A substituent on a benzene ring alters delta at ring C atoms o- and p- to it depending on electronic sigma values, while the m- position is little affected. Nitrobenzene is an exception. There delta for the o- position is less than for m-; the opposite is predicted. The p- C behaves normally. In acetophenone the o- and m- C signals differ by 0.1-0.3 ppm. A JACS paper [94, 3089 (1972)] states that o- and m- assignments are unconfirmed, but suggested that o- signal has higher delta value. All databases copied this suggestion. We found by C-H COSY the opposite assignment is true. This seems to be a through space effect as invoked to explain the effect of the nitro group o- to itself. Phenacyl chloride and propiophenone behave similarly to acetophenone but &alpha,&alpha-dichloroacetophenone doesn't. Benzaldehyde, benzoic acid and methyl benzoate behave normally. Supported in part by NSF's Division of Undergraduate Education grant #9650684.
Back to Organic Posters
Back to The 37th Middle Atlantic Regional Meeting (May 22-25, 2005)