Define the term hydrocarbon.

At the heart of organic chemistry are organic compounds—substances characterized by the presence of carbon atoms. Carbon is a versatile element, capable of forming strong covalent bonds with other elements, most notably hydrogen. While organic compounds may contain other elements like oxygen, nitrogen, and sulfur, hydrocarbons are a specific subset that includes only carbon and hydrogen.

Within the realm of organic compounds, hydrocarbons are recognized for their simplicity and prevalence. They serve as fundamental building blocks from which more complex molecules, such as proteins, DNA, and plastics, are constructed.

Aliphatic hydrocarbons are comprised of carbon and hydrogen atoms arranged in straight chains, branched chains, or non-aromatic rings. They are subdivided into three groups:

• Alkanes which possess single bonds and are saturated, such as propane (\(C_3H_8\)).
• Alkenes which feature at least one double bond, making them unsaturated; for instance, butene (\(C_4H_8\)).
• Alkynes where at least one triple bond is present, also unsaturated, like acetylene (\(C_2H_2\)).

The absence of aromatic rings means that aliphatic hydrocarbons are generally less stable than their aromatic counterparts, hence more reactive.

Aromatic hydrocarbons, or arenes, are a class of hydrocarbons distinguished by at least one benzene-like ring structure. The defining feature of these molecules is the delocalization of π-electrons across the ring, which grants them unique chemical properties such as increased stability and specific reactivity patterns. Benzene (\(C_6H_6\)) is the archetypal example of an aromatic hydrocarbon, and it is the basis for a multitude of derivative compounds used across various industries.

Hydrocarbons exhibit diverse chemical properties that are influenced by their molecular structure. The presence of double or triple bonds in alkenes and alkynes introduces possibilities for chemical reactions like polymerization and addition reactions. Conversely, the saturated alkanes are relatively inert, often requiring higher temperatures or catalysts to engage in reactions such as combustion or substitution.

Aromatic hydrocarbons also participate in unique chemical reactions, such as electrophilic aromatic substitution, that capitalize on the stable aromatic ring system. Understanding these properties is key for synthesizing new compounds and for predicting the reactivity of hydrocarbons in different environments.

The vast range of applications for hydrocarbons is a testament to their significance. Fuels like gasoline and diesel are refined from crude oil, which is primarily composed of hydrocarbons. These fuels are essential for transportation and energy generation.

In the petrochemical industry, hydrocarbons are the foundation for creating plastics, synthetic rubbers, and solvents—materials that are integral to modern life. In pharmaceuticals, hydrocarbons are used as starting materials in the synthesis of drugs.

Moreover, as a consideration for environmental sustainability, the combustion of hydrocarbons is a focus for the development of greener energy practices and the regulation of greenhouse gas emissions.