Planning a Photochemical Synthesis

A check must also be made that neither the material from which the apparatus is built nor the solvent absorb at that wavelength. In practice, with a low-pressure mercury arc, a quartz apparatus and a solvent chosen among alkanes, alcohols, ethers or 

Lamp Irradiation (nm) Einstein (min 1 cm 2) Volume irradiated (ml) Time taken to convert a 5 x 10 2 M solution ( D = 1) (h) 

l or the range indicated, the lamp emits also in the visible. 

When planning the scale of the experiment, the data listed in Table 1.2 might be of value. The quantities indicated are (i) the light flux (in Einstein, moles of photons) on a 1 cm2 surface close to the lamp and (ii) the time required for converting a 5 x 10 2 M solution by using that lamp, assuming that all of the flux is absorbed, the quantum yield is unitary, and that there is no inner filter effect (in other words, the minimal time required for converting the above amount). 

It can be seen that amounts of up to a few grams can be converted in 5-10 h in the case of complete absorption and unitary quantum yield (ignoring any internal filter effect). Lower quantum yields or an incomplete absorption proportionally lengthens the time required for the transformation. It is most likely safe to say that exploratory reactions on the 100 mg scale can be carried out in a reasonable time, provided that 0.1-0.05. Even reactions with a quantum yield at the lower limit or below may be interesting for a preparation if the reaction is clean. Given the minimal safety 

---