Alcohols hydrogen-bonding effects

T. Scharge, D. Luckhaus, and M. A. Suhm, Observation and quantification of the hydrogen bond effect on O H overtone intensities in an alcohol dimer. Chem. Phys. 346,167 175(2008). 

Attention should be paid to the additional hydrogen bonding effect in protic solvents like alcohols. It has indeed been observed that correlations of solvent-dependent properties (especially positions and intensities of absorption and emission bands) with the fcT(30) scale often follow two distinct lines, one for non-protic solvents and one for protic solvents. 

To investigate the effect of alcohol hydrogen-bond donor (HBD) strength in a systematic way, a series of modifiers possessing a common 4-teri-octylphenoxy ethanol core element were prepared, in which the properties of the substituent attached to the alcohol carbon atom were varied. These modifiers were evaluated with respect to both the cesium extraction strength they afforded when used in combination with BC6 and the HBD strength of the solvent as assessed using the solvatochromic parameter E. ... 

The continuum model has been applied to an experimental study of the solvent effect on the 6-chloro-2-hydroxypyridine/6-chloro-2-pyridone equilibrium in a variety of essentially non-hydrogen-bonding solvents (Beak et al., 1980). In this study, a plot of log A nh/oh) versus (e - 1)/ (2e + 1), the solvent dielectric term, yielded a linear least-squares fit with a slope of 2.5 0.2, an intercept of -1.71, and a correlation coefficient of 0.9944. This result was used to estimate the gas phase free-energy difference of 9.2 kJ mole-1, which compares favorably with the observed value of 8.8 kJ mole-1 for this system. The authors also reported that alcohol solvents are correlated fairly well in this study but that other solvents seem to be divided into two classes, those that are electron-pair donors and those that are electron-pair acceptors in a hydrogen bond. The hydrogen bonding effect is assumed to be independent from the reaction field effect and is included in the continuum model by means of the Kamlet and Taft (1976) empirical parameters. The interested reader is referred to the original paper for a detailed discussion of the method and its application. 

There is another type of phase diagram enconntered in some polymer solutions, where the LCST lies below the UCST. This is called closed-loop and appears for polymer solutions where hydrogen bonding effects are dominant snch as polyethylene gly-col/water and polyvinyl alcohol/water (Figure 16.4). 

Another reason for the double parentheses is that, as a consequence of uncertainties in unraveling multiple hydrogen-bonding effects, different sets of solvatochromic equations have given internally consistent but conflicting sets of results. As discussed earlier, this was the case with the j8 values for the more acidic alcohols and water. 

This resonance effect is described as conjugate chelation , to differentiate it from a normal hydrogen bonding effect such as that given by diacetone alcohol, and it can arise from any structure... 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Higher alcohols become more hydrocarbon like and less water soluble 1 Octanol for example dissolves to the extent of only 1 mL m 2000 mL of water As the alkyl chain gets longer the hydrophobic effect (Section 2 17) becomes more important to the point that It more than hydrogen bonding governs the solubility of alcohols... 

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