Hydrocarbons

Introduction

Hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. They form the backbone of organic chemistry and are classified into different types based on their structure and bonding. These compounds play a crucial role in fuels, industrial chemicals, and biological systems. Understanding hydrocarbons is essential for NEET aspirants, as they form the foundation of many organic reactions and mechanisms.

Classification of Hydrocarbons

Hydrocarbons are broadly classified into saturated and unsaturated hydrocarbons.

1. Saturated Hydrocarbons (Alkanes)

Saturated hydrocarbons, or alkanes, contain only single bonds between carbon atoms. They follow the general formula CₙH₂ₙ₊₂ and exhibit sp³ hybridization.

• Structure – Alkanes have a tetrahedral geometry around carbon atoms.

• Examples – Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈).

• Physical Properties – Alkanes are generally non-polar, have low boiling points, and are insoluble in water.

• Chemical Properties –

  • Combustion – Alkanes burn in oxygen to produce CO₂ and H₂O.

  • Substitution Reactions – Alkanes undergo halogenation in the presence of UV light.

  • Cracking – Large alkanes break down into smaller hydrocarbons at high temperatures.

2. Unsaturated Hydrocarbons

Unsaturated hydrocarbons contain at least one double or triple bond. They are classified into alkenes and alkynes.

a) Alkenes (Olefins)

Alkenes are hydrocarbons with one or more double bonds and follow the general formula CₙH₂ₙ.

• Structure – Alkenes have a trigonal planar geometry with sp² hybridization.

• Examples – Ethene (C₂H₄), Propene (C₃H₆).

• Physical Properties – They are slightly more reactive than alkanes and have moderate boiling points.

• Chemical Properties –

  • Addition Reactions – Hydrogenation, halogenation, and hydrohalogenation.

  • Oxidation – Cold KMnO₄ forms glycols, while hot KMnO₄ leads to cleavage.

  • Polymerization – Alkenes undergo polymerization to form plastics like polyethylene.

b) Alkynes

Alkynes contain at least one triple bond and follow the general formula CₙH₂ₙ₋₂.

• Structure – They have a linear geometry with sp hybridization.

• Examples – Ethyne (C₂H₂), Propyne (C₃H₄).

• Physical Properties – Similar to alkenes but with higher boiling points.

• Chemical Properties –

  • Addition Reactions – Hydrogenation and halogenation.

  • Oxidation – Cleavage with KMnO₄ forms carboxylic acids.

  • Acidic Nature – Terminal alkynes react with strong bases to form acetylides.

3. Aromatic Hydrocarbons

Aromatic hydrocarbons, or arenes, contain a benzene ring and exhibit resonance stability.

• Structure – Benzene (C₆H₆) is the simplest aromatic hydrocarbon with a hexagonal ring structure.

• Examples – Benzene, Toluene, Xylene.

• Physical Properties – They are non-polar, have a distinct smell, and are less dense than water.

• Chemical Properties –

  • Electrophilic Substitution – Nitration, halogenation, sulfonation, Friedel-Crafts reactions.

  • Combustion – Produces sooty flame due to high carbon content.

Sources of Hydrocarbons

1. Natural Sources

• Petroleum – Crude oil is a major source of alkanes, alkenes, and aromatics.

• Natural Gas – Mainly consists of methane, ethane, and propane.

• Coal – Source of aromatic hydrocarbons through destructive distillation.

2. Synthetic Sources

• Laboratory Synthesis – Hydrocarbons can be synthesized using organic reactions.

• Cracking of Alkanes – Used in the petroleum industry to produce smaller hydrocarbons.

Uses of Hydrocarbons

• As Fuels – Petrol, diesel, LPG, and CNG are hydrocarbons.

• Industrial Applications – Used in plastics, rubber, and synthetic fibers.

• Medical Field – Some hydrocarbons are used in drug synthesis.

Conclusion

Hydrocarbons form the backbone of organic chemistry and have widespread applications. Understanding their classification, properties, and reactions is crucial for NEET aspirants, as they frequently appear in exams.