An oxygen atom normally forms two σ bonds with other atoms; the water molecule, H2O, is the simplest and most common example. If one hydrogen atom is removed from a water molecule, a hydroxyl functional group (―OH) is generated. When a hydroxyl group is joined to an alkane framework, an alcohol such as ethanol, is produced.
When the hydroxyl group is joined to an aryl ring, a phenol results (shown above). Both alcohols and phenols are widespread in nature, with alcohols being especially ubiquitous. The hydroxyl group of alcohols and phenols is responsible for an interesting variety of physical and chemical properties. The biochemical action of vitamin E, for example, depends largely on the reactivity of the phenol functional group.
An oxygen atom is much more electronegative than carbon or hydrogen atoms, so both carbon-oxygen and hydrogen-oxygen bonds are polar. The oxygen atom is slightly negatively charged, and the carbon and hydrogen atoms are slightly positively charged. The polar bonds of the hydroxyl group are responsible for the major reaction characteristics of alcohols and phenols. In general, these reactions are initiated by reaction of electron-deficient groups with the negatively charged oxygen atom or by reaction of electron-rich groups with the positively charged atoms—namely, carbon or hydrogen—bonded to oxygen.
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