Infrared studies and thermodynamics of hydrogen bonding in 2-chloroethanol, 3-chloropropanol and 4-chlorobutanol

Abstract

An i.r. study in the hydroxyl stretching region of three chloroalcohols [Cl—(CH₂)_n—OH: n= 2, 3, 4] in CCl₄ solutions (concentrations up to 0.12 mol dm^–3) has been carried out in the temperature range 298–338 K. Apparent integrated intensities were measured and used to evaluate the equilibrium between free and intramolecularly hydrogen-bonded OH groups in dilute solution. The study revealed that ≈ 68 % of 2-chloroethanol molecules and ≈ 8 % of 3-chloropropanol molecules were present in the intramolecular H-bonded from (ΔH^°_{INTRA ⇌ FREE}= 15.3 and 26.1 kJ mol^–1, respectively). 4-Chlorobutanol, however, did not show intramolecular H-bonding in solution.

Association behaviour of these chloroalcohols was analysed by considering various possible structures (open/cyclic n-mers) for the species absorbing at ≈ 3500 cm^–1. A cyclic four-membered ring dimer satisfactorily explained the observed results (ΔH^°_{DIMER ⇌ FREE}= 35.9, 33.95 and 16.9 kJ mol^–1, respectively, for n= 2, 3 and 4) though in 2-chloroethanol a ten-membered ring dimer involving participation of chlorine is also possible. The tendency of chloroalcohols to associate was found to be low compared with simple alcohols and alkoxyalcohols and is explained by the large atomic size of chlorine.