Class 11 Chemistry Chapter 13 Hydrocarbons

Stuck trying to make sense of Hydrocarbons in Class 11 Chemistry? Don’t worry, we’ve got you covered. This chapter might seem like a lot at first, but it’s really just about carbon and hydrogen forming all kinds of compounds - from simple fuels to aromatic rings. Simple, right?

From alkanes and alkenes to alkynes and benzene, these hydrocarbons show up everywhere - in fuels, plastics, medicines, and even your exam questions. So if you want no-fluff, easy-to-grab Class 11 Hydrocarbons notes, you’re in the right place.

Hydrocarbons Class 11 Notes

Looking for Hydrocarbons notes that actually make sense before exams? You’re in the right place. No long-winded textbook explanations - just the important points, clearly explained so you can revise quickly.

Whether it’s alkanes, alkenes, alkynes, aromatic hydrocarbons, their preparation, reactions, or uses, we’ve covered it all in a way that’s easy to grasp and perfect for last-minute exam prepration.

What are Hydrocarbons?

hydrocarbons are basically compounds made of just carbon and hydrogen. Think of them as the building blocks of organic chemistry - almost everything else in organic chemistry comes from these guys. 

You’ll find them in petroleum, natural gas, coal, and of course in fuels like LPG, petrol, and diesel. Now, they aren’t all the same. They’re grouped based on what kind of bonds the carbon atoms have, which also decides how reactive or stable they are:

• Alkanes: Only single bonds (C–C). They’re chill, not very reactive, and mainly used as fuels.
• Alkenes: Have at least one double bond (C=C). These are more reactive and often used to make plastics.
• Alkynes: At least one triple bond (C≡C). Reactivity goes up even more, and they’re useful in things like welding.
• Aromatic hydrocarbons: Contain a benzene ring with special delocalised electrons. Think of benzene or toluene - these are important for dyes, drugs, and industrial chemicals.

Basically, if you get hydrocarbons down, the rest of organic chemistry becomes way easier. And trust me, these show up everywhere in exams and real life, so knowing them is super handy.

Classification of Hydrocarbons

Hydrocarbons are organic compounds made of only carbon and hydrogen. They’re classified based on the type of bond between carbon atoms. Understanding this helps you predict reactions and uses easily.

1. Saturated Hydrocarbons (Alkanes)

Alkanes have only single bonds between carbon atoms, making them stable and less reactive.

• General formula: CₙH₂ₙ₊₂
• Example: Methane (CH₄), Ethane (C₂H₆)
• Uses: Fuels like LPG and petrol, lubricants, solvents

2. Unsaturated Hydrocarbons

These have double or triple bonds, so they’re more reactive.

• Alkenes (C=C): General formula CₙH₂ₙ, e.g., Ethene (C₂H₄). They undergo additional reactions like hydrogenation, halogenation, hydrohalogenation (Markovnikov’s rule), and polymerisation.
• Alkynes (C≡C): General formula CₙH₂ₙ₋₂, e.g., Ethyne (C₂H₂). They show addition reactions and acidic reactions at the terminal hydrogen. Used in welding and organic synthesis.

3. Aromatic Hydrocarbons

These compounds contain benzene rings with delocalised π electrons, making them stable due to aromaticity.

• Examples: Benzene, Toluene
• Reactions: Electrophilic substitution like nitration, sulphonation, halogenation, Friedel–Crafts reactions
• Uses: Solvents, dyes, drugs, plastics

Preparation and Reactions of Hydrocarbons

Before learning reactions, you need to know how hydrocarbons are prepared. This section covers the common methods used to make different hydrocarbons and the basic reactions they show.

1. Alkanes (Saturated Hydrocarbons)

Alkanes are pretty stable, but we can still make them in the lab and use them in reactions.

Preparation:

• From Unsaturated Hydrocarbons: Adding hydrogen (H₂) in the presence of Ni/Pt/Pd catalysts converts alkenes to alkanes. Example: Ethene - Ethane
• Wurtz Reaction: React alkyl halides with sodium in dry ether to form higher alkanes.
• Decarboxylation: Sodium salts of carboxylic acids react with soda lime to give alkanes with one less carbon.

Reactions:

• Combustion: Alkanes burn to produce CO₂, H₂O, and heat.
• Substitution with Halogens: In sunlight, alkanes react with halogens like Cl₂ or Br₂.
• Oxidation: Mostly resistant to mild oxidising agents.

