Infrared studies and thermodynamics of hydrogen bonding in 2-halogenoethanols and 3-halogenopropanols

Abstract

From a non-linear least-squares curve analysis of the spectra of 2-halogenoethanols and 3-halogenopropanols in the hydroxy stretching region in dilute carbon tetrachloride solution at 298–338 K, the thermodynamics of the equilibrium between free and intramolecularly hydrogen-bonded species have been determined. The amount of intramolecularly hydrogen-bonded species decreased with increasing size of halogen and increasing steric repulsion. The similar frequency shifts of the intramolecularly hydrogen-bonded hydroxy group in both the 2-halogenoethanols (17–50 cm^–1) and 3-halogenopropanols (15–52 cm^–1) established the poor proton-acceptor properties of the halogens, including the highly electronegative fluorine. The absence of intramolecularly hydrogen-bonded species in 4-chlorobutanol is a case where the entropy change over-rides the enthalpy change due to hydrogen bonding. Hydroxy frequency shifts due to intramolecular hydrogen-bonding ran opposite to the enthalpy changes disobeying Badger–Bauer rule. These alcohols associate forming dimers (ca. 3500 cm^–1) in which the hydroxy groups alone participate in intermolecular hydrogen bonding.