Hydrocarbons: Alkanes, Alkenes, Alkynes and Aromatic Compounds

What are hydrocarbons?

Hydrocarbons are compounds of carbon and hydrogen only. LPG, CNG, petrol, kerosene and diesel are mixtures of hydrocarbons and are our main fuels.

Alkanes

In alkanes, each carbon is sp³ hybridised and tetrahedral, with bond angles of about 109.5°.

Preparation:

• Hydrogenation of alkenes and alkynes with H₂ over finely divided Pt, Pd or Ni: CH₂=CH₂ + H₂ → CH₃–CH₃.

• Wurtz reaction: an alkyl halide with sodium in dry ether gives a higher alkane with double the carbon atoms: 2CH₃Br + 2Na → CH₃–CH₃ + 2NaBr. It is used to make symmetrical alkanes.

• Decarboxylation: heating sodium salts of carboxylic acids with soda lime (NaOH + CaO) gives an alkane with one carbon less: CH₃COONa + NaOH → CH₄ + Na₂CO₃.

Reactions: alkanes are fairly unreactive (paraffins = "little affinity"), but:

• Halogenation in UV light is a free-radical substitution: CH₄ + Cl₂ → CH₃Cl + HCl, and further substitution gives CH₂Cl₂, CHCl₃ and CCl₄.

• Combustion gives CO₂ and water with much heat.

• Cracking (pyrolysis): heating higher alkanes breaks them into smaller ones; it is used to make petrol from heavier fractions.

Boiling points increase with molecular mass and decrease with branching, since branched molecules are more compact with less surface contact.

Conformations of ethane

Rotation about the C–C single bond gives different spatial arrangements called conformations. In ethane:

• Staggered: hydrogens on adjacent carbons are as far apart as possible. Most stable.

• Eclipsed: hydrogens are directly behind each other. Least stable, because of torsional strain.

The energy difference is only about 12.5 kJ/mol, so the conformations interconvert freely at room temperature. They are shown with Newman and sawhorse projections.

Alkenes

In alkenes, the doubly bonded carbons are sp² hybridised and planar (bond angle 120°). The double bond is one σ bond and one π bond. The π bond is weaker and its electrons are exposed, so alkenes undergo addition reactions.

Geometrical isomerism: rotation about C=C is restricted, so but-2-ene exists as cis (both CH₃ groups on the same side) and trans (on opposite sides) isomers. Trans isomers are usually less polar and have higher melting points.

Preparation: partial reduction of alkynes (Lindlar's catalyst gives cis alkenes; sodium in liquid ammonia gives trans); dehydrohalogenation of alkyl halides with alcoholic KOH; dehydration of alcohols with concentrated H₂SO₄.

Addition reactions:

• Hydrogen (with Ni/Pt): gives alkanes.

• Halogens: ethene decolourises the reddish-brown colour of bromine water, a test for unsaturation.

• Hydrogen halides: with unsymmetrical alkenes, Markovnikov's rule applies: the negative part of the reagent adds to the carbon with fewer hydrogens. Propene + HBr gives mainly 2-bromopropane, because the intermediate secondary carbocation is more stable than the primary one.

• Peroxide effect (Kharasch): in the presence of organic peroxide, HBr only adds against Markovnikov's rule, by a free-radical mechanism. Propene + HBr with peroxide gives 1-bromopropane.

• Ozonolysis breaks the double bond, giving aldehydes or ketones; it is used to locate double bonds.

• Oxidation: cold dilute KMnO₄ (Baeyer's reagent) gives diols and loses its purple colour, another test for unsaturation.

• Polymerisation: many ethene molecules join to form polythene.

Alkynes

The triply bonded carbons are sp hybridised and linear (180°). Ethyne (acetylene) is made by adding water to calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.

Acidic character: hydrogen on a triple-bonded carbon is weakly acidic, because an sp carbon (50% s-character) holds electrons tightly. Ethyne reacts with sodium to release H₂; alkenes and alkanes do not.

Alkynes also add H₂, halogens, HX (following Markovnikov's rule) and water. Passing ethyne through a red-hot iron tube gives benzene (cyclic polymerisation of three molecules).

Aromatic hydrocarbons

Benzene, C₆H₆, is a planar ring of six sp² carbons. Kekulé proposed alternating single and double bonds, but all six C–C bonds are actually the same length (139 pm), between single and double. The six π electrons are delocalised over the ring, which is shown by a circle inside the hexagon. This resonance makes benzene very stable.

Hückel's rule: a planar, cyclic, fully conjugated molecule is aromatic if it has (4n + 2) π electrons (n = 0, 1, 2, …). Benzene has 6 (n = 1).

Because of its stability, benzene prefers electrophilic substitution over addition:

Directive influence: groups already on the ring decide where a new group goes. –OH, –NH₂, –CH₃ and halogens are ortho and para directing; –NO₂, –CHO, –COOH and –CN are meta directing.

Some polynuclear aromatic hydrocarbons formed when organic matter burns incompletely (in tobacco smoke and vehicle exhaust) are carcinogenic.

Summary

• Alkanes (sp³) undergo substitution; alkenes (sp²) and alkynes (sp) undergo addition.

• Markovnikov: the negative part adds to the carbon with fewer H; with peroxide, HBr adds the opposite way.

• Bromine water and Baeyer's reagent test for unsaturation.

• Benzene is aromatic (6 π electrons, Hückel's rule) and undergoes electrophilic substitution.