During TLC, the starting material is less polar than the product and will be the highest spot on silica gel plate because the presence of the double bounded oxygen decreases the polarity of the substance because it more evenly distributes the electrons on the keytone as compared to an alcohol. The less polar substance has a less interaction with the silica gel plate and will travel up the silica gel plate more. Therefore, the product will be lower on silica gel plate because the starting material’s double bonded oxygen was reduced to an alcohol group via nucleophilic addition reaction. The other two spots seen on the crude material and co-spot TLC plate are the products of the nucleophilic addition of a hydride. Nucleophilic addition can attack the oxygen from either side and as a result, a mixture of diastereomers could be the product (refer to the image below as the possible diastereomer mix that could be formed from the nucleophilic addition of benzoin).
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