Structural Isomerism and Functional Groups

Structural isomers share the same molecular formula but display different arrangements of atoms. While they maintain similar chemical properties due to identical electron counts, their physical properties can vary significantly. Three main types of structural isomerism exist:

1. Chain isomerism: Different arrangements of the carbon backbone
2. Positional isomerism: Varying locations of functional groups
3. Functional group isomerism: Different functional groups with the same molecular formula

Highlight: Functional groups are specific atoms or combinations of atoms that determine the characteristic chemical properties of organic compounds.

Functional groups can act as either Brønsted-Lowry acids or bases. Acidic functional groups, like carboxylic acids, can donate protons to form carboxylate ions. Basic functional groups, typically containing nitrogen atoms with lone pairs, can accept protons.

Benzene Structure and Properties

Benzene (C₆H₆) represents a unique case in organic chemistry that challenged traditional structural theories. While the Kekulé structure suggested alternating single and double bonds, experimental evidence revealed a more complex reality.

The actual benzene structure features a ring of delocalized electrons, creating an electron density above and below the carbon atoms. This delocalization explains why benzene's properties differ significantly from what the Kekulé structure would predict.

Highlight: Benzene's actual structure shows equal bond lengths of 0.14nm, contrary to the Kekulé model's prediction of alternating bond lengths.

The stability of benzene is demonstrated through its enthalpy of hydrogenation, which at -210 kJ mol⁻¹ is significantly less than the -360 kJ mol⁻¹ predicted by the Kekulé structure. This enhanced stability results from electron delocalization.

Understanding Functional Groups and Chemical Reactions in Organic Chemistry

Characteristics of homologous series and their reactivity patterns form the foundation of organic chemistry. Compounds are classified as saturated when they contain only single carbon-carbon bonds, while unsaturated compounds feature double or triple bonds between carbon atoms. This fundamental distinction greatly influences their chemical behavior and reactions.

The homologous series of alkanes represents the simplest family of saturated hydrocarbons. These compounds demonstrate relatively low reactivity due to their non-polar nature and strong covalent bonds between carbon-carbon and carbon-hydrogen atoms. However, alkanes readily undergo combustion reactions, which are always exothermic processes. During complete combustion in excess oxygen, alkanes produce carbon dioxide and water vapor. In contrast, incomplete combustion occurs with limited oxygen supply, resulting in carbon monoxide, water vapor, and solid carbon (soot) formation.

Definition: Free-radical substitution is a type of reaction where atoms or groups of atoms are replaced through a mechanism involving radical intermediates, typically initiated by UV light.

One of the most significant reactions of alkanes is free-radical substitution, particularly with halogens like chlorine or bromine. This process occurs through distinct mechanisms: heterolytic fission, where both shared electrons transfer to one atom creating charged ions, and homolytic fission, where electrons split equally between atoms forming free radicals. The reaction proceeds through initiation (triggered by UV light) and propagation steps, ultimately producing halogenoalkanes and hydrogen halides.

Example: When methane (CH4) reacts with chlorine (Cl2) under UV light, it forms chloromethane (CH3Cl) and hydrogen chloride (HCl) through a free-radical substitution mechanism.