Hydrogen Bonding in Alcohols and Phenols

The boiling points (bp s) of alcohols are much higher than those of ethers or hydrocarbons with similar molecular weights. 

Two or more alcohol molecules thus become loosely bonded to one another through hydrogen bonds. 

This accounts for the complete miscibility of the lower alcohols with water. However, as the organic chain lengthens and the alcohol becomes relatively more hydrocarbonlike, its water solubility decreases. Table 7.1 illustrates these properties. 

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Figure 17.1 Hydrogen-bonding in alcohols and phenols. A weak attraction between a positively polarized OH hydrogen and a negatively polarized oxygen holds molecules together. The electrostatic potential map of methanol shows the positively polarized O-H hydrogen (blue) and the negatively polarized oxygen (red).

H hydrogen bonding in alcohols and phenols 3200-3600 strong, broad... 

The several detailed infrared studies of hydrogen bonding in alcohols and phenols are exemplified by the work of Stuart and Sutherland (1956), Lippincott and Schroeder (1955, 1956), " Jakobsen and Brasch (1965), and Lake and Thompson (1966). Investigators write about monomers, dimers, polymers, proton donors, proton acceptors, and free hydroxyl, but without reference to the formation of ions. 

The -OH group in alcohols and phenols is involved in intermolecular hydrogen bonding as shown below ... 

Since the hydroxyl group is present in alcohols and phenols, these compounds are polar. The polarity of the hydroxyl group, coupled with its ability to form hydrogen bonds, enables many alcohols and phenols to mix with water. Since these compounds also contain nonpolar portions, they show additional solubility in many organic solvents, such as dichloromethane and diethyl ether. 

This experiment describes a characterization analysis in which the degree of association, equilibrium constant, and hydrogen bond energy are measured for benzyl alcohol and phenol in CCI4. 

Examples of hydrogen-bonding-promoted Diels-Alder reactions obtained by using alcoholic and phenolic solvents are illustrated in Section 6.2.4. 

In terms of binding energy, classical hydrogen bonding may be enhanced by the increased acidity of the phenolic O—H, if the acceptor alcohol is non-phenolic [149, 182]. On the other hand, n interactions compete with classical hydrogen bonds [19, 54] and may even dominate the interaction, such as in the dimer of 1-naphthol [55]. This is not yet the case for phenol dimer, the prototype compound in this class [246, 247]. 

Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules as shown. The solubility decreases with increase in size of alkyl/aryl (hydro-phobic) groups. Several of the lower molecular mass alcohols are miscible with water in all proportions. 

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