Infrared spectroscopic studies of hydrogen bonding in substituted nitrophenols: substituent and solvent effects

• Autor(a) principal: Castilho, Paula C. M. F.
• Data de Publicação: 1992
• Outros Autores: Crampton, Michael R., Yarwood, Jack
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.13/3272
• topic: Infrared spectrometry; Hydrogen bonding; Nitrophenols; .; Faculdade de Ciências Exatas e da Engenharia

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 pKa). The 4-cyano (4CN) isomer shows two distinct NO2 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 NO2 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-NO2 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.