Abstract
A detailed infrared spectroscopic study of the substituted phenols 2-cyano-4,6-dinitrophenol and 4-cyano-2,6-dinitrophenol has been carried out (in several different solvents) in order to investigate the substituent and solvent effects on their intra- and inter-molecular hydrogen bonding properties. In benzene or dichloromethane it is found that both isomers form strong intramolecular hydrogen bonds with the 2-cyano (2CN) isomer having a stronger intramolecular interaction (in accordance with the higher p K a). The 4-cyano (4CN) isomer shows two distinct NO 2 groups and exchange between the two possible hydrogen bonding sites is probably slow on the infrared time-scale. In protic solvents such as methanol the intramolecular hydrogen bonds are broken (more easily for the 4CN isomer) by intermolecular hydrogen bonding to the solvent. The differential “reactivity” towards methanol may be associated with steric congestion in the 4CN isomer leading to the forcing of at least one of the NO 2 groups out of the aromatic plane. The use of mixed solvents (benzene-methanol) has established that the two hydrogen bonded species are observed together and that a high concentration of methanol is required to drive the equilibrium towards the intermolecular hydrogen bonded species. In dimethyl sulphoxide the behaviour of the two isomers is even more interesting. The 4CN isomer is ionised to produce the corresponding phenolate. However the 2CN isomer remains neutral (but highly solvated). We attribute this difference to the requirement for the 4CN isomer to allow the 2- and 6-NO 2 groups to recover planarity with the aromatic ring. The energy compensation involved in this process is clearly sufficient to break a stronger intramolecular hydrogen bond.
Highlights
A detailed infrared spectroscopic study of the substituted phenols 2-cyano-4,6-dinitrophenol and 4-cyano-2,6-dinitrophenol has been carried out in order to investigate the substituent and solvent effects on their intra- and inter-molecular hydrogen bonding properties
Has established that the two hydrogen bonded species are observed together and that a high concentration of methanol is required to drive the equilibrium towards the intermolecular hydrogen bonded species
We attribute this difference to the requirement for the 4CN isomer to allow the 2- and
Summary
Function of enviromnent.The compounds of interest for this study are shown in Scheme 1 Our interest in these compounds arose from our related kinetics study [14] on the dealkylation of ring activated alkyl aryl ethers in DMSO to produce the corresponding phenolate. It was the differences in IR spectra caused by such ionisation in DMSO which led us to a more detailed study of the corresponding phenols. As far as we are aware, detailed IR spectra of these materials have not been published previously. 4-Cyano-2,6_dinitrophenol was prepared by the reaction of 4cyanophenol with fuming nitric acid at room temperature for 24 h. Data were transferred to an IBM PC for further analysis and are reported here in dimensionless absorbance units: log&/l)
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