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How are aliphatic and aromatic hydrocarbons classified in modern chemistry?

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structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

In modern chemistry, the classification of hydrocarbons is based on their molecular structure rather than their historical origins or properties. While nineteenth-century chemists categorized them as aliphatic or aromatic based on their sources, modern terminology retains these names but defines them by their structural characteristics.12

Aliphatic Hydrocarbons

Aliphatic hydrocarbons are classified into three primary groups determined by the types of bonds present between carbon atoms:1
* Alkanes: Contain only single bonds and are considered saturated hydrocarbons.12
* Alkenes: Contain at least one carbon-carbon double bond.12
* Alkynes: Contain at least one carbon-carbon triple bond.12

Alkenes and alkynes are classified as unsaturated hydrocarbons.1

Aromatic Hydrocarbons

Aromatic hydrocarbons are defined by their "special stability," meaning they are significantly more stable than their Lewis structures would suggest. They are categorized as:13
* Arenes (Benzenoid): Compounds that contain a benzene ring as a structural unit.13
* Nonbenzenoid aromatic hydrocarbons: Compounds that lack a benzene ring but still possess special stability, often determined by the Hückel rule regarding cyclic conjugated systems.143