Isomerism

Section: Understanding Isomerism

• Isomerism is a key concept in chemistry that indicates when two or more compounds have the same molecular formula but a different arrangement of atoms.
• This phenomenon can greatly affect the physical and chemical properties of the compounds, including how they function as fuels.
• Isomerism can be broadly classified into two types: structural (or constitutional) isomerism, and stereoisomerism.

Section: Structural Isomerism

• Structural isomerism happens when the compounds have the same molecular formula but differ in their structural arrangement.
• This can be further classified into five types: chain isomerism, functional group isomerism, positional isomerism, tautomeric isomerism and ring-chain isomerism.
• Chain isomerism occurs when carbon compounds form different structural chains, for example butane and 2-methylpropane.
• Functional group isomerism involves the same atoms arranged into different functional groups, such as alcohol and ether.
• Positional isomerism occurs when functional groups exist at different positions on the same carbon chain.
• In fuels, the type of isomer can significantly affect combustion characteristics.

Section: Stereoisomerism

• Stereoisomerism occurs when compounds have the same structural formula but different spatial arrangement of atoms.
• This is further divided into geometrical and optical isomerism.
• Geometrical isomerism (or cis-trans isomerism) is seen in alkenes where rotation around the carbon-carbon double bond is not possible, leading to different spatial arrangements.
• Optical isomerism is associated with compounds that can rotate plane-polarised light, due to a molecule with non-superimposable mirror image, often marked by the presence of a chiral centre.
• Stereoisomers may have different combustion properties, influencing the efficiency and emissions of fuels.

Section: Importance of Isomerism in Fuels

• Understanding isomerism is crucial in the development of fuels as different isomers of the same hydrocarbon have different combustion properties.
• For instance, branched-chain isomers, which come under chain isomerism, generally burn more cleanly compared to straight-chain isomers.
• Similarly, the presence of different functional groups (functional group isomerism) will result in different combustion reactions and efficiencies.
• Thus, isomerism allows for the tailoring of fuel properties to enhance efficiency and reduce harmful emissions.