Unit 7: Comprehensive Summary

Organic Chemistry is the study of carbon compounds. Carbon's ability to form four strong covalent bonds and undergo catenation (linking to itself) allows for a vast diversity of molecules.

Organic molecules are classified by their carbon skeleton (aliphatic, alicyclic, branched, unbranched) and their functional group (the reactive part of the molecule). A homologous series is a family of compounds with the same functional group and general formula, showing similar chemical properties and a gradual trend in physical properties.

Molecules are represented using displayed (all bonds), structural (condensed), skeletal (stick), molecular (atom count), and empirical (simplest ratio) formulae. IUPAC nomenclature provides systematic rules for naming compounds based on their structure.

Isomers are molecules with the same molecular formula but different arrangements of atoms.

• Structural Isomerism: Different connectivity of atoms.
  • Chain Isomerism: Different carbon skeleton (e.g., butane and methylpropane).
  • Positional Isomerism: Same skeleton, but the functional group is in a different position (e.g., propan-1-ol and propan-2-ol).
  • Functional Group Isomerism: Different functional groups (e.g., an alkene and a cycloalkane like hexene and cyclohexane).
• Stereoisomerism: Same connectivity, different spatial arrangement.
  • Geometrical (Cis-Trans) Isomerism: Occurs in alkenes due to restricted rotation about the C=C double bond. Requires each carbon of the double bond to be attached to two different groups.

Crude oil is the primary source of hydrocarbons. It is separated into fractions (mixtures with similar boiling points) by fractional distillation.

Industrial processes are used to modify these fractions:

• Cracking: Breaks large hydrocarbons into smaller, more valuable alkanes and alkenes using high temperature and a catalyst.
• Reforming: Rearranges straight-chain alkanes into branched-chain and aromatic compounds to increase the octane number and improve fuel quality.

Combustion of alkanes is a major source of energy. Complete combustion (excess oxygen) produces $CO_2$ and $H_2O$. Incomplete combustion (limited oxygen) produces toxic $CO$ and/or soot (C). Burning fossil fuels contributes to environmental problems like global warming (from $CO_2$) and acid rain (from $SO_2$ and $NO_x$).

• Alkanes (Saturated): Relatively unreactive. Undergo free-radical substitution with halogens in the presence of UV light.
• Alkenes (Unsaturated): Reactive due to the C=C double bond. Undergo electrophilic addition reactions with $H_2$, halogens ($Br_2$), hydrogen halides ($HBr$), and water. The reaction with bromine water (decolourisation) is the test for unsaturation.
• Alkynes (Unsaturated): Also undergo addition reactions, similar to alkenes.
• Benzene (Aromatic): Unusually stable due to its delocalised pi electron ring. It does not readily undergo addition. Its characteristic reaction is electrophilic substitution, which preserves the stable aromatic system.