The torsional barrier in aromatic carbonyl compounds. VI. Carbon-13 and 1H DNMR study of protonated alkyl and haloalkyl p-tolyl ketones

✍ Scribed by Jean-Françlois Barthelemy; Roland Jost; Jean Sommer

• Book ID: 102951672
• Publisher: John Wiley and Sons
• Year: 1978
• Tongue: English
• Weight: 512 KB
• Volume: 11
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270110905

✦ Synopsis

The free energy of activation of the torsional barrier around the phenyl-carbonyl bond in a series of stable protonated a-substituted pmethylacetophenones (ptolyl-COHLR where R = CH,, C,H,, GCJ-I,, t -C4€&, , CHJX, CHCl,, CCl,, CHzF, CHF,, CF3 and CH,Br) has been evaluated by complete lineshape analysis of "C or proton low temperature NMR spectra. a-Substitution by an akyl group leads to a decrease of the barrier height due to both steric and electronic effects. For the halo compounds, steric and electronic effecls of the substibents are m competition, leading to a very high AG' value in protonated OL,CX,QL-Muom-pmethylacetophenone (63.2 W mol-I). In addition, the twist angle of the protonated carbonyl group has been tentatively evaluated by the use of the (modified) Stothers's method.

' H NMR band assignments are generally made on the basis of the 45 allylic couplings (Jcis > Jnam)4 and, with the exception of t-butyl p-tolyl ketone (5) (in which this coupling is not available), all the protonated ketones studied here exist as only one isomer of the Z configuration. The 'H (Fig. 1) and 13C spectra of protonated t-butyl p-tolyl ketone at -60 "C show clearly the existence of two stereoisomers A and B under slow exchange conditions of the OH+ proton with the acid solvent.