The Formation of Weak Intramolecular Hydrogen Bonds

In phenol and substituted phenols the C—O bond has some doublebond character, as discussed in Section 8-3. This tends to cause the hydrogen atom io lie in the plane of the benzene ring. The phenol molecule can thus assume either of the two configurations 

The infrared absorption spectrum thus showB that o-chlorophenol in solution in carbon tetrachloride consists of about 91 percent cis molecules and 9 percent trans molecules. The cis molecules are more stable than the trans molecules by a standard free-energy difference of about 1.4 kcal/mole (calculated from the ratio of the areas of the peaks). This is presumably the difference in free energy of the cis molecule with its intramolecular hydrogen bond and the trans molecule with a weaker hydrogen bond with a solvent molecule. 

The weak hydrogen bond in o-chlorophenol stabilizes the gas molecule relative to those of the meta and para isomers, whereas the crystalline and liquid phases of the three substances, in which hydrogen bonds can be formed between adjacent molecules, have about the same stabil- 

188 The equilibrium O—H distance is increased by about 0.01 A by interaction with fche chlorine atom. 

An absorption peak at 6620 cm l has been observed for liquid o-chlorophenol by Errera and Mollet.109 The further decrease in frequency below the value 6910 cm 1 for the trans form of the molecule can be explained by assuming that the liquid contains double molecules with the structure 

---