Definition
Alcohols are a group of organic compounds with the general chemical structure R-OH. The hydroxyl group (OH) is attached to an aliphatic carbon atom. Aromatic alcohols are called phenols. They are a separate group of substances. Alcohols can be obtained as derivatives of water (H
2
O) in which a hydrogen atom has been replaced by an organic radical. They can also be considered substituted (hydroxylated) alkanes.
Nomenclature
The names of alcohols are usually formed with the base and the suffix -ol:
- Methane methanol
- Ethane ethanol
- Propane Propanol
- Butane Butanol
- Cyclohexane Cyclohexanol
Common is also the former designation with the alkyl radical and the ending -alcohol (alkyl alcohol): methyl alcohol, ethyl alcohol, propyl alcohol etc. Aromatics that carry a hydroxyl group are called phenols. They do not belong to the alcohols: If a molecule contains several hydroxy groups, two groups are called a diol, three groups a triol and four groups a tetraol. These alcohols are also called monohydric, dihydric, trihydric and polyols. Depending on how many alkyl radicals the C atom with the hydroxyl group carries, the alcohols are divided into primary, secondary and tertiary alcohols:
- An alkyl radical: primary alcohol
- Two alkyl radicals: secondary alcohol
- Three alkyl radicals: tertiary alcohol
Representative
The best known example is ethyl alcohol or ethanol, also known as “the alcohol”. Ethanol is a well-known stimulant and intoxicant found in wine, beer and spirits. Methanol or “wood alcohol” (methyl alcohol) with the structure CH
3
-OH is the simplest alcohol. It is toxic and can cause blindness if ingested accidentally. Methanol is used primarily for technical purposes. Isopropanol (isopropy alcohol, propan-2-ol) is a disinfectant found, for example, in alcohol swabs and in hand sanitizers.
Properties
- Alcohols are more polar than the corresponding alkanes due to the hydroxy group and therefore especially the short-chain representatives are well miscible with water.
- Lower alcohols are liquids.
- The boiling point is higher than comparable alkanes because alcohols form intermolecular hydrogen bonds. The boiling point of thiols is also lower because the difference in electronegativity of sulfur and hydrogen is small.
- Alcohols are weak acids. For example, the pKa of ethanol is 16. The phenols are more acidic.
Reactions
Alcohols can be deprotonated with a strong base and protonated with a strong acid. With carboxylic acids, alcohols form esters. Alcohols are nucleophiles. Alcohols can be oxidized to aldehydes and carboxylic acids. Oxidation of benzyl alcohol to benzaldehyde and to benzoic acid:
In pharmacy.
Numerous active pharmaceutical ingredients contain a hydroxyl group. For example, the beta-blockers such as atenolol and metoprolol. Alcohols also play an important role as disinfectants, as solvents, extraction agents, and for chemical syntheses.
