You prepare benzhydrol via reduction of benzophenone with sodium borohydride. How can you use TLC to determine when the reaction is complete? How can you monitor the progress of the reaction with TLC?

1 Answer
Apr 1, 2017

I take it you can perform the steps of #"TLC"#.


Take a TLC of your starting material, benzophenone, and then run a chromatograph. You have to find appropriate conditions for your chromatograph: a typical mix of eluting solvent is 25% acetone, and 75% hexanes......but there are many choices.

Now run the TLC of your starting material benzophenone. And note its #R_f#: this should be characteristic for benzophenone. These days, TLC plates have UV phosphors so that you can see the spots under UV light.

Now do your reaction, and thruout the reaction, take samples of the reaction mixture, and spot new chromatographic plates. Ideally, (which thing never are!), the first few chromatographs you run should show two spots, one corresponding to the starting material (whose #R_f# you already have), and the reaction product, a new material, which should possess a different #R_f# value. And this helps you gauge the extent of the reaction.

If you aspire to be a synthetic chemist, this is a procedure you should (and will) get very good at. It will help you judge reaction times and conditions, and also help you to know when to turn off (i.e. quench) the reaction, and stop it cooking.

Anyway your library (or your lab) will have a copy of the book #"A Guide to the Perplexed Organic Chemist"#, or its later print version. Thin layer chromatography is explained in much better detail there.