Why Branched Alkanes Make Better Petrol Than Straight Chains

CNG in buses, LPG in kitchen cylinders, petrol, diesel and candle wax are all hydrocarbons — compounds of only carbon and hydrogen. They fuel homes and vehicles and are the raw material for plastics and medicines.

The simplest family, the alkanes, has only single bonds, yet shows rich chemistry once you understand how its bonds behave.

This part covers classification and isomers, preparation of alkanes, their properties and reactions, and conformations of ethane.

**Hydrocarbons are saturated (alkanes, C${}_n$H${}_{2n+2}$, only single bonds), unsaturated (alkenes C${}_n$H${}_{2n}$ and alkynes C${}_n$H${}_{2n-2}$), alicyclic (carbon rings such as cyclohexane) or aromatic (benzene and its relatives).

Worked example — isomers of C${}_6$H${}_{14}$. There are five** chain isomers:

- CH${}_3$(CH${}_2$)${}_4$CH${}_3$ — hexane
- CH${}_3$CH(CH${}_3$)CH${}_2$CH${}_2$CH${}_3$ — 2-methylpentane
- CH${}_3$CH${}_2$CH(CH${}_3$)CH${}_2$CH${}_3$ — 3-methylpentane
- CH${}_3$C(CH${}_3$)${}_2$CH${}_2$CH${}_3$ — 2,2-dimethylbutane
- CH${}_3$CH(CH${}_3$)CH(CH${}_3$)CH${}_3$ — 2,3-dimethylbutane

For comparison, C${}_4$H${}_{10}$ has two isomers and C${}_5$H${}_{12}$ has three.

An everyday example. CNG is mostly methane, while LPG is mainly propane and butane — members of the same alkane family with different chain lengths.

The substance. **Cycloalkanes share the general formula C${}_n$H${}_{2n}$ with alkenes**, so cyclopropane and propene are isomers.

Alkanes form when hydrogen adds to alkenes or alkynes over a metal catalyst, when alkyl halides are reduced, when two alkyl halides couple with sodium in dry ether (Wurtz reaction), or when a sodium salt of a carboxylic acid is heated with soda-lime, losing one carbon as carbonate.

- Hydrogenation: CH${}_2$=CH${}_2$ + H${}_2$ $\to$ CH${}_3$CH${}_3$ with Pt, Pd or Ni
- Reduction of alkyl halides: CH${}_3$CH${}_2$Cl + H${}_2$ $\to$ CH${}_3$CH${}_3$ + HCl, using zinc and acid
- Wurtz reaction: 2CH${}_3$Br + 2Na $\to$ CH${}_3$CH${}_3$ + 2NaBr, in dry ether
- Decarboxylation: CH${}_3$COONa + NaOH $\to$ CH${}_4$ + Na${}_2$CO${}_3$, heated with CaO

Worked example 1 — decarboxylation yield. $8.2$ g of sodium acetate ($82$ g/mol) is $0.1$ mol, so it gives $0.1$ mol of methane:

$$0.1\times 16=1.6\text{ g}0.1\times 22.4=2.24\text{ L at STP}$$

Worked example 2 — a mixed Wurtz reaction. CH${}_3$Br and C${}_2$H${}_5$Br together with sodium give three alkanes — ethane, propane and butane — which are hard to separate.

An everyday example. Vanaspati ghee is made by hydrogenating vegetable oils over a nickel catalyst, turning liquid oils into a solid fat.

The substance. The Wurtz reaction is suited to making symmetrical alkanes with an even number of carbons, not odd-carbon alkanes.

Alkanes are non-polar, insoluble in water, and boil at higher temperatures as chains grow; they react mainly by free-radical substitution with halogens in light, and also undergo combustion, controlled oxidation, isomerisation, aromatisation and pyrolysis.

Boiling points. Branching lowers the boiling point: pentane $309$ K, 2-methylbutane $301$ K, 2,2-dimethylpropane about $282.5$ K — more compact molecules have smaller surface contact.

Free-radical chlorination of methane:

- Initiation: Cl${}_2$ $\to$ 2Cl${}^{\bullet }$ in light
- Propagation: Cl${}^{\bullet }$ + CH${}_4$ $\to$ ${}^{\bullet }$CH${}_3$ + HCl, then ${}^{\bullet }$CH${}_3$ + Cl${}_2$ $\to$ CH${}_3$Cl + Cl${}^{\bullet }$
- Termination: two radicals combine, such as ${}^{\bullet }$CH${}_3$ + ${}^{\bullet }$CH${}_3$ $\to$ C${}_2$H${}_6$

Further substitution gives CH${}_2$Cl${}_2$, CHCl${}_3$ and CCl${}_4$; reactivity of halogens is F${}_2$ > Cl${}_2$ > Br${}_2$ > I${}_2$.

Other reactions:

- Combustion: CH${}_4$ + 2O${}_2$ $\to$ CO${}_2$ + 2H${}_2$O, releasing about $890$ kJ per mole
- Controlled oxidation: methane to methanol or methanal with suitable catalysts
- Isomerisation: hexane to branched isomers with anhydrous AlCl${}_3$ and HCl
- Aromatisation: hexane to benzene over metal oxide catalysts at high temperature
- Pyrolysis (cracking): long alkanes break into smaller alkanes, alkenes and hydrogen when heated strongly

An everyday example. Burning LPG on a gas stove is alkane combustion, releasing the heat that cooks food.

The substance. Chlorination of methane gives a mixture of products, not a single chloromethane.

Rotation about the C–C single bond of ethane gives conformations: in the staggered form the hydrogens on the two carbons are as far apart as possible, in the eclipsed form they line up, and the staggered form is more stable because the eclipsed form suffers torsional strain.

Projections.

- Sawhorse — the C–C bond is drawn at an angle, with three hydrogens on each carbon
- Newman — looking straight along the C–C bond: the front carbon is a point where three bonds meet, the back carbon a circle with three bonds coming from its edge

Dihedral angle. In the staggered form, front and back C–H bonds are $60°$ apart; in the eclipsed form they are $0°$ apart.

Worked example — the energy difference. The eclipsed form is about $12.5$ kJ/mol higher in energy. At $298$ K, $RT$ is about $8.314\times 10^{-3}\times 298\approx 2.5$ kJ/mol, and collisions easily supply the rest, so the two forms interconvert rapidly.

Torsional strain is the repulsion between bonding electrons of the C–H bonds on neighbouring carbons when they are eclipsed.

An everyday example. Twisting one half of a spinning top against the other passes through positions where the parts line up and positions where they alternate — like eclipsed and staggered ethane.

The substance. Conformations of ethane cannot be separated at room temperature, because rotation about the single bond is too easy.