Physical and chemical properties
The hydroxyl group generally makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds. Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and of the carbon chain to resist it. Thus, methanol, ethanol, and propanol are highly miscible in water because the hydroxyl group predominates. Butanol is moderately soluble because of a balance between the two trends. Pentanol and branched butanols are effectively insoluble because of the hydrocarbon chain's dominance. Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. All simple alcohols are miscible in organic solvents.
Alcohols are very weakly acidic, and are so called "protic" solvents. They can lose the proton H+ of the hydroxyl group and are very weak acids, weaker than water except for methanol, but still stronger than ammonia or acetylene.
One important class of reactions undergone by alcohols is nucleophilic substitution, where one nucleophilic group attached to a carbon atom is replaced by another. So, for instance, alcohols react with hydrochloric acid to produce alkyl halides, where the hydroxyl group is replaced by a chlorine atom. The equilibrium lies to the right, since chlorine is a stronger nucleophile, but can be driven to the left using an alkaline medium, which is one way of synthesizing alcohols.
Alcohols are themselves nucleophilic, so can react with one another to produce ethers and water. They also react with hydroxy acids (or acid halides) to produce compounds called esters, of which the esters of organic acids are the most important. At high temperatures, alcohols can undergo an elimination reaction to produce alkenes. The reverse of this, the addition of water to an alkene to produce an alcohol, is catalyzed by acids but is of limited use for synthesis because it generally results in mixtures. Some other techniques exist to convert alkenes to alcohols more reliably.
Toxicity
Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanol in the form of alcoholic beverages has been consumed by humans since pre-historic times, for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. While it is widely considered relatively harmless or even benefical in small quantities, large enough doses result in a state known as drunkenness or intoxication, which may lead to permanent health damage or death (see ethanol for further discussion).
Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized, and often their metabolism produces even more toxic subtances. Methanol, or "wood alcohol", for instance, is oxidized by alcohol dehydrogenase enzymes in the liver to the poisonous formaldehyde, which can cause blindness or death. Interestingly, an effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. This will bind to alcohol dehydrogenase, preventing methanol from binding and thus its acting as a substrate.
See also
External links
Source | Copyright
