Sciencemadness Discussion Board

UV wavelength

greenyppols - 4-2-2006 at 04:25

As dumb as this sounds, I figure the truly dumb question is the one not asked.

Does anyone know if there is a "preference" of UV wavelength when chlorinating toluene? I noticed UV sources had some different wavelengths and this prompted my question. If it doesnt matter, it doesnt matter BUT if a certain UV wavelength was, say 40%, faster than another different UV wavelength then it would make a significant difference so thats why I ask.

I noted the points to stop at during chlorination, either "weightwise" or the "finishing temp" but this was just a curiousity question if anyone knew. I couldnt/didnt find any info concerning this but I may have used the wrong search terms, or missed it completely somewhere.

Thanks all

garage chemist - 4-2-2006 at 04:50

I used a 500 watt halogen floodlight with removed front glass (normally filters out UV, which is undesired here) 50cm in front of the flask.
All chlorine reacted, not the tiniest bit escaped. Only HCl fumes came out of the dimroth condenser.

The boiling point (mixture constantly heated to reflux from below, thermometer installed inside flask) climbed slowly but steadily and I stopped after it reached the temperature given in literature (stopping earlier gives less byproducts but lower yields calculated from toluene).
Fractionation gave a good yield of benzyl chloride.

Don't bother with UV light sources (way too expensive anyway), just use a bright ordinary light source with UV protection removed.

To answer your question: There is a wavelight preference which is the same for all organic chlorinations with elemental chlorine. It is the wavelength at which chlorine absorbs, since the light serves to cleave the chlorine molecules, giving radicals.
This should help in searching for it.

[Edited on 4-2-2006 by garage chemist]

greenyppols - 4-2-2006 at 14:16

Garage Chemist - Thanks for the info. Being able to get away with a halogen will work out. Got one layin' around, somewhere. I know glass stops UV in the sense you cant tan thru it, but a halogen never occurred to me.

And thanks for the info on the wavelength preference, or rather the way you presented it. Something to figure out without have it spoonfed to me. (entirely spoonfed anyway. lol )

Being able to get away with "regular" light is ok. Does it speed up the reaction? Not that Im going to try it if a halogen works, just more of a curiousity now, I guess.

Thanks again

Edit: I guess it would "speed" it up, if it cleaves it, right?

[Edited on 4-2-2006 by greenyppols]

garage chemist - 4-2-2006 at 14:43

Light speeds up the reaction a lot. It also helps to limit the time the chlorine has to attack the aromatic ring giving undesired ringchlorinated products.
With enough light, the chlorine doesn't really dissolve into the toluene since it reacts instantly upon contact.
Without light, the chlorine dissolves into the toluene before reacting, giving the ringchlorination more time and chance to happen.
Light is actually a MUST in the production of benzyl chloride from toluene. From the reaction mechanism you can see that light initiates the first step, and without light you'd get mostly ringchlorinated products.
It is also very important that the toluene is distilled before the synthesis, as traces of iron compounds (from storage in a steel can) strongly catalyse ringchlorination.

If you just want to make benzyl chloride, there is a much easier alternative though: reaction of benzyl alcohol with hydrochloric acid (look in rhodium archive under benzyl chloride). This way a product superior to the one from toluene is obtained, much cleaner.

If your desired product is benzal chloride for benzaldehyde production, you may consider oxidising benzyl alcohol to the aldehyde with MnO2/H2SO4, yields are acceptable and the product is much purer than by the chlorination route.

Benzyl alcohol is used for cosmetics and the like, you can buy it in the pharmacy. It is neither watched nor controlled.

[Edited on 4-2-2006 by garage chemist]

vulture - 5-2-2006 at 02:24

You can calculate the theoretical wavelength by taking the bond energy of chlorine and applying E = hv.

This only accounts for chlorine radicals though. The UV light might also help speeding up the reaction by increasing the concentration of benzyl radicals.

Does anybody know if this reaction proceeds through the singlet or triplet state?

Bill - 19-2-2006 at 19:05

Fun stuff.

mantis - 20-2-2006 at 02:51

is that all your benzyl chloride?

IrC - 20-2-2006 at 03:31

This may help?

Wavelength range: 245-300nm

F. Caralp, V. Focher, R. Lesclaux, T.J. Wallington, and M.D. Hurley, "Atmospheric chemistry of benzaldehyde: UV absorption spectrum and
reaction kinetics and mechanisms of the C6H5C(O)O2 radical," Phys. Chem. Chem. Phys. 1, 3509-3517 (1999)

runlabrun - 20-2-2006 at 04:43

jesus thats alot of benzylchloride....
Dare i ask if its yours and y u have so much? :o:o
So flooding toluene with 245-300nm UV is best for promoting chlorination? On basis of what? yields? time?


IrC - 22-2-2006 at 17:45

Get the paper and read it.

Phel - 22-2-2006 at 18:07

Here's the paper: (Click PDF)

leu - 23-2-2006 at 06:29

The wavelength of the ultraviolet radiation isn't critical, gamma radiation will also enable free radical halogenation :cool: