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The torsional barrier in aromatic carbonyl compounds. VI . Carbon‐13 and 1 H DNMR study of protonated alkyl and haloalkyl p ‐tolyl ketones
Author(s) -
Barthelemy JeanFrançlois,
Jost Roland,
Sommer Jean
Publication year - 1978
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270110905
Subject(s) - steric effects , protonation , chemistry , alkyl , electronic effect , medicinal chemistry , carbon 13 nmr , proton nmr , crystallography , photochemistry , stereochemistry , organic chemistry , ion
The free energy of activation of the torsional barrier around the phenyl–carbonyl bond in a series of stable protonated α‐substituted p ‐methylacetophenones ( p ‐tolyl‐ where R = CH 3 , C 2 H 5 , i ‐C 3 H 7 , t ‐C 4 H 9 , CH 2 Cl, CHCl 2 , CCl 3 , CH 2 F, CHF 2 , CF 3 and CH 2 Br) has been evaluated by complete lineshape analysis of 13 C or proton low temperature NMR spectra. α‐Substitution by an alkyl group leads to a decrease of the barrier height due to both steric and electronic effects. For the halo compounds, steric and electronic effects of the substituents are in competition, leading to a very high ΔG ≠ value in protonated α,α,α‐trifluoro‐ p ‐methylacetophenone (63.2 kJ mol −1 ). In addition, the twist angle of the protonated carbonyl group has been tentatively evaluated by the use of the (modified) Stothers's method.