Board exam focus — Hydrocarbons (CBSE & HBSE)

Hydrocarbons are compounds of carbon and hydrogen only and are the parent compounds of all organic chemistry. This chapter classifies them into saturated (alkanes), unsaturated (alkenes, alkynes) and aromatic types, and develops their preparation, physical and chemical properties, and mechanisms. Key examinable themes for CBSE and HBSE include Markovnikov's rule and the peroxide (anti-Markovnikov) effect, the acidic nature of terminal alkynes, conformations of ethane, and the mechanism and directive influence of electrophilic aromatic substitution.

Classification and Alkanes: Preparation, Properties and Conformations

Classification of Hydrocarbons

• Saturated — alkanes (only C\u2013C single bonds), general formula C\u2099H\u2082\u2099\u208a\u2082.
• Unsaturated — alkenes (C=C, C\u2099H\u2082\u2099) and alkynes (C\u2261C, C\u2099H\u2082\u2099\u208b\u2082).
• Aromatic — benzene and its derivatives, containing delocalised \u03c0 systems.

Preparation of Alkanes

1. Hydrogenation of alkenes/alkynes (Sabatier-Senderens): CH\u2082=CH\u2082 + H\u2082 \u2192 (Ni) CH\u2083CH\u2083.
2. From alkyl halides: (a) Wurtz reaction — 2R\u2013X + 2Na \u2192 R\u2013R + 2NaX (gives symmetrical alkanes); (b) Reduction with Zn/HCl.
3. Decarboxylation of sodium salt of carboxylic acid with soda lime: CH\u2083COONa + NaOH \u2192 (CaO) CH\u2084 + Na\u2082CO\u2083.
4. Kolbe electrolysis of carboxylate salts.

Chemical Properties

Alkanes are relatively inert ('paraffins') but undergo:

• Free-radical halogenation (substitution) by a chain mechanism: initiation (X\u2082 \u2192 2X\u2022 by UV), propagation, termination. Reactivity F\u2082 > Cl\u2082 > Br\u2082 > I\u2082; ease of substitution 3\u00b0 > 2\u00b0 > 1\u00b0.
• Combustion to CO\u2082 and H\u2082O.
• Pyrolysis (cracking) and isomerisation.

Conformations of Ethane

Rotation about the C\u2013C single bond produces an infinite number of spatial arrangements called conformations.

Tip: The energy difference between staggered and eclipsed ethane (~12.5 kJ/mol) is the torsional strain, too small to isolate separate conformers at room temperature. CBSE often asks why conformers cannot be separated — the barrier is too low.

Alkenes and Alkynes: Preparation, Properties, Markovnikov and Peroxide Effect

Alkenes — Preparation

• Dehydrohalogenation of alkyl halides with alcoholic KOH (\u03b2-elimination; follows Saytzeff rule \u2192 more substituted alkene).
• Dehydration of alcohols with conc. H\u2082SO\u2084.
• Dehalogenation of vicinal dihalides with Zn.

Electrophilic Addition and Markovnikov's Rule

Alkenes are electron-rich (\u03c0 bond) and undergo electrophilic addition.

Markovnikov's rule: in the addition of an unsymmetrical reagent (HX) to an unsymmetrical alkene, the negative part adds to the carbon bearing fewer hydrogens (the H adds to the carbon with more H). This is because addition proceeds through the more stable carbocation.

Example: CH\u2083\u2013CH=CH\u2082 + HBr \u2192 CH\u2083\u2013CHBr\u2013CH\u2083 (2-bromopropane, via the more stable 2\u00b0 cation).

Peroxide (Kharasch) Effect — Anti-Markovnikov

In the presence of organic peroxides, HBr (only HBr) adds to alkenes by a free-radical mechanism, giving the anti-Markovnikov product: CH\u2083\u2013CH=CH\u2082 + HBr \u2192 (peroxide) CH\u2083\u2013CH\u2082\u2013CH\u2082Br (1-bromopropane).

Tip: The peroxide effect is observed only with HBr, not HCl or HI. The H\u2013Cl bond is too strong (radical chain step endothermic) and the H\u2013I bond too weak (the I\u2022 step is too slow), so only HBr gives an efficient radical chain. This selectivity is a classic CBSE/HBSE trap.

Alkynes

• Preparation: from CaC\u2082 (CaC\u2082 + 2H\u2082O \u2192 C\u2082H\u2082 + Ca(OH)\u2082); dehydrohalogenation of vicinal/geminal dihalides.
• Acidic nature: terminal alkynes (\u2013C\u2261C\u2013H) have an acidic hydrogen because the sp carbon (50% s-character) holds the bonding pair tightly, stabilising the conjugate carbanion. They react with Na, and with ammoniacal AgNO\u2083 / Cu\u2082Cl\u2082 to form metal acetylides (a test for terminal alkynes).
• Addition: HX, H\u2082O (Markovnikov, gives carbonyl via enol), H\u2082 (to alkene then alkane).

Aromatic Hydrocarbons: Benzene, Aromaticity and Electrophilic Substitution

Structure of Benzene and Aromaticity

Benzene (C\u2086H\u2086) is a planar, regular hexagon with all C\u2013C bond lengths equal (139 pm), intermediate between single and double bonds. Each carbon is sp\u00b2 hybridised; the six unhybridised p orbitals overlap to form a delocalised \u03c0 cloud above and below the ring. The real structure is a resonance hybrid of the two Kekul\u00e9 structures, giving extra stability (resonance/delocalisation energy ~152 kJ/mol).

H\u00fcckel's rule: a planar, cyclic, fully conjugated ring is aromatic if it contains (4n + 2) \u03c0 electrons (n = 0, 1, 2 ...). Benzene has 6 \u03c0 electrons (n = 1), so it is aromatic.

Electrophilic Aromatic Substitution (EAS)

Benzene undergoes substitution (not addition) to preserve aromaticity. General mechanism:

1. Generation of the electrophile (E\u207a).
2. Attack of E\u207a on the \u03c0 cloud to form a resonance-stabilised arenium ion (\u03c3-complex / carbocation) — this disrupts aromaticity temporarily.
3. Loss of H\u207a restores aromaticity, giving the substituted product.

Directive Influence

• Ortho/para-directing, activating groups donate electrons (+I, +M): \u2013OH, \u2013NH\u2082, \u2013OR, \u2013CH\u2083. (Halogens are o/p-directing but deactivating.)
• Meta-directing, deactivating groups withdraw electrons (\u2013I, \u2013M): \u2013NO\u2082, \u2013COOH, \u2013CN, \u2013SO\u2083H.

Tip: Halogens are the exception that students miss: they are deactivating (\u2013I dominates rate) yet o/p-directing (+M directs position). State both effects in the exam.

Carcinogenicity

Polynuclear aromatic hydrocarbons (PAHs) such as benzo[a]pyrene and 1,2-benzanthracene, formed by incomplete combustion of tobacco, coal and fuels, are carcinogenic (cancer-causing).

Frequently asked questions

Are these Hydrocarbons notes free?

Yes — the Hydrocarbons notes for Chemistry (Class 11) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Hydrocarbons notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Hydrocarbons chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Hydrocarbons.