Reduction of Benzoin

          In this experiment, a purified sample of 1, 2-diphenylethane-1, 2-diol was obtained through the reduction of benzoin with sodium borohydride and the recrystallization of the product with acetone, resulting in 71.73% yield. The product was identified to be 1, 2-diphenylethane-1, 2-diol via TLC analysis and comparison of the experimental melting point to the known melting point. The known melting point of 1, 2-diphenylethane-1, 2-diol is 138 °C, while the experimental melting point was 135-137 °C. That the experimental range is slightly lower than the known range indicates the presence of impurities in the sample, but the relatively narrow experimental range indicates that whatever impurities remain in the final sample are likely present in only small amounts. During TLC analysis, visualization was achieved via short-wave UV light and iodine staining. Iodine produced yellow stains when reacted with the pure benzoin sample, but not the samples of only the recrystallized or crude product. This contributed to the identification of the spots derived from a mixture of benzoin and my experimental samples. Rf numbers were obtained for benzoin, the crude product, and the recrystallized product. Higher Rf numbers indicate higher polarity. Benzoin is less polar than 1,2-diphenylethane-1,2-diol because alcohols are more polar than carbonyls and 1,2-diphenylethane-1,2-diol has two alcohol groups where benzoin only has one. Since benzoin is less polar, it would be expected to travel farther on the TLC plate and have higher Rf values because it has less absorption with the polar silica gel on the TLC plate than 1,2-diphenylethane-1,2-diol. In the experimental TLC analysis, spots derived from benzoin did move further than spots derived from 1,2-diphenylethane-1,2-diol.