Classification

9.1 Classification of Hydrocarbons

Hydrocarbons are the simplest organic compounds — they contain only carbon and hydrogen. But the way those carbon atoms link to each other changes everything. The classification of hydrocarbons rests entirely on the types of carbon-carbon bonds present in the molecule.

There are three broad categories: saturated hydrocarbons, unsaturated hydrocarbons, and aromatic hydrocarbons.

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Saturated Hydrocarbons

A saturated hydrocarbon contains only carbon-carbon single bonds and carbon-hydrogen single bonds. Every carbon atom is bonded to four other atoms (it is tetravalent), and every hydrogen atom is bonded to one carbon (it is monovalent). No carbon-carbon double or triple bonds exist.

Saturated hydrocarbons are further divided into two structural types:

Alkanes — Here, the carbon atoms form an open chain (a straight or branched chain) with only single bonds. You have already studied alkanes in Unit 8. Their general formula is $C_nH_{2n+2}$.

Cycloalkanes — Here, the carbon atoms form a closed chain or a ring. The general formula for a cycloalkane with one ring is $C_nH_{2n}$.

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Unsaturated Hydrocarbons

Unsaturated hydrocarbons contain at least one carbon-carbon multiple bond — a double bond ($C=C$), a triple bond ($C\equiv C$), or both types in the same molecule.

The presence of a multiple bond means the molecule has fewer hydrogen atoms than the corresponding saturated hydrocarbon. This "unsaturation" makes these compounds more reactive — they can add more hydrogen atoms (or other atoms) across the multiple bond.

The two main families are:

• Alkenes — contain at least one carbon-carbon double bond. General formula: $C_nH_{2n}$ (for one double bond, open chain).
• Alkynes — contain at least one carbon-carbon triple bond. General formula: $C_nH_{2n-2}$ (for one triple bond, open chain).

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Aromatic Hydrocarbons

Aromatic hydrocarbons are a special type of cyclic compound. They are not simply unsaturated rings — they have a unique stability and reactivity pattern that comes from a delocalised electron system (a concept you will study in detail later). The simplest example is benzene, $C_6H_6$.

Aromatic compounds do not behave like typical alkenes or alkynes, even though they appear to contain multiple bonds. Their classification as a separate category is essential because their chemistry is fundamentally different.

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Building Molecular Models

You can construct physical models of hydrocarbons to understand their three-dimensional structure. The textbook suggests a simple approach: …