Introduction to Alkanes and Nomenclature

Introduction to Alkanes

• Alkanes are a class of saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms and all carbon-carbon bonds are single bonds.
• They are the simplest organic compounds.
• The general formula for alkanes is CnH2n+2, where 'n' represents the number of carbon atoms.
• Alkanes are relatively unreactive, due to the strong C-H and C-C sigma bonds.

Nomenclature of Alkanes

• Alkanes are named according to the number of carbon atoms in the longest continuous chain.
• Prefixes are used to indicate the number of carbon atoms in the chain: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10), etc.
• Suffix -ane indicates that the compound is an alkane.
• Branches (alkyl groups) are named as prefixes and their location specified by numbers.
• The location of the branch is determined by the carbon atom on the main chain that the branch is attached to, with the lowest possible number being used.
• Example: 2-methylpentane (a 5-carbon chain with a methyl group on the second carbon).

Isomerism in Alkanes

• Isomers are compounds with the same molecular formula but different structural formulas.
• Alkanes exhibit structural isomerism, where the same number of atoms arrange differently in space.
• Different isomers have different physical and chemical properties.
• Branched-chain alkanes have lower boiling points than straight-chain alkanes with the same number of carbon atoms (due to weaker dispersion forces).

Physical Properties of Alkanes

• Alkanes are nonpolar molecules.
• They are generally colorless and odorless.
• They are insoluble in water, but soluble in non-polar solvents.
• Boiling points increase with increasing molecular weight due to stronger London dispersion forces.
• Melting points increase with increasing chain length but branches decrease melting point due to the more compact structure.

Chemical Properties of Alkanes

• Alkanes are generally unreactive; they do not readily undergo reactions like addition reactions.
• The dominant reaction type for alkanes is combustion (reaction with oxygen).
• Alkanes undergo combustion to form carbon dioxide and water, releasing a large amount of energy.
• Complete combustion requires a sufficient supply of oxygen. Incomplete combustion produces carbon monoxide (a toxic gas) and carbon soot.
• Alkanes can undergo substitution reactions in the presence of light or heat with certain halogens; this reaction replaces a hydrogen atom of the alkane with a halogen atom.
• The reaction with halogens is a free radical chain reaction.
• Halogenation reactions are initiated by ultraviolet (UV) light or heat.

Uses of Alkanes

• Alkanes are a major source of energy.
• They are used as fuels (petrol, diesel, kerosene).
• Alkanes are also used as raw materials for producing other chemicals.
• They have various industrial applications such as solvents, waxes, and lubricants.

Important Concepts

• The concept of structural formula, isomers, and the impact of structure on the physical and chemical properties of alkanes is a central and essential part of organic chemistry.