Knowledge Bank

A single conformation exists in ethyl benzenes with the ethyl C-C bond lying in a plane orthogonal to the benzene ring. This conclusion results from the observation that asymmetrically substituted ethyl benzenes each display only a single band series in low resolution microwave (LRMW) spectra whereas any other conformation of the ethylphenyl fragment would result in two spectroscopically distinguishable species . LRMW spectra of Isopropyl benzene derivatives display three spectroscopically distinguishable species. Two of these species correspond to the conformation with the methiane $C-H$ bond eclipsed with the benzene ring and syn or anti with respect to an unsymmetrical para substituent. The third species is much more intense and has a B+C value which is the average of the syn and anti forms. This species is composed of all the torsionally excited species in states above the internal rotation barrier between the phenyl and isopropyl groups. From intensity measurements the 2-fold barrier is determined to be $250\pm 140$ cal/mol in p-isopropyl benzaldehyde and p-isopropyl benzoyl chloride.