[Zensation]

I have a reaction that is to use UV light to excite halides towards nucleophilic substitution. I read elsewhere that the strength of the light is not pertinent since the UV light directly excites/breaks the halide bonds, as if implying that the UV light is incredibly effective at exciting the halides, regardless of the strength, and that stirring rate is more important that UV light strength.

This was reasoned that it only takes a single photon to enter the electron orbits of the halide and to excite them to break the bonds. With the photons being produced in extreme abundance and moving far faster and more penetrating than any of the molecules in the solution it was implied the strength of the light can only help so much.

Would a 200w UV light run a reaction any more rapidly than a 400w light? I want to get a better understanding of this from the mechanical perspective of the photon. I also don't want to spend an extra $900 on a stronger light if the reaction will only be sped up by a few minutes (for multiple mole batches 15-30+).

[Corribus]

This is not easy to answer in a general way, as the rate of a photochemical reaction depends on a lot of things, only one of which is the light intensity. What is sure is that light must be absorbed in order to make a photochemical reaction go, but the relationship is not only linear. In fact, the rate of absorption of photons isn't even linear, and it can be more or less linear depending on the molecule that's doing the absorbing. If your molecule has a large absorption cross-section and is already efficiently photoactivated at the lower light intensity, doubling the intensity isn't going to do much because your rate is limited by other factors. If, however, your cross-section is small and your light absorption efficiency is low (and your quantum yield of reaction is fast), then your rate limiting step is absorption, and doubling your intensity will certainly make a substantial difference.  You've also got other kinetic things to worry about because, unlike a traditional reaction, your reagents aren't already necessarily premixed. This kind of inner filter effect essentially limits the rate of mixing of your reagents (one of which is light). This is why vigorous stirring MAY be important.... it also depends on how clear your solution is! So, physical theory isn't necessarily going to give you the practical answer you're looking for.

If you tell a little bit more about the reaction you're doing, this will help. If you prefer to look into it on your own, I suggest finding a copy of Turro's excellent book Modern Molecular Photochemistry, which has a whole chapter on rates and also sections on homolytic cleavage reactions.  Otherwise, if you don't really care about learning all you're probably going to need to learn, I suggest the trial and error method: try the lower Watt bulb and if you're happy with the result, move on with your life.

[Zensation]

Learning is important to me a great deal. I don't particularly like to do anything that I don't have a good grasp of understanding on.

Vigorous stirring would of course be a must. For our purposes we use powerful overhead stirrers. All substrates/solvents are clear. We are doing SRN1 reactions of aryl bromides reacting with NaOH. This reaction should go on its on without light since bromine is an OK leaving group, but we figure given these conditions UV input would only accelerate the process.