HydrocarbonsNEET Chemistry · Class 11 · NCERT Chapter 9

Classification of Hydrocarbons

Hydrocarbons are organic compounds that contain only carbon and hydrogen. They are the simplest class of organic compounds and form the basis for understanding all of organic chemistry. NEET asks about their reactions every year.

General formula: alkane CₙH₂ₙ₊₂; alkene CₙH₂ₙ; alkyne CₙH₂ₙ₋₂; benzene series CₙH₂ₙ₋₆.

Alkanes: Conformations and Reactions

Rotate the dihedral angle to see how conformation affects relative energy. Key: staggered conformations (60° apart) are more stable than eclipsed.

Alkanes are saturated hydrocarbons (all single bonds). Their main reactions are combustion and free radical halogenation. They are chemically less reactive than alkenes or alkynes.

Conformational Isomers

In ethane, the two CH₃ groups can rotate around the C-C bond. Different spatial arrangements arising from rotation around single bonds are called conformations. Two extreme conformations of ethane:

• Staggered conformation: H atoms on adjacent carbons are as far apart as possible. This has minimum torsional strain and is the most stable conformation.
• Eclipsed conformation: H atoms on adjacent carbons are directly aligned. This has maximum torsional strain and is less stable.

Conformations are not permanent structures (they interconvert rapidly at room temperature) and are not structural isomers.

Halogenation of Alkanes: Free Radical Mechanism

Alkanes react with chlorine or bromine in the presence of UV light (hν) via a free radical chain mechanism. For methane + Cl₂:

1. Initiation: UV light breaks the Cl-Cl bond to form chlorine radicals.

2. Propagation: The chain is carried forward.

3. Termination: Two radicals combine to form a stable molecule.

Reactivity of C-H bonds toward halogenation: 3° > 2° > 1° (due to stability of the radical intermediate formed). Chlorination is less selective than bromination (bromine is more selective because radical formation by Br• is more endothermic, making the reaction more sensitive to radical stability).

Fluorination is violent and uncontrollable; iodination is too slow (thermodynamically unfavourable).

Alkenes: Reactions and Markovnikov's Rule

Quick questions

Alkenes contain a C=C double bond (one σ and one π bond). The π bond is the site of most reactions. Alkenes undergo electrophilic addition reactions because the π electrons act as a Lewis base and attract electrophiles.

Addition of HX (Hydrogen Halides)

HX adds across the double bond. For unsymmetrical alkenes, Markovnikov's rule applies:

The H goes to the carbon with more H atoms; X goes to the carbon with fewer H atoms. The reason: the more substituted carbocation intermediate (formed when H⁺ adds first) is more stable.

Anti-Markovnikov Addition (Peroxide Effect)

In the presence of peroxides (ROOR or H₂O₂), HBr adds via a free radical mechanism to give the anti-Markovnikov product:

This is called the Kharasch effect or peroxide effect. It only works with HBr (not HCl or HI). HCl: Cl• too reactive (indiscriminate). HI: I• too stable (unreactive toward the alkene).

Addition of H₂O (Hydration)

Alkenes react with water in the presence of an acid catalyst (H₃PO₄ or H₂SO₄) to give alcohols. Markovnikov's rule applies: OH goes to the more substituted carbon.

Addition of Cl₂ / Br₂ (Halogenation)

Halogens add across the double bond. The reaction with Br₂ in CCl₄ is used as a test for unsaturation (the orange-brown colour of Br₂ decolourises).

Ozonolysis

Treatment with ozone (O₃) followed by zinc and water (reductive workup) cleaves the C=C bond to give carbonyl compounds:

• Terminal =CH₂ gives formaldehyde (HCHO).
• =CHR gives an aldehyde (RCHO).
• =CR₂ gives a ketone (R₂C=O).

Ozonolysis is used to determine the position of the double bond in an unknown alkene.

Oxidation with KMnO₄

Cold dilute KMnO₄ (Baeyer's reagent) adds two OH groups across the double bond (gives a diol). The purple colour of KMnO₄ decolourises — another test for unsaturation. Hot concentrated KMnO₄ cleaves the double bond completely (similar to oxidative ozonolysis).

Combustion

Alkynes: Acidic Character and Reactions

Alkynes have a C≡C triple bond (one σ and two π bonds). The sp hybridised carbon holds electrons closer to the nucleus, giving the terminal C-H bond a slightly acidic character.

