Hydrocarbons

In this chapter: classification, alkanes (preparation, properties, free radical halogenation, conformations of ethane), alkenes (structure, geometrical isomerism, preparation, addition reactions, Markovnikov rule, peroxide effect, oxidation, ozonolysis, polymerisation), alkynes (preparation, acidic character, addition reactions), aromatic hydrocarbons (structure of benzene, aromaticity, preparation, electrophilic substitution, directive influence of groups) and carcinogenicity.

Classification

Hydrocarbons contain only carbon and hydrogen. Saturated hydrocarbons (alkanes, C_nH_2n+2, and cycloalkanes) have only C−C single bonds. Unsaturated hydrocarbons have double bonds (alkenes, C_nH_2n) or triple bonds (alkynes, C_nH_2n−2). Aromatic hydrocarbons (arenes) contain benzene rings. Our main fuels, LPG and CNG, are alkanes, and the petrochemical industry uses hydrocarbons to make polymers, dyes and medicines.

Alkanes

Each carbon is sp³ hybridised and tetrahedral (bond angle 109.5°); the C−C bond is 154 pm. Alkanes are also called paraffins because they are fairly unreactive.

Preparation

Properties

Alkanes are non-polar, so they dissolve in non-polar solvents and not in water. Boiling points rise with molecular mass. Branching lowers the boiling point because the molecule becomes more spherical and the surface area for van der Waals forces falls: pentane boils at 309 K, 2-methylbutane at 301 K and 2,2-dimethylpropane at 282.5 K.

• Halogenation (in UV light or at high temperature) is a free radical substitution. Initiation: Cl₂ → 2Cl• (hν). Propagation: CH₄ + Cl• → CH₃• + HCl, then CH₃• + Cl₂ → CH₃Cl + Cl•. Termination: two radicals combine (Cl• + Cl•, CH₃• + Cl•, or CH₃• + CH₃•, which is why some ethane forms). Reactivity is F₂ > Cl₂ > Br₂ > I₂, and replacement of hydrogen is easiest at 3° > 2° > 1° carbon.
• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O; Δ_cH° = −890 kJ mol⁻¹.
• Controlled oxidation: 2CH₄ + O₂ → 2CH₃OH (Cu, 523 K, 100 atm); CH₄ + O₂ → HCHO + H₂O (Mo₂O₃); (CH₃)₃CH is oxidised by KMnO₄ to (CH₃)₃COH.
• Isomerisation: n-hexane with anhydrous AlCl₃ and HCl gives 2-methylpentane and 3-methylpentane.
• Aromatisation: n-hexane over Cr₂O₃, V₂O₅ or Mo₂O₃ on alumina at 773 K and 10 to 20 atm gives benzene.
• With steam: CH₄ + H₂O → CO + 3H₂ (Ni, 1273 K); this is how hydrogen is made industrially. Pyrolysis (cracking) breaks higher alkanes into smaller alkanes and alkenes on heating.

Conformations of ethane

Rotation about the C−C single bond gives different spatial arrangements called conformations. In the eclipsed form, the hydrogens on the two carbons are directly behind each other; in the staggered form they are as far apart as possible. They are drawn as Sawhorse or Newman projections. The staggered form is more stable because it has the least torsional strain (repulsion between the bond pairs). The energy difference is only about 12.5 kJ mol⁻¹, so the conformations interconvert freely at room temperature and cannot be separated.

Alkenes

The C=C bond (134 pm) is one σ bond (sp²-sp²) and one π bond. The π electrons are loosely held, so alkenes behave as a source of electrons and undergo electrophilic addition. Rotation about C=C is restricted, which gives geometrical isomers when each doubly bonded carbon carries two different groups. cis-But-2-ene (groups on the same side) is polar, with a higher boiling point; trans-but-2-ene is nearly non-polar and packs better, with a higher melting point.

Preparation

• From alkynes: partial hydrogenation with Lindlar's catalyst (Pd on CaCO₃, poisoned with sulphur compounds or quinoline) gives the cis alkene; Na in liquid NH₃ gives the trans alkene.
• From alkyl halides: heating with alcoholic KOH removes HX (dehydrohalogenation, a β-elimination).
• From vicinal dihalides: heating with zinc removes both halogens (dehalogenation): CH₂Br−CH₂Br + Zn → CH₂=CH₂ + ZnBr₂.
• From alcohols: heating with conc. H₂SO₄ removes water (acidic dehydration): CH₃CH₂OH → CH₂=CH₂ + H₂O at 443 K.

Addition reactions

• Hydrogen (Ni, Pt or Pd) gives alkanes.
• Bromine in CCl₄ adds to give a vicinal dibromide, and its reddish-orange colour disappears. This is the standard test for unsaturation.
• Hydrogen halides (reactivity HI > HBr > HCl): with unsymmetrical alkenes the product follows Markovnikov's rule, the negative part of the addendum goes to the carbon with fewer hydrogen atoms.
• Peroxide effect (Kharash effect): in the presence of a peroxide, HBr adds against Markovnikov's rule by a free radical mechanism: CH₃CH=CH₂ + HBr → CH₃CH₂CH₂Br. This happens only with HBr, not with HCl or HI.
• Cold conc. H₂SO₄ gives alkyl hydrogen sulphates (Markovnikov), and water in the presence of a few drops of conc. H₂SO₄ gives alcohols, also by Markovnikov's rule.

