Hydrocarbons: Class 12 Chemistry

Key points

Hydrocarbons are compounds of carbon and hydrogen only. Alkanes (CₙH₂ₙ₊₂) are saturated and have only single bonds. Alkenes (CₙH₂ₙ) contain a C=C double bond and alkynes (CₙH₂ₙ₋₂) contain a C≡C triple bond. Both are unsaturated. Aromatic hydrocarbons such as benzene contain a stable ring with delocalised electrons.

In IUPAC naming, find the longest carbon chain that holds the functional group, number it so the double or triple bond gets the lowest number, then name and number the side groups in alphabetical order. Examples: CH₃CH₂CH=CH₂ is but-1-ene and CH₃C≡CCH₃ is but-2-yne. Branched alkanes are named as alkyl derivatives, as in 2-methylpropane.

Types of isomerism:

• Chain isomerism: different carbon skeletons, such as butane and 2-methylpropane (both C₄H₁₀).
• Position isomerism: the double bond is at a different place, such as but-1-ene and but-2-ene.
• Geometric (cis-trans) isomerism: in but-2-ene each carbon of the double bond carries two different groups, and rotation about C=C is not possible. The cis form has similar groups on the same side and the trans form has them on opposite sides.

In alkanes carbon is sp³ hybridised with bond angles of 109.5°. In alkenes carbon is sp² with a planar shape and angle 120°, and the double bond is one sigma and one pi bond. In alkynes carbon is sp with a linear shape, 180°, and the triple bond is one sigma and two pi bonds. The C=C bond length is about 134 pm, which is shorter than C-C (154 pm).

Preparation of alkanes, alkenes and alkynes:

• Alkanes: hydrogenation of alkenes with a nickel catalyst, reduction of alkyl halides, or Wurtz reaction, 2R-X + 2Na → R-R + 2NaX in dry ether.
• Alkenes: dehydration of alcohols with hot concentrated H₂SO₄ (C₂H₅OH → C₂H₄ + H₂O at about 170 °C), or dehydrohalogenation of alkyl halides with alcoholic KOH.
• Alkynes: ethyne from calcium carbide, CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂, or by dehydrohalogenation of vicinal dihalides.

Alkanes are fairly unreactive because C-C and C-H bonds are strong and non-polar. Their main reactions are combustion, CH₄ + 2O₂ → CO₂ + 2H₂O, and free radical substitution with halogens in sunlight. Substitution goes through initiation (Cl₂ splits into two Cl· radicals), propagation (Cl· + CH₄ → HCl + CH₃·, then CH₃· + Cl₂ → CH₃Cl + Cl·) and termination (two radicals combine).

Alkenes and alkynes are reactive because the pi bond is weaker and electron rich. They undergo electrophilic addition. Examples: addition of H₂ (Ni, giving an alkane), Br₂ (giving a dibromoalkane), HBr and water (with acid catalyst). Ethyne adds two moles of reagent, first forming an alkene derivative and then a saturated product.

Markovnikov's rule: when HX adds to an unsymmetrical alkene, the hydrogen goes to the carbon that already has more hydrogen atoms. Reason: the more substituted carbocation is more stable. So propene plus HBr gives 2-bromopropane as the major product.

Tests for unsaturation:

• Bromine water (orange or brown) is decolourised by alkenes and alkynes.
• Cold dilute alkaline KMnO₄ (Baeyer's reagent, purple) is decolourised and a brown precipitate of MnO₂ may form. Alkenes give a diol.
• Terminal alkynes such as ethyne form a precipitate with ammoniacal silver nitrate (white) or ammoniacal cuprous chloride (red).

Benzene C₆H₆ is a planar ring of six sp² carbons, all C-C bonds equal (139 pm), with delocalised pi electrons above and below the ring. It is stable and follows Hückel's rule (4n + 2 pi electrons, here 6). It prefers substitution to addition. Kekulé's structure shows alternating double bonds, but the real molecule is a resonance hybrid.

Electrophilic substitution of benzene keeps the ring intact. Nitration uses conc. HNO₃ and conc. H₂SO₄ at about 50 °C to give nitrobenzene. Halogenation uses Cl₂ or Br₂ with FeCl₃ or FeBr₃. Friedel-Crafts alkylation with CH₃Cl and anhydrous AlCl₃ gives toluene. Addition happens only under severe conditions, such as hydrogenation to cyclohexane.

A group already on the ring directs the next substituent. The CH₃, OH and halogen groups direct to ortho and para positions. The NO₂, CHO and COOH groups direct to the meta position. Most groups that direct ortho and para activate the ring, while the others deactivate it (halogens deactivate slightly but still direct ortho and para).

Study tip: Learn each reaction with its reagent, condition and product, as a short chain: reagent, then condition, then product. Practise naming five or six alkenes and alkynes every day. For mechanism questions write each step on its own line and show the movement of the species clearly.

Definitions and flashcards

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Formula sheet

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Short questions with model answers

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Common mistakes

• Writing the general formula of alkynes as CₙH₂ₙ₊₂ or of alkanes as CₙH₂ₙ.
• Applying Markovnikov's rule to the wrong carbon, or to symmetrical alkenes such as ethene where it does not matter.
• Showing benzene undergoing addition under normal conditions. It gives substitution.
• Forgetting the catalyst or condition in a reaction, such as Ni for hydrogenation, FeBr₃ for bromination of benzene or alcoholic KOH for elimination.
• Numbering the chain from the wrong end, so the double bond does not get the lowest number.

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