Acidic Character of Terminal Alkynes

Terminal alkynes (HC≡CR) react with strong bases (Na, NaNH₂) to release H⁺ and form acetylide ions (RC≡C⁻). This makes alkynes more acidic than alkenes or alkanes.

Acidic order: alkyne (pKa ≈ 25) >> alkene (pKa ≈ 44) > alkane (pKa ≈ 50). Water has pKa = 15.7 and is more acidic than alkynes in the common sense, but among hydrocarbons, alkynes are the most acidic.

Addition Reactions of Alkynes

Alkynes undergo electrophilic addition, but less readily than alkenes (the triple bond is less reactive toward electrophiles because the sp carbons are more electronegative and hold the π electrons more tightly).

• HX addition follows Markovnikov's rule. For acetylene + HCl: HC≡CH + HCl → CH₂=CHCl (vinyl chloride, first addition) → CH₃CHCl₂ (second HCl addition).
• Addition of H₂O (in presence of Hg²⁺/H₂SO₄) gives an enol which immediately tautomerises to a carbonyl compound. HC≡CH + H₂O → [CH₂=CHOH] → CH₃CHO (acetaldehyde).
• Hydrogenation: partial (Lindlar's catalyst, poisoned Pd) gives cis-alkene. Complete H₂ + Pt gives alkane.

Benzene: Structure and Stability

Benzene (C₆H₆) has a cyclic structure with three alternating double bonds (Kekulé structure). However, the actual structure is a resonance hybrid — all six C-C bond lengths are equal (1.40 Å, between single 1.54 Å and double 1.34 Å), and all bond angles are 120°. The six π electrons are fully delocalised over the ring.

Evidence for Delocalisation

• All C-C bonds in benzene are equal in length (1.40 Å).
• Benzene does not decolourise Br₂ in CCl₄ (unlike alkenes) — it does not undergo addition readily.
• The heat of hydrogenation of benzene (−208 kJ/mol) is much less than the expected value for cyclohexatriene (−360 kJ/mol). The difference (≈ 152 kJ/mol) is the resonance energy.

Hückel's Rule

A monocyclic, planar compound with (4n + 2) π electrons (where n = 0, 1, 2, ...) is aromatic. For benzene: 6 π electrons, n = 1 → aromatic. Cyclooctatetraene (8 π electrons, 4n with n = 2) is anti-aromatic and non-planar in practice.

Electrophilic Aromatic Substitution (EAS)

Benzene undergoes substitution (not addition) because the delocalized ring is too stable to add across. An electrophile attacks the ring, and a proton is lost to restore aromaticity. General mechanism:

1. Formation of the electrophile (E⁺) using a Lewis acid catalyst.
2. The electrophile attacks the benzene ring, forming a non-aromatic sigma complex (arenium ion, also called a Wheland intermediate or sigma complex). Aromaticity is temporarily lost.
3. A proton is lost from the carbon that bonded to E⁺, restoring aromaticity. This step is fast.

Important EAS Reactions of Benzene

Electrophiles formed in these reactions: NO₂⁺ (nitronium) in nitration; Cl⁺ or FeCl₄⁻Cl complex in halogenation; R⁺ in alkylation.

Directive Influence of Substituents

When a substituent is already on the benzene ring, it directs an incoming electrophile to specific positions.

Ortho/Para Directors (Ring-Activating Groups)

Groups that donate electrons to the ring (by +M effect or +I effect) activate it toward EAS and direct the electrophile to the ortho and para positions. These positions become relatively electron-rich due to resonance donation.

• Examples: −OH, −OR, −NH₂, −NHR, −NR₂, −NHCOCH₃, alkyl groups (−CH₃).
• Halogens (−F, −Cl, −Br, −I) are ortho/para directors but ring-deactivating (−I effect dominates over +M effect for rate, but position still ortho/para).

Meta Directors (Ring-Deactivating Groups)

Groups that withdraw electrons from the ring (by −M effect) deactivate it and direct the electrophile to the meta position. The ortho and para positions are made more electron-poor by the withdrawing group's resonance, leaving meta as the preferred site.

• Examples: −NO₂, −COOH, −SO₃H, −CHO, −COR (ketone), −CN, −CF₃, −NR₃⁺.

A useful memory tip: groups that contain a positive charge or a highly electronegative atom directly attached to the ring (especially through a double bond toward oxygen or nitrogen) are meta directors.

Worked NEET Problems

Summary Cheat Sheet