Uses: Fuel (LPG, petrol), solvents, and lubricants.

2. Alkenes (Unsaturated Hydrocarbons with C=C)

Alkenes are more reactive due to the double bond.

Preparation:

• Dehydration of Alcohols: Removing water using concentrated H₂SO₄.
• Dehydrohalogenation: Alkyl halide + alcoholic KOH.

Reactions:

• Addition Reactions: Double bond opens up to add H₂, halogens, or HX.
• Markovnikov’s Rule: In unsymmetrical alkenes, H attaches to the carbon that already has more H atoms.
• Oxidation: Can produce diols or break the double bond.
• Polymerisation: Forms long-chain polymers like plastics.

3. Alkynes (Unsaturated Hydrocarbons with C≡C)

Alkynes are highly reactive due to the triple bond.

Preparation:

• From Calcium Carbide: CaC₂ + H₂O → Ethyne
• Dehydrohalogenation of Dihalides: Removes HX to form triple bonds.

Reactions:

• Addition Reactions: Similar to alkenes but can happen twice because of triple bond.
• Acidic Nature: Terminal H in alkynes is slightly acidic.
• Oxidation: Can give carboxylic acids or CO₂ + H₂O depending on conditions.

Uses: Welding (oxy-acetylene flame) and organic synthesis.

4. Aromatic Hydrocarbons

Benzene and its derivatives are special because of their planar ring structure with delocalised π electrons, which gives extra stability (aromaticity).

Preparation:

• From Sodium Benzoate
• From Phenol
• From Acetylene

Reactions:

• Electrophilic Substitution: Nitration, sulphonation, halogenation, Friedel–Crafts alkylation & acylation.

Uses: Solvents, dyes, drugs, plastics.

Aromatic Hydrocarbons (Benzene and Its Family)

Aromatic hydrocarbons are a special class of hydrocarbons that contain a benzene ring. They are different from alkanes, alkenes, and alkynes because of their extra stability, which comes from delocalised electrons in the ring.

Benzene (C₆H₆) is the simplest and most important aromatic compound.

Why benzene is special:

• It has a planar ring structure
• Each carbon is sp² hybridised
• π-electrons are delocalised over the ring, making it unusually stable

Aromaticity conditions (easy way to remember):

• The structure must be cyclic and planar
• It must be fully conjugated
• It should follow Hückel’s rule → (4n + 2) π electrons

Chemical behaviour: Instead of addition reactions, aromatic hydrocarbons mainly show electrophilic substitution reactions, such as:

• Nitration
• Halogenation
• Sulphonation
• Friedel–Crafts alkylation and acylation

Environmental Effects of Hydrocarbons

Hydrocarbons make our daily life easier because they’re used as fuels, but they do come with some side effects. When fuels like petrol, diesel, or LPG are burned, they don’t always burn perfectly.

If burning is incomplete (which happens a lot in vehicles), harmful gases are released into the air. This is where most environmental problems start.

Some effects you should remember for exams:

• Carbon monoxide (CO) is released during incomplete combustion and is dangerous to breathe.
• Smoke and unburnt hydrocarbons from vehicles add to air pollution.
• Carbon dioxide (CO₂) increases the greenhouse effect, leading to global warming.
• Continuous pollution can cause health issues like breathing problems and eye irritation.

FAQs

Q1. What are hydrocarbons?

Ans. Hydrocarbons are organic compounds made up of only carbon and hydrogen. They are the simplest organic compounds and form the base of organic chemistry.

Q2. Why are alkanes called saturated hydrocarbons?

Ans. Alkanes are called saturated because they contain only single bonds between carbon atoms, meaning they already have the maximum number of hydrogen atoms attached.

Q3. What types of reactions are shown by alkenes and alkynes?

Ans. Alkenes and alkynes mainly show additional reactions because of the presence of double and triple bonds, which are more reactive than single bonds.

Q4. Why is benzene considered an aromatic hydrocarbon?

Ans. Benzene is aromatic because it has a stable ring structure with delocalised electrons and follows Hückel’s rule, which gives it extra stability.

Q5. Which hydrocarbons are the most reactive and why?

Ans. Alkynes are generally the most reactive because the triple bond between carbon atoms contains more energy and reacts easily.