A NUMBER of workers have observed various anomalous properties in nuclear-substituted derivatives of mesitylene, such anomalies usually being ascribed to restricted rotation about the bond connecting the substituent to the benzene ring caused by the two ortho-methyl - groups. In particular, Adams and Theobald1 have resolved β-bromo-β-(3-bromo-2,4,6-trimethylphenyl)-acrylic acid and the corresponding β-chloro - compound into optically active forms, and have shown the half-life period of the latter to be much greater for the bromo- than for the chloro - compound (64 hr. and 200 min. respectively). Substitution of hydrogen for the halogen atom would clearly make resolution impossible. However, we have recently shown that restriction of rotation is still appreciable even with such a small blocking group as hydrogen, by a study of the simplest possible compound of this class, namely, vinyl-mesitylene.