Sciencemadness Discussion Board

Benzyl Bromide Synthesis

Damitwhy - 23-1-2014 at 18:49

Strange Out Come! ?!?

After making 200ml of 48% Hydrobromic Acid via Sodium Bromide and Sulfuric Acid and redistilling twice to constant boiling temp of round 123-124°C, I decided to perform a Benzyl Bromide synthesis using 202g of previously mentioned HBr Acid and 100g Benzyl Alcohol... I set up for reflux and first added Hydrobromic Acid to flask then I added all of Benzyl Alcohol and almost instantly the reaction mixture separated into two phases but quite unexpectedly both phases turned dark amber in colour which threw me little... I continued stirring with magnetic stirrer under fume hood for a few minutes while I did a wiki search to find what colour Benzyl Bromide should be... after discovering it should be colourless I came back turned off stirrer to see how well the phases would separate again only now the top phase (Benzyl Bromide ) was a dark amber and the bottom phase (Hydrobromic Acid) was now a milky white to clear transparent... I turned on the heat and stirrer and continued to heat reaction mixture to 85°C for 20 minutes to be sure of completion...

After cooling to room temp I used separation column to take off bottom acid layer and washed top product layer with first wash of distilled water and found the heavier Benzyl Bromide lost Amber colour and the now top water layer only had a slight off yellow tint... continued to wash bottom Benzyl Bromide layer twice more with distilled water and then dried over Magnesium Sulphate to be left with clear transparent liquid which truly is a pungent tear producing lacrimator...

My question is, why had the reaction mixture turned dark amber to begin with? is it normal? Could it be free radical Bromine formation? if not it could of been a result of purchasing the Benzyl Alcohol from ebay, would be interesting to know for sure...

Dr.Bob - 24-1-2014 at 08:13

Often bromide reactions will produce some free bromine, which will quickly color the reaction. If it is just some free bromine, then a wash with aqueous thiosulfate solution (or a similar reducing agent) would remove the bromine coloration. I would do that after the water wash, then dry it, and if you are really concerned, distill the benzyl bromide, but it may not be stable to high temps. You also may want to store the benzyl bromide over copper wire, K2CO3, or some other HBr scavenger, as that will help to keep it stable in storage. This can happen even with very pure reagents, although less pure ones will likely be worse.

Damitwhy - 24-1-2014 at 09:03

Quote: Originally posted by Dr.Bob  
Often bromide reactions will produce some free bromine, which will quickly color the reaction. If it is just some free bromine, then a wash with aqueous thiosulfate solution (or a similar reducing agent) would remove the bromine coloration. I would do that after the water wash, then dry it, and if you are really concerned, distill the benzyl bromide, but it may not be stable to high temps. You also may want to store the benzyl bromide over copper wire, K2CO3, or some other HBr scavenger, as that will help to keep it stable in storage. This can happen even with very pure reagents, although less pure ones will likely be worse.


Thank You Dr.Bob ;-) I'll use your suggestions to help keep the product stable and maybe set up for vacuum distillation for the next round of experimentation :-D