Electric Dipole Moment Investigation of Buttressing in Alkyl Duryl Ketones

Abstract

Dipole moments (D) determined for the following alkyl duryl ketones in benzene at 30° were: Me, 2.83; Et, 2.80; i-Pr, 2.74; t-Bu, 2.65. The decrease in the series is explained by increasing rotational restriction about the carbonyl group due to an increasing interference by the ortho methyl groups with the alkyl group. This results in a greater twist from coplanarity between the ring and carbonyl and less π-electron delocalization and consequently a lower dipole moment as the size of the alkyl increases. The increase observed for the duryl ketone moments over those of corresponding mesityl ketones is tentatively explained as due to a higher duryl group moment relative to the mesityl moment. The interplanar angles (angles of twist) for the duryl ketones calculated with the Braude–Sondheimer equation were slightly larger than those calculated for the mesityl ketones, the magnitude depending on the method of calculation. It is tentatively concluded that evidence from dipole moment data indicates that a small buttressing effect exists in the duryl ketone system.