Maarg Manthan · Topic 7.4

Hydrocarbons and Their Derivatives

Science & Technology › Chemistry in Daily Life and Biomolecules · Topic 7.4

Hydrocarbons are organic compounds made only of carbon and hydrogen. Alkanes are saturated hydrocarbons with single bonds and the formula CnH2n+2, such as methane, ethane, propane and butane. Alkenes have a carbon-carbon double bond and the formula CnH2n, such as ethene, and alkynes have a triple bond, such as ethyne, also called acetylene. Aromatic hydrocarbons, such as benzene, contain a ring with delocalised electrons. Hydrocarbons are the main part of fossil fuels such as petroleum and natural gas. Derivatives are formed when hydrogen atoms are replaced by functional groups, such as the hydroxyl group in alcohols, the carboxyl group in acids, and halogens in haloalkanes. Crude oil is separated into fuels by fractional distillation.

5 min read · UPSC Prelims · Science & Technology · Chemistry in Daily Life and Biomolecules

At a Glance

Alkanes CnH2n+2; single bonds
Alkenes CnH2n; double bond
Alkynes CnH2n-2; triple bond
LPG Propane and butane
Ethanol blending Petrol mixed with ethanol
Exam link UPSC Prelims, Science & Technology: Chemistry in Daily Life and Biomolecules

Where Does It Fit?

This is the fourth topic of the macro on chemistry in daily life and biomolecules. It builds on the topic on carbon and prepares for biomolecules, and links to energy and environment. Previous topic: Carbon and Its Allotropic Forms. Next topic: Biomolecules: Carbohydrates, Proteins, Lipids, Nucleic Acids.

What Are the Types of Hydrocarbons?

Type	Features and examples
Alkanes	Saturated and fairly unreactive. Methane in natural gas and biogas, ethane, propane and butane in LPG, and heavier ones in petrol and diesel
Alkenes	Unsaturated and more reactive. Ethene is used to make polythene and to ripen fruits
Alkynes	Ethyne burns with a very hot flame, used in welding, and is made from calcium carbide and water
Aromatic hydrocarbons	Benzene, a ring of six carbons, is a solvent and a carcinogen. Naphthalene is used as moth balls
Isomers	Compounds with the same formula but different structures, such as butane and isobutane
Cycloalkanes	Alkanes in rings, such as cyclohexane

What Are the Important Functional Groups and Derivatives?

Group	Compounds and uses
Alcohols (-OH)	Ethanol in beverages, as a fuel and solvent. Methanol is highly toxic and causes blindness and death. Denatured spirit has additives to prevent drinking
Aldehydes and ketones (-CHO, -CO-)	Formaldehyde as a preservative, and acetone as a solvent in nail polish removers
Carboxylic acids (-COOH)	Acetic acid in vinegar, formic acid in ant stings, and fatty acids in soaps
Esters (-COO-)	Pleasant smell of fruits and flowers, used in perfumes and food flavours. Formed from an acid and an alcohol
Halogen derivatives	Chloroform, carbon tetrachloride and chlorofluorocarbons, which harm the ozone layer and are controlled under the Montreal Protocol
Ethers	Diethyl ether was used as an anaesthetic

How Is Crude Oil Processed and What Else Should You Remember?

Fractional distillation: crude oil is heated and separated by boiling point into gases, petrol, naphtha, kerosene, diesel, lubricating oil, fuel oil and bitumen.
Octane and cetane numbers: the octane number measures how well petrol resists knocking, and the cetane number measures the ignition quality of diesel.
LPG and CNG: LPG is propane and butane stored under pressure for cooking, and CNG is mostly methane, a cleaner fuel for vehicles, as in States of Matter.
Ethanol blending: ethanol made from sugarcane, grain and other biomass is blended with petrol to cut oil imports and emissions. India has aimed for 20 per cent blending and reported reaching it in 2025.
Bharat Stage norms: India moved to BS-VI emission norms in April 2020 with cleaner fuel, to cut vehicle pollution.
Biogas and biodiesel: methane from dung and waste, and diesel substitutes from vegetable oils, are renewable options.
Combustion: complete burning gives carbon dioxide and water, while incomplete burning gives carbon monoxide and soot.
Petrochemicals: hydrocarbons are the raw material for plastics, fertilisers and drugs, as in Polymers and Biopolymers.
Hooch tragedies: illicit liquor containing methanol causes deaths, so the quality of alcohol is a public health issue.
Isomerism: isomers have the same molecular formula but different structures, so butane has two forms, and isomerism explains why there are so many organic compounds.
Greenhouse gases: methane is a potent greenhouse gas released from marshes, livestock, landfills and gas leaks, and carbon dioxide is released when hydrocarbons burn.
Cracking: large hydrocarbons are broken into smaller ones, called cracking, to make more petrol and raw materials for plastics.
Natural gas in India: India has been raising the share of natural gas in its energy mix through city gas distribution and pipelines.

