Synthesis and Stereochemistry of 2,2-dimethyl-1,3-dioxolane

In this experiment, Benzil was reduced with NaBH4, and the diol—Hydrobenzoin—was converted to an acetal of hydrobenzoin—2,2-dimethyl-1,3-dioxolane. The hydryobenzoin product weighed 0.290 g, or 0.00135 mol. The theoretical yield of hydrobenzoin was 0.32139 g, and the percent yield was 90.3%. The melting point was 134.8 degC. According to the melting point, the hydrobenzoin isomer was meso—R,S. because the product was meso, the proton NMR for the acetal revealed 2 methyl peaks. The Jones test for hydrobenzoin was negative, but because the benzyl starting product had such a low mp of ~95 degC, the test was not reliable for the hydrobenzoin product. The basis of alcohol (diol) should focus on mp of hydrobenzoin, which obtained an R,S meso configuration. The weight of the acetal product—dioxolane—was 0.222g, or 8.73 x 10-4 mol. The theoretical yield of the acetal was 0.343 g, and the percent yield was 64.7%. The melting point of the acetal product was 57.8 degC. The low percent of product could have been caused by boiling away acetone during reflux, boiling away product because of its low mp (57.8), or mixture of product and hexane soaked up in cotton. Because the hydrobenzoin that reacted with the acetone to form the acetal was meso (1R,2S), the the hydrogens on the methyl groups of the acetal were diastereotopic; therefore, two methyl peaks were seen in the NMR for 2,2-dimethyl-1,3-dioxolane.

Conclusion:

In this lab, the synthesis of a carbonyl protecting group, dioxolane was introduced. The stereoselective reduction of benzyl using sodium borohydride was demonstrated in order to determine the identity of the diol hydrobenzoin. The 134.8 mp of hydrobenzoin determined it to be R,S stereochemistry, which is the meso configuration; therefore, the meso diol reacted with acetone to form the acetal 2,2-dimethyl-1,3-dioxolane with diastereotopic hydrogen on the methyl groups. The NMR further characterized the diastereotopic hydrogens by representing two different peaks for the different hydrogens on the two methyl groups.