Chemistry · Organic chemistry · Grade 10

Alkanes and cycloalkanes

Methane in a gas hob and propane in a cylinder belong to one family — so why is one a gas and the other a liquid?

The situation

The gas stove in the kitchen burns natural gas that comes through the yellow pipe — mainly methane CH₄; houses on the edge of the city use propane from the red cylinder. A lighter contains butane, car petrol contains octane and its isomers, and candle paraffin has molecules of twenty to thirty carbons. They are all members of one family — the alkanes. Why are those at the head of the family gases, those in the middle liquids and those at the end solids? What reactions do they undergo, and how can methane be made in the laboratory?

Topic

Alkanes and cycloalkanes. Gas hob (methane) and propane cylinder: the homologous series of alkanes, their combustion and reaction with chlorine

Key ideas

Homologous series CₙH₂ₙ₊₂
Combustion and chlorination
Ring strain

Formulas

C_{n} H_{2 n + 2}, M = 14 n + 2 g/mol

The homologous series of alkanes: each member differs from the previous one by a CH₂ group (14 g/mol).

CH_{4} + 2 O_{2} \overset{}{\to} CO_{2} + 2 H_{2} O Δ H = - 890 kJ/mol

Combustion of methane: 1 L (STP) of methane releases ≈ 39.8 kJ of heat.

CH_{3} COONa + NaOH \overset{\overset{}{t}}{\to} CH_{4} ↑ + Na_{2} CO_{3}

Making methane in the laboratory: sodium ethanoate is heated with soda lime.

CH_{4} + Cl_{2} \overset{\overset{}{hν}}{\to} CH_{3} Cl + HCl

Radical chlorination (in light): it can continue as far as CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.

2 CH_{3} Cl + 2 Na \overset{}{\to} C_{2} H_{6} + 2 NaCl

The Wurtz reaction: an alkane with a longer carbon chain from haloalkanes.

Theory

Alkanes: structure and homologous series (2.1)

Alkanes (saturated hydrocarbons, paraffins) are hydrocarbons whose molecules contain only single C–C and C–H bonds; the general formula is CₙH₂ₙ₊₂. The carbon atoms are sp³-hybridised: the four bonds point to the corners of a tetrahedron (109.5°), C–C 154 pm, C–H 109 pm; so the chain is “zigzag” in shape, and there is free rotation about C–C. A series of substances with similar structure and properties whose composition differs by one or more CH₂ groups (the homologous difference, 14 g/mol) is a homologous series: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane. The isomerism of alkanes (from C₄ onwards) and their naming are covered in the “Isomers and nomenclature” experiment.

Preparation and physical properties (2.3)

In industry alkanes are separated from natural gas (mainly methane), from associated petroleum gases and from oil by distillation; in Uzbekistan the gas of the Shurtan, Mubarek and Ustyurt fields is processed. In the laboratory: heating sodium ethanoate with soda lime (NaOH + CaO) — $CH_{3} COONa + NaOH \overset{}{\to} CH_{4} + Na_{2} CO_{3}$ ; hydrolysis of aluminium carbide — $Al_{4} C_{3} + 12 H_{2} O \overset{}{\to} 3 CH_{4} + 4 Al (OH)_{3}$ ; the Wurtz reaction — $2 CH_{3} Cl + 2 Na \overset{}{\to} C_{2} H_{6} + 2 NaCl$ (the chain doubles in length). Physical properties: C₁–C₄ are gases, C₅–C₁₅ are liquids, and the later ones are solids (paraffin wax). As the molecule gets longer, the van der Waals attraction increases and the boiling point rises: methane −161.5 °C, butane −0.5 °C, pentane 36.1 °C, octane 125.6 °C, decane 174.1 °C. Alkanes are colourless, almost insoluble in water, and less dense than water (pentane 0.626, decane 0.730 g/mL). Methane is odourless and lighter than air (M = 16 g/mol, air — 29).

Practical: making methane and studying its properties

A mixture of sodium ethanoate and soda lime (≈ 1 : 2) is put into a dry test tube, which is clamped with its mouth slightly downwards and heated. The gas is collected under water in an inverted cylinder (methane is almost insoluble in water). Calculation: $n (C H_{4}) = n (C H_{3} C O O N a) = m / M$ , at STP $V_{0} = 22.4 \cdot n$ L; at room temperature (20 °C) the volume is slightly larger: $V = n R T / p$ . Properties: methane does not decolourise bromine water or KMnO₄ solution — alkanes are saturated and under ordinary conditions are not oxidised and do not undergo addition; it burns in air with a pale blue flame. Safety: a mixture of methane with air (5–15 %) explodes — before lighting, the purity of the gas is checked, and it is burned in a small vessel; soda lime is caustic, so wear goggles; before heating is stopped, the delivery tube is taken out of the water.

