What are Hydrocarbons? Types, Examples, and Formulas

As the name says, Hydrogen+Carbons (Hydrocarbons) are organic compounds made of only hydrogen (H) and Carbon (C) atoms.

Hydrocarbons can be of three types: Alkanes, Alkenes, and Alkynes.

Hydrocarbons are the main compounds of petroleum and natural gases, used as a fuel and raw material in various resources such as rubber, fuel, fabric, etc. They are highly combustible, releasing carbon dioxide and heat when burned.

Hydrocarbons are classified in three categories: Saturated, Unsaturated, and Aromatic Hydrocarbons. The classification of hydrocarbons is based on the type of carbon bond present.

For example,

Saturated Hydrocarbons (Alkanes) have C-C and C-H single bonds.

Unsaturated Hydrocarbons have C-C multiple bonds such as double bonds, triple bonds, or both.

Aromatic Hydrocarbons are cyclic compounds, i.e., Benzene (C6H6).

Alkanes are saturated open-chain compounds that contain only single bonds. The general formula for alkanes is CnH2n+2. Each carbon is sp3 hybridised with a bond angle of 109.5 degrees.

For example-

Methane, Ethane, and Propane

The physical properties of alkanes are nonpolar, insoluble in water, and soluble in organic solvents. The chemical properties are Combustion (CO2+ H2O), Halogenation (free radical substitution with halogens in the presence of light/heat), and Isomerism (structural isomerism from C4H10 onwards).

The general formula for alkenes is CnH2n. Alkenes are unsaturated hydrocarbons with a C=C double bond. Each carbon is sp2 hybridised with a bond angle of 120 degrees.

Its physical properties are Nonpolar, insoluble in water, and soluble in nonpolar solvents, and its chemical properties are:

Addition reactions:

Hydrogenation → alkanes

Halogenation → dihaloalkanes

Hydrohalogenation → Markovnikov’s rule; Peroxide effect (anti-Markovnikov)

Hydration → alcohols

Oxidation:

Baeyer’s test (alkaline KMnO₄) → glycols

Ozonolysis → aldehydes/ketones

Polymerisation: Ethene → polythene

Isomerism: Structural and geometrical (cis-trans)

Alkynes are unsaturated hydrocarbons containing a Carbon-Carbon triple bond. The general formula is CnH2n-2. Each carbon of the triple bond is sp-hybridised, linear, 180°.

Physical Properties of Alkynes are similar of alkanes and alkenes, and the chemical properties are:

Addition reactions:

With hydrogen → alkenes → alkanes

With halogens/halogen acids → substituted alkenes/alkanes

Oxidation: Strong oxidising agents → acids or CO₂ + H₂O

Acidic nature: Terminal alkynes are acidic (react with Na, AgNO₃, Cu₂Cl₂).

C6H6 (Benzene) is an ideal example of Aromatic Hydrocarbons. The Hydrocarbons that contain a benzene ring and similar structures are Aromatic Hydrocarbons.

Benzene structure:

Molecular formula: C₆H₆

Planar, cyclic, conjugated system

All C-C bonds are equal in length due to resonance

Obeys Hückel’s rule (4n+2 π electrons, n=1 → 6 π electrons)

The chemical properties of Aromatic Hydrocarbons are:

Lectrophilic Substitution Reactions (EAS):

Nitration:
C₆H₆ + HNO₃ → C₆H₅NO₂ (with H₂SO₄)

Halogenation:
C₆H₆ + Cl₂ → C₆H₅Cl (FeCl₃ catalyst)

Sulphonation:
C₆H₆ + SO₃ → C₆H₅SO₃H (conc. H₂SO₄)

Friedel-Crafts alkylation:
 C₆H₆ + CH₃Cl → C₆H₅CH₃ (AlCl₃ catalyst)

Side chain oxidation:
 C₆H₅–CH₃ → C₆H₅–COOH (benzoic acid, by KMnO₄ oxidation).

Many polynuclear aromatic hydrocarbons (PAHs) with more than two benzene rings combine and become cancer-causing, such as benz[a]pyrene, benzanthracene, which are present in tobacco smoke, coal tar, and automobile exhaust. They are carcinogenic.