC or flash chromatography? Clearly they should be easily seperable if stable? I'm guessing it goes off in the presence of light? Not much experience in these compounds but just a suggestion.

R.

[ptb969]

Hi,
well, I have about 100g so I think it is enough for HPLC or flash chromatography. Nevertheless, benzhydrylbromide is quite sensitive to humidity so that in a commerciably bought bottle there is always some benzhydrol as a result of the contact of benzhydryl bromide with water. So I do not know whether HPLC or flash chromatography may help.
I am now thinking of adding some HBr/MeCOBr. HBr will convert the remaining benzhydrol into benzhydryl bromide, MeCOBr helps take the released water and subsequently doing distillation. Do you think it may work?
By the way, could you tell me in more details of the so-called flash chromatography since I have never heard about it.
Thank you for discussion.
PTB969

[russellm72]

Hi,

Firstly if you have about 100g HPLC is a no go as it will take you ages. Flash chromatography is possibly probably in two batches but if you dont know what it is you probably best not to tackle. I pressume you know about TLC? If you product and the benzhydrol are easily separated on TLC it could be a good idea to try a silica plug. You need a system that will leave the benzhydrol on the baseline but will elute your product.

On your scale recrystallisation is optimal but I understand the problems associated with low melting solids. Have you got access to Beilstein commander or the Merck Index? Could be useful for giving you a suitable solvent to recrystallise from. DONT WASTE LOADS OF TIME IF SOMEONE HAS PURIFIED BEFORE! Massive ammount of literature out there.

R.

[russellm72]

In addition. Never tackle purification of all your material unless you know it works on a small scale.

As for your idea with HBr/acetyl bromide.... In theory HBr would convert the alcohol into the bromide producing water as by-product but acetyl bromide is nasty stuff. Can you not dry your product first in vauco over phosphorous pentoxide or spin it down a few times from toluene which is great at removing water?

If it were me I'd do a small scale silica plug and isolate the pure bromide. Remember to keep under N2 and protected from light using foil. Then you can get a recovery and a purity figure by HPLC.

R.

[ptb969]

Hi there,
thank you for the suggestions.
You are right.  If I put attempt to purify 100 gr of my compound by HPLC, it would take my ages  .
Now I am preparing for my presentation for a meeting in 2 weeks. After that, I will come back to my experimental work.
I will try your advices, either by column or by recrystalisation, or by distillation.
I do hope one of the ideas works.
I will inform you when I see you in the forum.
My best regards,
PTB969