Hydrocarbons – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Classification of Hydrocarbons

Hydrocarbons are organic compounds that consist solely of hydrogen and carbon atoms. They are classified into:

• Alkanes (Saturated hydrocarbons): Compounds with only single bonds between carbon atoms (e.g., methane, ethane).
• Alkenes (Unsaturated hydrocarbons): Compounds with at least one double bond between carbon atoms (e.g., ethene, propene).
• Alkynes (Unsaturated hydrocarbons): Compounds with at least one triple bond between carbon atoms (e.g., ethyne, propyne).
• Aromatic hydrocarbons: Compounds that contain a benzene ring as a part of their structure (e.g., benzene, toluene).

2. Isomerism in Hydrocarbons

Hydrocarbons exhibit different types of isomerism, including:

• Structural Isomerism: Same molecular formula but different structural arrangements (e.g., butane and isobutane).
• Geometrical Isomerism: Occurs in alkenes due to the restricted rotation around the double bond (e.g., cis and trans forms).
• Optical Isomerism: Occurs when there is a chiral center in the molecule, as seen in some alkynes and substituted benzene compounds.

3. IUPAC Nomenclature of Hydrocarbons

Hydrocarbons are named according to IUPAC rules. For alkanes, alkenes, and alkynes, the name is derived from the longest chain of carbon atoms, followed by the appropriate suffix (-ane for alkanes, -ene for alkenes, -yne for alkynes). Substituents are named based on their position in the chain.

4. General Methods of Preparation of Hydrocarbons

• Alkanes: Alkanes are typically prepared by the reduction of alkenes or alkynes, or by the Wurtz reaction.
• Alkenes: Alkenes are prepared by elimination reactions such as dehydrohalogenation of alkyl halides or dehydration of alcohols.
• Alkynes: Alkynes are prepared by the dehydrohalogenation of dihalides or by the reaction of alkynyl compounds with alkyl halides.
• Aromatic Hydrocarbons: Aromatic hydrocarbons like benzene are prepared by catalytic reforming or from coal tar.

5. Properties and Reactions of Hydrocarbons

Hydrocarbons undergo various types of reactions:

Alkanes

Alkanes are relatively unreactive but can undergo reactions such as:

• Halogenation: The reaction of alkanes with halogens in the presence of heat or light, resulting in alkyl halides.
• Conformations: The study of the different spatial arrangements of atoms in a molecule due to rotation about single bonds, such as in ethane. These can be visualized using Sawhorse and Newman projections.

Alkenes

Alkenes are more reactive than alkanes due to the presence of a double bond and can undergo:

• Electrophilic Addition: Addition of halogens, hydrogen halides, and water across the double bond.
• Hydrogenation: The addition of hydrogen to alkenes in the presence of a catalyst, resulting in alkanes.
• Markovnikov’s Rule: When hydrogen halides are added to alkenes, the hydrogen atom adds to the carbon with the greater number of hydrogen atoms.
• Peroxide Effect: Addition of hydrogen halides in the presence of peroxides gives anti-Markovnikov products.
• Ozonolysis: The cleavage of alkenes with ozone, producing aldehydes or ketones.
• Polymerization: The process where alkenes undergo chain growth to form polymers.

Alkynes

Alkynes also undergo reactions similar to alkenes:

• Acidic Character: Alkynes exhibit acidic behavior due to the sp hybridization of carbon, making the hydrogen attached to the triple bond relatively acidic.
• Addition of Hydrogen: Alkynes can undergo hydrogenation to form alkanes.
• Halogenation: Addition of halogens to alkynes forms tetrahalides.
• Addition of Water: Alkynes undergo hydration to form carbonyl compounds (ketones or aldehydes).
• Polymerization: Alkynes can undergo polymerization to form larger carbon-based compounds.

Aromatic Hydrocarbons

Aromatic hydrocarbons like benzene have unique reactions due to their resonance and aromaticity:

• Electrophilic Substitution: The primary reaction mechanism of aromatic hydrocarbons, including halogenation, nitration, and sulfonation.
• Friedel-Crafts Alkylation and Acylation: Reactions in which an alkyl or acyl group is introduced into the benzene ring using a catalyst like AlCl₃.
• Directive Influence: Substituents on the benzene ring influence the positions of incoming groups. Electron-donating groups activate the ring at the ortho- and para-positions, while electron-withdrawing groups activate it at the meta-position.