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| - Conformation and Steric Effects in Mono- and DimethoxybenzoicAcids
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| - The conformation of mono- and dimethoxybenzoic acids in solution was determined from their IRspectra and 13C and 1H NMR spectra. The main feature is a strong intramolecular hydrogen bondin all 2-methoxy derivatives that persists even in polar aprotic solvents but not in methanol. Themethyl groups are mostly coplanar with the ring plane, the two planar conformations are almostequally abundant, and their conformation is not responsible for some enhanced values of theenthalpy of formation. The hydrogen bond (and possibly dimerization) is the reason for differencesin the relative acidities (δpK = pK − pKH) measured either in dimethyl sulfoxide or in methanoland also for the differences between δpK values of methoxybenzoic acids and correspondingmethylbenzoic acids. Nevertheless, it was possible to separate with good approximation the polarand steric effects on the acidity. The steric effects in methanol are then parallel for methyl- andmethoxybenzoic acids despite variable conformation of the latter. Steric effects in dimethyl sulfoxideare smaller and less regular. The acidity enhancement by ortho substituents is better described aselectrostatic induction in the anion rather than a steric hindrance.
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