Chemical properties and uses (2.4)

The C–C and C–H bonds are strong and weakly polar, so alkanes are inert under ordinary conditions (“paraffin” means “little affinity”). Combustion: $CH_{4} + 2 O_{2} \overset{}{\to} CO_{2} + 2 H_{2} O$ , ΔH = −890 kJ/mol; with too little air burning is incomplete — poisonous carbon monoxide CO and soot form (a yellow, smoky flame): $2 CH_{4} + 3 O_{2} \overset{}{\to} 2 CO + 4 H_{2} O$ . Radical substitution (halogenation) takes place in light or on heating: Cl₂ → 2Cl• (initiation), Cl• + CH₄ → HCl + CH₃•, CH₃• + Cl₂ → CH₃Cl + Cl• (propagation); when radicals meet, the chain is terminated. With enough chlorine it goes as far as CH₂Cl₂, CHCl₃ (chloroform) and CCl₄; to get more CH₃Cl, methane is taken in excess. Cracking (450–550 °C): a large molecule breaks into a smaller alkane and an alkene — $C_{10} H_{22} \overset{}{\to} C_{5} H_{12} + C_{5} H_{10}$ . Dehydrogenation: $C_{2} H_{6} \overset{}{\to} C_{2} H_{4} + H_{2}$ — one of the main routes to ethylene. Isomerisation (AlCl₃) improves the quality of petrol. Uses: fuel (domestic gas, propane–butane, petrol, diesel), making hydrogen and synthesis gas, solvents, paraffin wax, ethylene for polyethylene.

Cycloalkanes (2.5–2.6)

Cycloalkanes are saturated hydrocarbons with a closed chain, CₙH₂ₙ (interclass isomers of the alkenes: cyclobutane and but-1-ene). The name is “cyclo-” + the name of the alkane: cyclopropane, cyclobutane, cyclopentane, cyclohexane; substituents are numbered on the ring (methylcyclopropane, 1,2-dimethylcyclohexane). Baeyer strain: the ideal angle for sp³ carbon is 109.5°, but in cyclopropane the C–C–C angle is 60° — the bonds are “bent” and the ring is strained (≈ 115 kJ/mol, from the heat of combustion); in cyclobutane ≈ 110 kJ/mol. Cyclopentane (“envelope”, ≈ 26 kJ/mol) is only slightly strained, and cyclohexane (“chair”, ≈ 111°) is almost strain-free. In the chair form each carbon has one axial hydrogen (parallel to the ring axis) and one equatorial hydrogen; in the chair–chair flip they swap places. Preparation: $C_{6} H_{6} + 3 H_{2} \overset{\overset{}{Ni, t}}{\to} C_{6} H_{12}$ ; cyclopropane from 1,3-dibromopropane by the action of zinc. Properties: small rings (C₃, C₄) undergo addition and open ( $C_{3} H_{6} + Br_{2} \overset{}{\to} C_{3} H_{6} Br_{2}$ , 1,3-dibromopropane), whereas cyclohexane undergoes substitution like the alkanes. Cyclohexane is used to make the raw material for nylon (kapron).

About the model

Boiling and melting points are at 101.3 kPa (CRC Handbook); the state in the ampoules follows these temperatures. Making methane: soda lime is in excess and the reaction goes to completion; the gas in the cylinder is at 20 °C and 101.3 kPa, the water vapour in it and the air in the test tube are not taken into account; the heating time is shortened. Gas stove: burner power 2 kW, ≈ 40 % of the heat passes to the water, and in the kettle mode time is speeded up 10 times; when air is short, part of the carbon becomes CO (according to the oxygen balance). The chlorination is schematic: every C–H bond is assumed equally reactive (a statistical model) — in reality the chloromethanes react at somewhat different rates. The strain energy is the excess in the heat of combustion (compared with 658.6 kJ/mol for CH₂); the chair–chair animation is schematic (the intermediate “boat” forms are not shown).

Did you know?

Natural gas itself is odourless. To notice a leak, a very small amount of a sulfur-containing organic compound with a sharp smell (an odorant, for example ethanethiol) is added to it — this is the “smell of gas” that we notice.
On the octane rating scale of petrol, 2,2,4-trimethylpentane is taken as 100 and normal heptane as 0: branched alkanes burn in the engine without knocking.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Chemistry in our body — Grade 7
Organic chemistry: structure — Grade 10
Isomers and nomenclature — Grade 10
Alkenes and alkadienes — Grade 10
Polymers, rubber and fibres — Grade 10
Alkynes, benzene and styrene — Grade 10
Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10