Q6(e). IR spectra of cis- and trans-cyclohexane-1,3-diol

Introduction. The O–H stretching band shows how an OH group is hydrogen-bonded. A non-bonded OH absorbs sharply at 3590–3650 cm⁻¹; hydrogen bonding lowers the wavenumber. Intermolecular hydrogen bonds depend on concentration and break on dilution in CCl₄; intramolecular hydrogen bonds do not change on dilution. The test is to record the spectra in very dilute CCl₄ solution.

1. cis-Diol: in one chair both OH are axial on the same face (1,3-diaxial). The O atoms are then close enough for an intramolecular O–H…O hydrogen bond (a six-membered ring of O–C–C–C–O plus H).
2. trans-Diol: one OH is axial and the other equatorial in every chair; the two O atoms point away from each other, so no intramolecular H-bond is possible.

Figure: cis-1,3-Diol, diaxial chair: the two OH groups face each other.

1. In concentrated solution both isomers show a broad band from intermolecular hydrogen bonding, so they cannot be told apart.
2. On dilution with CCl₄ the intermolecular bonds break. The trans isomer then shows only the sharp non-bonded O–H band; the cis isomer keeps a second, concentration-independent band at lower wavenumber from the intramolecular O–H…O bond.
3. The ratio of the two cis bands also shows the population of the diaxial chair (which is less favoured sterically but is stabilised by the hydrogen bond).

Conclusion. In very dilute CCl₄, cis-1,3-diol shows a non-bonded and a dilution-independent intramolecularly bonded O–H band, whereas trans-1,3-diol shows only the non-bonded O–H band.

Application. The dilution test separates intra- from intermolecular hydrogen bonding in general (e.g. o- versus p-nitrophenol, cis- versus trans-cyclopentane-1,2-diol) and is used to assign the relative configuration of diols in natural products.