Introduction
Alcohols are a family of organic compounds in which one or more hydroxyl (-OH) functional groups are bound to a saturated carbon atom. According to the amount of hydroxyl, they can be classified into monohydric alcohol, diol or polyol compound. A carbon atom does not normally contain two hydroxyl groups, which is unstable and easy to lose water to form a carbonyl compound. According to the number of hydrogens on the carbon atoms linked to the hydroxyl group, alcohols can also be classified into primary alcohols, secondary alcohols and tertiary alcohols (Fig 1). Alcohols can be prepared by fermentation from starchy crops such as wheat, or be prepared by chemical synthesis.
Fig.1. The classification of alcohols
Physical and chemical properties
Generally, alcohols are colorless liquids or solids: the ones with carbon atoms below 12 are liquid, the ones with 12 or more carbon atoms are solid, and polyols (such as glycerin) are syrupy liquid. The melting point and boiling point of lower alcohols are much higher than those alkanes with the same numbers of carbon atoms, which is due to the existence of hydrogen bonds between alcohol molecules. There are two opposing solubility trends in alcohols: the tendency of the polar OH to promote solubility in water, and the tendency of the carbon chain to resist it. Thus, methanol, ethanol, propanol and butanol are miscible in water, while alcohols with five or more carbons are insoluble in water.
The alcohol-chemical properties are active, like water, can show either acidic or basic properties at the -OH group. Their acidity is slightly weaker than water, but still able to react with strong bases such as sodium hydride. They can be oxidized to give aldehydes, ketones or carboxylic acids, or can be dehydrated to alkenes. In addition, they can react with carboxylic acids to form ester compounds, or undergo nucleophilic substitution reactions as a nucleophile.
Applications
Fig 2 Applications of alcohols
Solvent: Alcohols have a wide range of applications in industry and science. Because of their ability to dissolve non-polar substances, alcohols can be used as a solvent in medical drugs, perfumes, and vegetable essences. In organic synthesis, alcohols serve as versatile solvents or reagents.
Medical: Ethanol can be used as an antiseptic to disinfect the skin before injections are given, often using along with iodine. Ethanol-based soaps are becoming common in restaurants and are convenient because they do not require drying due to the volatility of the compound.
Alcohol fuel: Some alcohols, mainly ethanol and methanol, can be used as fuel. Fuel performance can be increased in forced induction internal combustion engines by injecting alcohol into the air intake after the turbocharger or supercharger has pressurized the air.
Alcoholic beverages: Alcoholic beverages typically containing 3–50% alcohol by volume, have been produced and consumed by humans. Natural fermentation also produces trace amounts of alcohols since pre-historic times.
