Hydrocarbons: Comprehensive NEET Chemistry Notes

1. Introduction to Hydrocarbons

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon atoms. They are the primary constituents of many fuels and are essential in various industrial applications.

1.1 Types of Hydrocarbons

Hydrocarbons are classified into three main categories:

Saturated Hydrocarbons (Alkanes): Contain only single bonds between carbon atoms.
Unsaturated Hydrocarbons: Contain one or more double (alkenes) or triple (alkynes) bonds between carbon atoms.
Aromatic Hydrocarbons: Contain one or more aromatic rings, such as benzene.

Did You Know?

LPG (Liquefied Petroleum Gas) and CNG (Compressed Natural Gas) are common fuels derived from hydrocarbons.

Real-life Application:

Hydrocarbons are used in the production of polymers like polyethylene and in pharmaceuticals.

2. Alkanes

Alkanes are saturated hydrocarbons with the general formula $C_{n} H_{2 n + 2}$

2.1 Structure and Properties

Alkanes have a tetrahedral geometry around each carbon atom with bond angles of 109.5°. They are relatively inert but undergo reactions like combustion and substitution.

2.2 Nomenclature and Isomerism

Alkanes are named using the IUPAC system based on the number of carbon atoms. Isomerism in alkanes includes chain isomerism.

Mnemonic:

"My Elephant Prefers Bananas" for the first four alkanes: Methane, Ethane, Propane, Butane.

2.3 Preparation

Alkanes can be prepared by hydrogenation of alkenes and alkynes, reduction of alkyl halides, and decarboxylation of carboxylic acids.

2.4 Reactions

Combustion: Alkanes burn in oxygen to produce carbon dioxide, water, and energy. $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$
Substitution: In the presence of halogens and UV light, alkanes undergo substitution reactions.

Common Misconception:

Alkanes are often thought to be completely unreactive, but they do react under certain conditions, such as high temperatures or with specific catalysts.

3. Alkenes

Alkenes are unsaturated hydrocarbons containing at least one double bond, with the general formula $C_{n} H_{2 n}$ .

3.1 Structure and Properties

Alkenes have a planar structure around the double bond with bond angles of approximately 120°. The presence of the double bond makes them more reactive than alkanes.

3.2 Nomenclature and Isomerism

The IUPAC names of alkenes are derived by replacing the '-ane' suffix of alkanes with '-ene'. Alkenes exhibit both structural isomerism and geometrical (cis-trans) isomerism.

3.3 Preparation

Alkenes can be prepared by:

Dehydrohalogenation of alkyl halides.
Dehydration of alcohols.

3.4 Reactions

Addition Reactions: Alkenes undergo electrophilic addition reactions, such as hydrogenation, halogenation, and hydrohalogenation. $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$
Oxidation and Polymerization: Alkenes can be oxidized to form glycols and can polymerize to form materials like polyethylene.

NEET Tip:

For electrophilic addition reactions, remember Markovnikov's rule: "The rich get richer" - the hydrogen atom attaches to the carbon with more hydrogen atoms.

4. Alkynes

Alkynes are unsaturated hydrocarbons with at least one triple bond, with the general formula $C_{n} H_{2 n - 2}$ .

4.1 Structure and Properties

Alkynes have a linear structure around the triple bond with bond angles of 180°. The triple bond makes them more reactive than both alkanes and alkenes.

4.2 Nomenclature and Isomerism

Alkynes are named by replacing the '-ane' suffix with '-yne'. They exhibit structural isomerism and positional isomerism.

4.3 Preparation

Alkynes can be prepared by:

Dehalogenation of dihalides.
Partial reduction of alkenes.

4.4 Reactions

Addition Reactions: Alkynes undergo similar addition reactions to alkenes but require specific conditions. $C H \equiv C H + H_{2} P d C H_{2} = C H_{2} N i C H_{3} C H_{3}$
Polymerization: Alkynes can also polymerize to form various products.

NEET Problem-Solving Strategy:

Practice the addition reactions of alkenes and alkynes by identifying the electrophile and nucleophile in each reaction.

Quick Recap

Hydrocarbons are classified into alkanes, alkenes, and alkynes based on the type of bonds between carbon atoms.
Alkanes are saturated and undergo substitution and combustion reactions.
Alkenes and alkynes are unsaturated and undergo addition reactions.
Understanding the preparation and reactions of hydrocarbons is crucial for NEET.

Practice Questions

Question: Define isomerism and give examples of chain isomerism in alkanes. Solution: Isomerism is the phenomenon where compounds have the same molecular formula but different structures. For example, butane $C_{4} H_{10}$ has two isomers: n-butane and isobutane.
Question: Explain Markovnikov's rule with an example. Solution: Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms. For example, in the addition of HBr to propene, the product is 2-bromopropane.
Question: Describe the preparation of ethyne from calcium carbide. Solution: Ethyne is prepared by reacting calcium carbide with water: $C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$
Question: What is the product of the hydration of ethene in the presence of an acid catalyst? Solution: The hydration of ethene in the presence of an acid catalyst produces ethanol: $C H_{2} = C H_{2} + H_{2} O H^{+} C H_{3} C H_{2} O H$
Question: How do alkenes react with bromine water? Solution: Alkenes react with bromine water to form dibromoalkanes, decolorizing the bromine water: $C H_{2} = C H_{2} + B r_{2} \to C H_{2} B r - C H_{2} B r$

Quick Reference Guide and Glossary

Saturated Hydrocarbons: Hydrocarbons with only single bonds.
Unsaturated Hydrocarbons: Hydrocarbons with double or triple bonds.
Aromatic Hydrocarbons: Hydrocarbons containing one or more aromatic rings.
Isomerism: Compounds with the same molecular formula but different structures.
Electrophilic Addition: A reaction where an electrophile reacts with a double or triple bond.