Schooling · Class 11 · Chemistry · Hydrocarbons · Chapter on a page
CBSE Class 11 · Chemistry · NCERT Chemistry Part II
Hydrocarbons: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Saturated vs unsaturated
- Hybridisation and shape
- Isomerism
- General formulas
- Characteristic reactions
- Markovnikov's rule
- Aromaticity
- Formulas and facts
- Alkane general formula
- Degree of unsaturation (DBE)
- Markovnikov's rule
- Anti-Markovnikov (peroxide effect)
- Benzene resonance energy
- Hückel's rule for aromaticity
- Combustion of hydrocarbons
- Worked examples
- IUPAC naming of a branched alkene
- Predicting the major product using…
- Degree of unsaturation
- Watch out for
- Forgetting that geometric isomerism…
- Numbering the parent chain without…
- Applying Markovnikov's rule to…
- Assuming alkenes and alkynes burn…
- Treating benzene like a typical…
What this chapter is about
Hydrocarbons are organic compounds containing only carbon and hydrogen atoms. This chapter introduces the major classes of hydrocarbons: alkanes, alkenes, alkynes, and aromatic compounds. You learn how to name them using IUPAC rules, write their structures, understand their bonding, and predict their physical and chemical behaviour.
Key ideas
- Saturated vs unsaturated: Alkanes have only C–C single bonds (saturated). Alkenes have at least one C═C double bond; alkynes have at least one C≡C triple bond (both unsaturated). Unsaturated compounds undergo addition reactions.
- Hybridisation and shape: In alkanes every carbon is sp³-hybridised with bond angles near 109.5°. In alkenes the doubly-bonded carbons are sp²-hybridised (bond angle about 120°). In alkynes the triply-bonded carbons are sp-hybridised (bond angle 180°).
- Isomerism: Chain isomers differ in branching; positional isomers differ in the location of a functional group or multiple bond; geometrical (cis-trans) isomerism arises when rotation about a double bond is restricted.
- General formulas: Alkanes CₙH₂ₙ₊₂; cycloalkanes CₙH₂ₙ; alkenes CₙH₂ₙ; alkynes CₙH₂ₙ₋₂.
- Characteristic reactions: Alkanes mainly undergo free-radical substitution (e.g., halogenation in sunlight). Alkenes and alkynes undergo electrophilic addition (e.g., addition of HBr, H₂O; H₂ adds instead by metal-catalysed hydrogenation). Aromatic compounds prefer electrophilic substitution over addition to preserve their stable ring.
- Markovnikov's rule: When an unsymmetrical reagent HX adds to an unsymmetrical alkene, the hydrogen attaches to the carbon already holding more hydrogen atoms.
- Aromaticity: Benzene and related compounds are aromatic because they have a planar, cyclic ring of sp²-hybridised atoms with (4n + 2) π electrons (where n is a non-negative integer). This delocalization gives extra stability.
Formulas and facts to remember
- Alkane general formula: CₙH₂ₙ₊₂. Each successive member differs by CH₂ (homologous series).
- Degree of unsaturation (DBE): For CₓHᵧ, DBE = (2x + 2 − y) / 2. One double bond or one ring equals one DBE; one triple bond equals two.
- Markovnikov's rule: In the addition of HX to an unsymmetrical alkene, H goes to the carbon with more H atoms; the halide goes to the carbon with fewer H atoms.
- Anti-Markovnikov (peroxide effect): With HBr in the presence of peroxides, the opposite orientation occurs via a free-radical mechanism. HCl and HI do not show this effect.
- Benzene resonance energy: Benzene is about 150 kJ mol⁻¹ more stable than a hypothetical "cyclohexatriene" with localised double bonds. This extra stability explains its preference for substitution over addition.
- Hückel's rule for aromaticity: A planar, cyclic, fully conjugated system is aromatic if it has (4n + 2) π electrons (n = 0, 1, 2, …).
- Combustion of hydrocarbons: Complete combustion gives CO₂ and H₂O. Example: CH₄ + 2 O₂ → CO₂ + 2 H₂O.
- Ozonolysis: Alkenes react with ozone followed by zinc/water to cleave the double bond and yield aldehydes or ketones; useful for locating the double bond position.
Common mistakes
- Forgetting that geometric isomerism requires two different groups on each doubly-bonded carbon → check both carbons before labelling cis or trans.
- Numbering the parent chain without giving the principal functional group the lowest locant → always give double or triple bonds lower numbers than substituents.
- Applying Markovnikov's rule to symmetrical alkenes → it only affects orientation in unsymmetrical alkenes.
- Assuming alkenes and alkynes burn incompletely because they are unsaturated → with sufficient oxygen, they undergo complete combustion like alkanes.
- Treating benzene like a typical alkene and expecting easy addition → benzene prefers electrophilic substitution to preserve aromaticity.
Quick revision
- Alkanes: CₙH₂ₙ₊₂, sp³, single bonds, substitution reactions.
- Alkenes: CₙH₂ₙ, sp², double bonds, electrophilic addition, follow Markovnikov's rule.
- Alkynes: CₙH₂ₙ₋₂, sp, triple bonds, can add two moles of reagent.
- Aromatic compounds: planar ring with (4n + 2) π electrons, prefer substitution over addition.
- DBE = (2C + 2 − H) / 2 tells you the sum of rings and multiple bonds.