Question

Suppose that you were to convert benzophenone (the starting material) into benzhydrol (the product) by reaction with NaBH4 or some comparable reducing agent. Note that no reaction occurs before the NaBH4 is added. Also note that NaBH4 does not appear on your TLC plate. On the templates provided below, sketch the TLC plates you would obtain for: a) the reaction mixture before adding the sodium borohydride; b) a sample taken from the reaction mixture when the reaction is about half complete c) a sample taken from the reaction vessel after the reaction is complete On each plate, show two lanes: the left lane will show, for reference, a mixture of the starting material (benzophenone) and the product (benzhydrol). The reference mixture does not contain NaBH4 and there is no reaction occurring in the reference. The right lane should show the contents of your reaction vessel. Your Rf values should approximate those you found.

Answer 1

The reduction of benzophenone using sodium borohydride is usually performed using methanol as the solvent (it also is invisible in TLC) and the reaction is quite fast and even when charged at zero degrees, it tends to achieve spot-to-spot conversion (100% conversion) by 10-15min. Hence, the TLC of the reaction will clearly show the appearance of a more polar diphenylmethanol spot with the disappearance of the less polar benzophenone spot. In TLC, a more polar compound moves with a lower Rf than one with lower polarity and in this case, the alcohol "tails" the ketone when TLC is checked around halfway of reaction progress due to its binding with the borohydride. The typical Rf of the ketone arrives around 0.6 with the alcohol at 0.3 with the retention factors varying with higher or lower percentage composition of ethyl acetate in hexane. The TLC patterns for the reaction are as follows, drawn in accordance with the requisites mentioned.

Finally, after reaction completion, there should be only one spot in the TLC, now corresponding to diphenylmethanol and the sodium borohydride, being a basic compound is taken out of the medium by acidic workup or its high solubility in water can be exploited and a simple aqueous workup is done to expel. Afterwards, the crude diphenylmethanol i recrystallized, usually from ethylacetate or diethyl ether to afford the pure product.