Nomenclature and Properties of Alcohols - SS2 Chemistry Lesson Note
Alcohols are organic compounds characterised by the presence of a hydroxyl (-OH) functional group attached to a carbon atom. They are versatile compounds widely used in various applications, ranging from solvents and disinfectants to pharmaceuticals and fuels.
Nomenclature of Alcohols:
The International Union of Pure and Applied Chemistry (IUPAC) establishes systematic rules for naming alcohols based on the parent hydrocarbon and the hydroxyl group.
IUPAC Naming Rules:
● Identify the parent hydrocarbon chain, which contains the carbon atom attached to the hydroxyl group.
● Assign the appropriate ending "-ol" to the parent hydrocarbon name.
● Number the carbon atoms in the chain to locate the position of the hydroxyl group.
● Indicate the position of the hydroxyl group by the lowest possible number.
● If there are multiple hydroxyl groups, use prefixes like di-, tri-, etc., and include their positions in the name.
Examples of IUPAC Names:
● Methanol: The simplest alcohol, with the structure CH3OH.
● Ethanol: A two-carbon alcohol, with the structure CH3CH2OH.
● 2-Propanol: A three-carbon alcohol with the hydroxyl group attached to the second carbon atom.
Properties of Alcohols:
Alcohols possess several important properties that arise from the presence of the hydroxyl group. These properties contribute to their diverse applications and reactivity.
Physical Properties:
● Solubility: Lower-molecular-weight alcohols (methanol, ethanol) are miscible with water due to hydrogen bonding between the hydroxyl group and water molecules. Solubility decreases with increasing carbon chain length.
● Boiling Points: Alcohols have higher boiling points compared to hydrocarbons of similar molecular weight due to intermolecular hydrogen bonding between alcohol molecules.
Chemical Properties:
● Reactivity: The hydroxyl group imparts reactivity to alcohols, making them prone to various chemical reactions.
● Acid-Base Behaviour: Alcohols can act as both weak acids and weak bases. They can donate a proton from the hydroxyl group, behaving as a Brønsted-Lowry acid, or accept a proton, acting as a base.
● Oxidation: Alcohols can undergo oxidation reactions to form aldehydes, ketones, or carboxylic acids, depending on the oxidising agent and reaction conditions.
● Esterification: Alcohols react with carboxylic acids to form esters, a reaction important in the synthesis of fragrances, flavours, and solvents.
● Dehydration: Alcohols can undergo dehydration reactions to form alkenes, where a water molecule is eliminated from the alcohol molecule.
Classification of Alcohols:
Alcohols can be classified based on the number of alkyl groups attached to the carbon atom bearing the hydroxyl group:
● Primary Alcohols: The carbon attached to the hydroxyl group is bonded to one alkyl group.
● Secondary Alcohols: The carbon attached to the hydroxyl group is bonded to two alkyl groups.
● Tertiary Alcohols: The carbon attached to the hydroxyl group is bonded to three alkyl groups.
The classification of alcohols influences their reactivity and properties. For example, tertiary alcohols are more sterically hindered and exhibit different oxidation and elimination reactions compared to primary and secondary alcohols.
Alcohols are important organic compounds with diverse applications. Nomenclature provides a systematic approach to name alcohols based on the IUPAC rules, allowing for clear and unambiguous identification.
The hydroxyl group in alcohols imparts unique properties, such as solubility, boiling points, and the ability to undergo oxidation, esterification, and dehydration reactions. The classification of alcohols based on the number of alkyl groups attached to the hydroxyl-bearing carbon atom further influences their behaviour and reactivity.