Oxidation, ozonolysis and polymerisation

• Baeyer's reagent (cold, dilute, aqueous 1% KMnO₄) converts alkenes to vicinal glycols and loses its pink colour; this is another test for unsaturation. CH₂=CH₂ gives ethane-1,2-diol.
• Acidic KMnO₄ or K₂Cr₂O₇ breaks the double bond: but-2-ene gives two molecules of ethanoic acid; 2-methylpropene gives propanone and CO₂.
• Ozonolysis: O₃ forms an ozonide, which Zn and water split into aldehydes and/or ketones. Each doubly bonded carbon becomes a C=O. Propene gives ethanal and methanal.
• Polymerisation: ethene at high temperature and pressure with a catalyst gives polythene, n CH₂=CH₂ → −(CH₂−CH₂)_n−.

Worked example: An alkene C₅H₁₀ on ozonolysis gives ethanal and propanone. Identify it.
Solution: Join the two carbonyl carbons with a double bond after removing the oxygens. Ethanal (CH₃CHO) provides CH₃CH=, and propanone ((CH₃)₂CO) provides =C(CH₃)₂. The alkene is (CH₃)₂C=CHCH₃, 2-methylbut-2-ene. Carbon count check: 2 + 3 = 5.

Alkynes

The C≡C bond (120 pm) is one σ and two π bonds; each carbon is sp hybridised, so ethyne is linear (H−C−C 180°).

• From calcium carbide: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. Calcium carbide itself is made by heating quicklime with coke.
• From vicinal dihalides: alcoholic KOH removes one HX to give a vinylic halide, and sodamide (NaNH₂) removes the second to give the alkyne.

Acidic character: the hydrogen on a triple-bonded carbon is weakly acidic, because the sp carbon (50% s-character) holds the C−H bonding electrons tightly. Ethyne reacts with sodium: HC≡CH + Na → HC≡C⁻Na⁺ + ½H₂. Acidity order: HC≡CH > H₂C=CH₂ > CH₃−CH₃. Only terminal alkynes (with ≡C−H) show this; but-2-yne does not.

Additions: alkynes add H₂, halogens and HX (Markovnikov) in two steps, first to the alkene stage. With water in the presence of HgSO₄ and dilute H₂SO₄ at 333 K, ethyne gives ethanal and propyne gives propanone. Passing ethyne through a red-hot iron tube at 873 K gives benzene (cyclic polymerisation).

Worked example: 2.6 g of ethyne is completely hydrogenated to ethane. Find the mass of hydrogen used and of ethane formed. (C = 12, H = 1)
Solution: HC≡CH + 2H₂ → CH₃−CH₃. Moles of C₂H₂ = 2.6/26 = 0.10 mol, so H₂ needed = 0.20 mol = 0.40 g, and ethane formed = 0.10 mol = 0.10 × 30 = 3.0 g. Mass check: 2.6 + 0.4 = 3.0 g.

Aromatic hydrocarbons

Benzene, C₆H₆, is unusually stable for such an unsaturated molecule: it prefers substitution to addition. Kekulé proposed a ring of alternating single and double bonds, but X-ray studies show all six C−C bonds are the same length. Each carbon is sp²; the six unhybridised p orbitals overlap sideways all round the ring, and the six π electrons are delocalised above and below the plane.

Aromaticity (Hückel rule): a compound is aromatic if it is planar, has complete delocalisation of π electrons in the ring, and has (4n + 2) π electrons, n = 0, 1, 2 and so on. Benzene (6 π electrons, n = 1) is aromatic.

Preparation of benzene: cyclic polymerisation of ethyne (red-hot iron tube, 873 K); heating sodium benzoate with soda lime (decarboxylation); and heating phenol vapour with zinc dust (reduction).

Electrophilic substitution

The mechanism has three steps: the electrophile is generated, it attacks the ring to form a carbocation (the arenium ion or σ-complex, in which one carbon is sp³ and aromaticity is lost), and a proton is removed to restore the aromatic ring. Benzene does show addition under harsh conditions: with H₂ and Ni it gives cyclohexane, and with Cl₂ in UV light it gives benzene hexachloride (C₆H₆Cl₆).

Directive influence of groups

• Ortho and para directing (and activating, by releasing electrons into the ring): −OH, −NH₂, −NHR, −NHCOCH₃, −OCH₃, −CH₃, −C₂H₅. Halogens are ortho and para directing but slightly deactivating, because their −I effect outweighs their +R effect.
• Meta directing (and deactivating, by withdrawing electrons from the ring): −NO₂, −CN, −CHO, −COR, −COOH, −COOR, −SO₃H.

Carcinogenicity and toxicity: benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and are known to be carcinogenic. They form by incomplete combustion of organic matter such as tobacco, coal and petroleum.

Common mistakes: (1) Applying the peroxide effect to HCl or HI; it works only with HBr. (2) Mixing up Lindlar's catalyst (cis) and Na/liquid NH₃ (trans). (3) Forgetting that decarboxylation removes one carbon while Wurtz doubles the carbon count. (4) Hydration of ethyne gives ethanal, but of other alkynes gives ketones. (5) Calling halogens meta directing because they deactivate the ring; they are ortho and para directing.

JEE and NEET focus

• Preparation of alkanes (Wurtz, Kolbe, decarboxylation) with the number of carbons in the product.
• Free radical halogenation mechanism and products; conformations of ethane.
• Markovnikov and anti-Markovnikov products, and the carbocation reasoning behind them.
• Ozonolysis products and working backwards from products to the alkene.
• Acidity of terminal alkynes, hydration of alkynes, and cis/trans alkenes from alkynes.
• Electrophilic substitution of benzene with electrophiles, Hückel rule, and ortho/para vs meta directing groups.

Practice questions