Self-association of phenols in inert solvents. Apparent heat capacities of phenol, substituted phenols and aromatic alcohols in n-heptane

Abstract

Apparent heat capacities have been measured for phenol, substituted phenols and aromatic alcohols in dilute n-heptane at 25 "C. The alcohols are phenol, o-cresol, rn-cresol, p-cresol, 2,6-dimethylphenol, 3,5-dimethylp he no I, 3,5-d i -tert-b u ty I p he n o I, 2,6-d i-tert-b u t y I p he n o I, 2,4,6-t r i-tert-bu ty I p h e no I, 2,6-d i -tert-b u ty I -4-met h y I p hen o I, 2-te rtb u t y I p he no I, 4-te rtb u t y I p h e n o I, 2-e t h y I p h e no I , 2 -prop y I p h e no I, 2-p h en y I p h e no I, be n z y I a I c o h o I , 2phenylpropan-2-01 and phenethyl alcohol. In addition, cyclohexanol, 3-ethylpentan-3-01 and hexan-1-01 were also studied. The associational part of the apparent molar heat capacity @,(assoc) shows a maximum against concentration which decreases and moves to higher alcohol concentration as the hydroxyl group on the alcohol becomes increasingly hindered, effectively reducing the self-association capability. For the 2,6-disubstituted phenols the @, maximum disappears, indicating the absence of self-association. The Treszczanowicz-Kehiaian theory for associated liquids was used to obtain the volumetric equilibrium constants and the enthalpy of hydrogen-bond formation (AH'). For ptienols AHo is lower than for other alcohols, indicating weaker hydrogen bonding. In contrast to alkan-1-ols, where tetramers are the predominant species, for the present alcohols as the steric hindrance increases tetramer population is severely reduced, dimers and trimers being more important.

Effect of steric hindrance and π electrons on alcohol self-association

“…This is in fact what was found in ref. 4, where it was shown that benzyl alcohol has a AHo value similar to that for linear alcohols. In ref.…”

“…The results in Table 2 allow the qualitative discussion of several observations made in ref. 4 and 5 regarding the selfassociation behaviour of alcohols. For benzyl alcohol, which has an aromatic ring, but whose hydroxyl group is separated from it by a CH, group, the MNDO calculations show that the oxygen atom does not transfer electronic charge to the aromatic ring, and as a consequence it must form a strong hydrogen bond.…”

“…2 where it is seen that these three alcohols have the same AG4f. 4 Although the correlation between q and AGZ in Fig. 2 is able to explain fine details of the self-association behaviour of different alcohols, it is clear that it fails to be general, i.e.…”

“…Corresponding to this increase in q, AGZ values for the cyclohexanols are larger than for the corresponding phenols, indicating their better ability to form t e t r a m e r ~. ~ An exception to this behaviour is given by the alcohols 2-tert-butylphenol and 2-tert-butylcyclohexanol, where the phenolic alcohol has the largest AG: ; (2) comparison between the alcohols 2-tert-butylphenol, 2-ethylphenol and 2-propylphenol shows that their 4 values are very similar. Hence it is predicted that these alcohols should have the same ability to form tetramers.…”

“…For the alcohols 2,6,-di-tertbutylphenol, 2,4,6-tri-tert-butylphenol and 2,6,-di-tert-butyl-4-methylphenol q values are well below this critical value in accordance with the results in ref. 4, where it was found that these alcohols do not self-associate in solution. For alcohols whose q values are between 0.7 and 0.75, $,(assoc) increases slightly with concentration and its maximum is displaced to high alcohol concentrations [Fig.…”