[kamiyu]

I have been recently doing bromomethylation on 1,3,5-triethylbenzene and I experience a problem. Here are the detials:

1. All the literature use the key reagent which is 33% HBr in HOAc. But because my lab (even the whole department) does not have this and I do not want to wait for shipping, I try the following recipe, after considering the mechanism of the reaction (which should be very simple): paraformaldehyde, KBr, HOAc as solvent and H2SO4 as acid catalyst and reagent. Heat at 90oC for overnight.

2. The result is that I did obtain products but it is always contaminated by another thing which has 7.00ppm, 4.67ppm (same as ArCH2-Br) and a slight lower ppm of CH2CH3 compared with my desired product. This impurity cannot be removed by recrystallization and column chromatography (one spot)

This makes me even wonder I obtained a single product or mixture. But it is too hard to explain if it is a single pure substance.

[discodermolide]

What is the yield?
Why not check the purity by GC?

[kamiyu]

1) The yield is resonable (60-70%)

2) GC has too high temp the product cannot survive. I would suggest HPLC instead.

[discodermolide]

If you are getting 60-70% what is the rest of the material?
Why do you think this compound will not go through the GC?
If you suggest HPLC why have you not used it?

[kamiyu]

1. Please understand that it is always not easy to spot the every product of a reaction.

2. The molecular weight is above 400 and may have too high b.p.

3. I am going to do HPLC.

[discodermolide]

Finding and separating the products of a reaction are really what the science is all about.
Surely the bromethyl derivative of 1,3,5 triethyl benzene should be quite a stable compound, so it should go through the GC.
But HPLC as also ok.

[kamiyu]

I am sorry I cannot agree with you because I think your comment that separating all products of a reaction is something too idealistic/theoretical.

I have looked at your research profile and I think you are a synthetic chemist. It really suprises me when you say one should check all the products. Probably you are referring to brand new reactions.

Please note that the bromomethylation is an old reaction. Finding out all the products can be done, but what is the point of doing so, for me??

As I mentioned in my post, I struggle with the unknown impurity that always come with my product.

[kamiyu]

I am sorry to say that benzyl bromide is an unstable compound, for its ability to carry out hydrolysis.

Indeed, benzyl bromide has been used as chemical weapon. When benzyl bromide reaches our respiratory tract, it reacts with water to release HBr and this is what something hurts.

[discodermolide]

Sorry but separating the products of a reaction is a fundamental part of an organic chemistry experiment, it is neither idealistic or theoretical. It needs to be done.
The point of finding the products for you is to help you identify the unknown material in your product, try GC/MS.
If the impurity is unknown then you need to find out what it is then perhaps you can devise reaction conditions that will avoid its formation.
Is you starting material pure?

[discodermolide]

I am sorry to say that benzyl bromide is an unstable compound, for its ability to carry out hydrolysis.

Indeed, benzyl bromide has been used as chemical weapon. When benzyl bromide reaches our respiratory tract, it reacts with water to release HBr and this is what something hurts.

Please tell me how HBr is released from a benzyl bromide.
They are not pleasant compounds, I know that but you are in a lab environment so exposure must be limited.

[kamiyu]

Yes, I made the 1,3,5-triethylbenzene by FC alkylation. Very easy. One trial, success.

[discodermolide]

What was the yield and purity of the FC product?