JEE Chemistry Hydrocarbons Complete Guide

Hydrocarbons are the foundation of organic chemistry and a crucial topic in JEE syllabus. They consist entirely of carbon and hydrogen atoms, classified mainly as saturated and unsaturated compounds. Mastering hydrocarbons is essential for solving a variety of organic chemistry problems in JEE Mains and Advanced exams.

1. Classification of Hydrocarbons

Hydrocarbons are broadly classified into two categories based on the type of bonding between carbon atoms:

• Saturated Hydrocarbons (Alkanes): All carbon-carbon bonds are single bonds (sigma bonds).
• Unsaturated Hydrocarbons: Contain one or more carbon-carbon double or triple bonds.
  • Alkenes: Contain at least one double bond (C=C).
  • Alkynes: Contain at least one triple bond (C \equiv C).
• Aromatic Hydrocarbons: Contain benzene rings or related conjugated ring structures.

2. Nomenclature of Hydrocarbons

The IUPAC system provides a systematic way to name hydrocarbons. Key rules include:

• Identify the longest carbon chain as the parent name (meth-, eth-, prop-, but-, etc.).
• Number the chain from the end nearest to a substituent or multiple bond.
• Name substituents and their positions.
• For multiple bonds, suffixes are -ene for double bonds and -yne for triple bonds.
• Use prefixes like di-, tri-, tetra- for multiple identical substituents.

Example:

CH_3-CH=CH-CH_3 is named But-2-ene.

3. Alkanes (Saturated Hydrocarbons)

Alkanes have the general formula C_nH_{2n+2}. They are saturated and mostly unreactive except in certain reactions.

3.1 Physical Properties

• Nonpolar molecules, insoluble in water.
• Boiling points increase with molecular weight.
• Low density compared to water.

3.2 Chemical Properties

• Combustion: Complete and incomplete combustion reactions.
• Substitution Reactions: Halogenation under UV light.
• Cracking: Thermal and catalytic cracking to produce smaller alkanes and alkenes.

3.3 Important Reactions

\mathrm{CH_4} + \mathrm{Cl_2} \xrightarrow{h\nu} \mathrm{CH_3Cl} + \mathrm{HCl}

\mathrm{C_4H_{10}} \xrightarrow{\text{Cracking}} \mathrm{C_2H_4} + \mathrm{C_2H_6}

4. Alkenes (Unsaturated Hydrocarbons)

Alkenes have at least one carbon-carbon double bond, with general formula C_nH_{2n}. The double bond introduces reactivity and geometric isomerism.

4.1 Physical Properties

• Nonpolar, with boiling points slightly lower than corresponding alkanes.
• Soluble in organic solvents but insoluble in water.

4.2 Chemical Properties

• Addition Reactions: Electrophilic addition across the double bond, e.g., hydrogenation, halogenation.
• Polymerization: Alkenes can undergo addition polymerization.
• Oxidation: Mild oxidation to form diols or cleavage by strong oxidants.

4.3 Important Reactions

\mathrm{CH_2=CH_2} + \mathrm{H_2} \xrightarrow{\text{Ni}} \mathrm{C_2H_6}

\mathrm{CH_2=CH_2} + \mathrm{Br_2} \rightarrow \mathrm{CH_2Br-CH_2Br}

5. Alkynes (Unsaturated Hydrocarbons)

Alkynes contain at least one carbon-carbon triple bond, with general formula C_nH_{2n-2}. The triple bond adds unique chemical properties.

5.1 Physical Properties

• Similar to alkenes but with higher acidity at terminal alkynes.

5.2 Chemical Properties

• Addition Reactions: Similar to alkenes but can add two equivalents of reagents.
• Acidity: Terminal alkynes have acidic hydrogen capable of forming acetylide ions.
• Reduction: Alkynes can be reduced to alkenes or alkanes selectively.

5.3 Important Reactions

\mathrm{HC \equiv CH} + \mathrm{2H_2} \xrightarrow{\text{Pd/BaSO_4}} \mathrm{CH_3-CH_3}

\mathrm{HC \equiv CH} + \mathrm{NaNH_2} \rightarrow \mathrm{HC \equiv C^- Na^+} + \mathrm{NH_3}

6. Aromatic Hydrocarbons

Aromatic hydrocarbons contain benzene rings or similar conjugated cyclic systems with special stability due to resonance.

6.1 Benzene Structure

Benzene has a planar hexagonal ring with alternating double bonds represented by resonance structures.

\mathrm{C_6H_6}

6.2 Properties

• Unusual stability known as aromaticity.
• Undergoes substitution rather than addition reactions to preserve aromaticity.

6.3 Important Reactions

• Nitration: \( \mathrm{C_6H_6} + \mathrm{HNO_3} \xrightarrow{H_2SO_4} \mathrm{C_6H_5NO_2} + \mathrm{H_2O} \)
• Halogenation: \( \mathrm{C_6H_6} + \mathrm{Cl_2} \xrightarrow{FeCl_3} \mathrm{C_6H_5Cl} + \mathrm{HCl} \)
• Friedel-Crafts Alkylation and Acylation

7. Mechanisms of Key Reactions

Understanding reaction mechanisms helps solve organic problems efficiently.

7.1 Free Radical Substitution in Alkanes

• Initiation: Formation of radicals under UV light.
• Propagation: Radical reacts with alkane forming new radicals.
• Termination: Radicals combine to form stable molecules.

7.2 Electrophilic Addition in Alkenes

• Electrophile attacks the pi bond forming a carbocation intermediate.
• Nucleophile attacks the carbocation to complete the addition.

8. Important Tips for JEE Preparation

• Memorize general formulas and nomenclature rules.
• Practice naming and drawing structures of hydrocarbons.
• Understand mechanisms, especially free radical substitution and electrophilic addition.
• Revise reactions of benzene and its derivatives thoroughly.
• Solve previous years' JEE problems on hydrocarbons for better conceptual clarity.

9. Summary Table of Hydrocarbons

This guide provides a thorough understanding of hydrocarbons essential for JEE Chemistry preparation. Continuous practice and conceptual clarity are the keys to mastering this topic.