Alcohols, Phenols, and Thiols - Cengage Learning - page 123

7
Alcohols, Phenols, and Thiols
Chapter Summary
The functional group of alcohols and phenols is the hydroxyl group . In alcohols, this group is
connected to an aliphatic carbon, whereas in phenols, it is attached to an aromatic ring.
In the IUPAC system of nomenclature, the suffix for alcohols is -ol . Alcohols are
classified as primary , secondary , or tertiary depending on whether one, two, or three
organic groups are attached to the hydroxyl-bearing carbon. The nomenclature of alcohols
and phenols is summarized in Secs. 7.1-7.3.
Alcohols and phenols form hydrogen bonds . These bonds account for the relatively
high boiling points of these substances and the water solubility of lower members of the
series.
Brønsted-Lowry and Lewis definitions of acids and bases are reviewed in Sec. 7.6.
Alcohols are comparable in acidity to water, but phenols are much more acidic. This increased
acidity is due to charge delocalization (resonance) in phenoxide ions. Electron-withdrawing
groups, such as − F and − NO 2 , increase acidity, through either an inductive or a resonance
effect, or both.
Alkoxides , the conjugate bases of alcohols, are prepared from alcohols by reaction
with reactive metals or metal hydrides. They are used as organic bases. Because of the
greater acidity of phenols, phenoxides can be obtained from phenols and aqueous base.
Alcohols and phenols are also weak bases. They can be protonated on the oxygen by
strong acids. This reaction is the first step in the acid-catalyzed dehydration of alcohols to
alkenes and in the conversion of alcohols to alkyl halides by reaction with hydrogen halides.
Alkyl halides can also be prepared from alcohols to alkyl halides by reaction with hydrogen
halides. Alkyl halides can also be prepared from alcohols by reaction with thionyl chloride or
phosphorus halides .
Primary alcohols can be oxidized to carboxylic acids using Jones' reagent , whereas
secondary alcohols give ketones . Primary alcohols can be oxidized to aldehydes using
pyridinium chlorochromate (PCC).
Glycols have two or more hydroxyl groups on adjacent carbons. Ethylene glycol ,
glycerol , and sorbitol are examples of glycols that are commercially important. Three
important industrial alcohols are methanol , ethanol , and 2-propanol .
Phenols readily undergo aromatic substitution since the hydroxyl group is ring-
activating and ortho , para -directing. Phenols are easily oxidized to quinones . Phenols with
bulky ortho substituents are commercial antioxidants.
Examples of biologically important alcohols are geraniol , farnesol , and cholesterol .