As an analytical point, hydrocarbons power the modern economy but also drive climate change. The shift to cleaner fuels such as ethanol, biogas and hydrogen is a key goal.

Why Does It Matter for the Exam?

Formula and name questions: general formulae and examples for each family.
Fuel questions: LPG, CNG, octane rating and refinery fractions.
Policy questions: ethanol blending and Bharat Stage norms.
Environment link: methane and carbon dioxide emissions and ozone-depleting compounds.
Common errors: mixing up ethanol with methanol or alkenes with alkanes.

Why Is This Topic Important?

Facts Formulae and fuels asked
Policy Ethanol blending
Link Plastics and pollution

What Came Next?

The next topic covers biomolecules: carbohydrates, proteins, lipids and nucleic acids. Next topic: Biomolecules: Carbohydrates, Proteins, Lipids, Nucleic Acids.

Key People and Terms

Hydrocarbon Compound of carbon and hydrogen
Functional group Group that defines properties
Isomerism Same formula, different structure
Fractional distillation Separation by boiling point
Octane number Anti-knock rating
CFC Chlorofluorocarbon

Exam Corner

Points to Remember

Alkane CnH2n+2; alkene CnH2n.
LPG: propane and butane.
Methanol is toxic.
Ethyne for welding.
BS-VI since April 2020.

Do Not Confuse With

Ethanol and methanol Drinkable fuel alcohol versus toxic alcohol
Alkane and alkene Single versus double bond
LPG and CNG Propane-butane versus methane

Memory Hook

Alkane single, alkene double, alkyne triple bond; LPG propane-butane, CNG methane; ethanol blend and BS-VI clean.

Mains Angle

Questions link organic fuels with energy security and the environment. These points add depth.

Families Alkanes to aromatics
Derivatives Functional groups
Fuels Blending and norms

A Question You May Face

An original practice question, not a past paper question.

“Describe the classes of hydrocarbons and their derivatives, and discuss the role of ethanol blending in India’s energy policy.”

How to Answer

Classes Alkane to alkyne.
Groups Alcohol, acid.
Refining Fractions.
Policy Blending.

GS Relevance

Prelims: types, formulas, uses, functional groups, fuel grades, ethanol blending. GS Paper 3: science and technology, energy.

Frequently Asked Questions

What are hydrocarbons?

Hydrocarbons are organic compounds made only of carbon and hydrogen. They are grouped as alkanes with single bonds, alkenes with a double bond, alkynes with a triple bond and aromatic compounds with a benzene ring, and they are the main components of fossil fuels such as petroleum and natural gas.

What is the difference between LPG and CNG?

LPG, or liquefied petroleum gas, is a mixture of propane and butane stored as a liquid under pressure and used mainly for cooking. CNG, or compressed natural gas, is mostly methane stored as a gas at high pressure and is used as a cleaner fuel in vehicles.

What is a functional group?

A functional group is an atom or group of atoms in an organic molecule that decides its chemical properties. Examples are the hydroxyl group in alcohols, the aldehyde group, the ketone group, the carboxyl group in acids and halogen atoms, and compounds with the same group behave in a similar way.

What is ethanol blending?

Ethanol blending is the mixing of ethanol, made from sugarcane, grain or other plant matter, with petrol. It reduces the use of imported crude oil and vehicle emissions, and India has targeted 20 per cent blending, a level reported to have been reached in 2025 under the Ethanol Blended Petrol programme.

Why is methanol dangerous?

Methanol is a toxic alcohol which the body changes into formaldehyde and formic acid, damaging the optic nerve and other organs. Even small amounts can cause blindness and death, and deaths from illicit liquor contaminated with methanol are a recurring public health problem.

PYQ Practice — Statement Analysis

1 Alkenes contain a carbon-carbon double bond.

True

General formula CnH2n.

2 CNG consists mainly of propane and butane.

False

It is mainly methane.

3 Methanol is safe to drink in small amounts.