Halogenation, combustion and cracking of alkanes

Why alkanes are slow to react

Alkanes are not touched by acids, bases or the usual oxidising agents. The C-C and C-H bonds are strong, and they are nearly nonpolar. There is no region of the molecule that attracts an ion. So the reactions that alkanes do show need a push, such as light, heat or a catalyst. The main ones are substitution by halogens, combustion, cracking and a few others that we list at the end.

Halogenation: a free-radical chain

Methane and chlorine do not react in the dark. In ultraviolet light, in diffused sunlight, or at high temperature, they react:

A hydrogen atom is substituted by a chlorine atom. The reaction can go on. The product $\text{CH}_{3}\text{Cl}$ gives $\text{CH}_{2}\text{Cl}_{2}$, then $\text{CHCl}_{3}$ (trichloromethane, also called chloroform), then $\text{CCl}_{4}$ (tetrachloromethane). A large excess of methane favours $\text{CH}_{3}\text{Cl}$. A large excess of chlorine favours $\text{CCl}_{4}$.

The mechanism

The reaction runs by free radicals, as you saw in chapter 8. It is a chain reaction with three stages.

1. Initiation. Light supplies the energy to break the weak Cl-Cl bond. Each chlorine atom keeps one electron, so we get two chlorine atoms with an unpaired electron: $\text{Cl}^{\bullet}$.
2. Propagation. A chlorine atom takes a hydrogen atom from methane. This makes a methyl radical and hydrogen chloride. The methyl radical takes a chlorine atom from a chlorine molecule. This makes $\text{CH}_{3}\text{Cl}$ and a new chlorine atom. The new atom starts the cycle again.
3. Termination. Two radicals meet and join. No new radical is made, and the chain stops.

One photon starts a chain that makes thousands of molecules. That is why a flash of light can be enough. Two facts support the mechanism. The reaction needs light. And a small amount of oxygen slows it, because oxygen traps radicals.

The halogens compared

• Fluorine reacts explosively, even in the dark, with the release of a great deal of heat.
• Chlorine and bromine react at a rate that can be controlled. Chlorine is faster.
• Iodine reacts slowly, and the reaction goes backwards, because hydrogen iodide reduces the alkyl iodide back to the alkane. An oxidising agent such as iodic acid or nitric acid is added to destroy the hydrogen iodide.

The order of reactivity is $\text{F}_{2} > \text{Cl}_{2} > \text{Br}_{2} > \text{I}_{2}$.

Which hydrogen is replaced

Propane has two kinds of hydrogen. Six are primary, on the end carbon atoms. Two are secondary, on the middle carbon. The chlorine atom takes a hydrogen from the carbon that gives the more stable radical. Tertiary radicals are more stable than secondary ones, and these are more stable than primary ones.

Measured at room temperature, the relative rates for one hydrogen atom are

The share of each product is the number of hydrogen atoms of that kind, times their relative rate, divided by the total. For propane and chlorine:

That is 44 per cent and 56 per cent. With bromine it is $6 : 164$, so 2-bromopropane is 96 per cent of the product.

Bromine is less reactive and more selective than chlorine. A bromine atom is slower to take a hydrogen atom, so it is more choosy. It goes for the hydrogen that gives the most stable radical. A chlorine atom reacts so easily that it takes almost any hydrogen it meets.

Practise with new numbers

Combustion

Alkanes burn in air. This is the reaction that gives them their value as fuels. Complete combustion gives carbon dioxide and water, with a large release of heat:

Written for methane, $\text{CH}_{4}\ +\ 2\text{O}_{2}\ \longrightarrow \ \text{CO}_{2}\ +\ 2\text{H}_{2}\text{O}$ gives out $890\ \text{kJ}$ for each mole. For ethane the half in the formula is cleared by doubling: $2\text{C}_{2}\text{H}_{6}\ +\ 7\text{O}_{2}\ \longrightarrow \ 4\text{CO}_{2}\ +\ 6\text{H}_{2}\text{O}$.

With too little oxygen, the combustion is incomplete. Carbon monoxide and soot form. Carbon monoxide is a poison, which is why a gas heater needs fresh air.

The program below shows that the heat per gram falls slowly as the chain gets longer. Methane gives $55.5\ \text{kJ/g}$ and octane $47.9\ \text{kJ/g}$.

Other reactions, briefly

• Controlled oxidation. With a limited supply of oxygen and a catalyst, methane gives methanol (copper tube, about $523\ \text{K}$, $100\ \text{atm}$) or methanal (molybdenum(III) oxide as catalyst). A tertiary alkane such as 2-methylpropane is oxidised by potassium permanganate to a tertiary alcohol.
• Isomerisation. A straight-chain alkane is heated with anhydrous aluminium chloride and hydrogen chloride. It changes into a branched isomer: butane gives 2-methylpropane.
• Aromatisation. An alkane with six or more carbon atoms can lose hydrogen and close into a ring. The conditions are about $773\ \text{K}$, a pressure of $10$ to $20\ \text{atm}$, and a catalyst of chromium, vanadium or molybdenum oxide on alumina. Hexane gives benzene: $\text{C}_{6}\text{H}_{14}\ \longrightarrow \ \text{C}_{6}\text{H}_{6}\ +\ 4\text{H}_{2}$.
• Steam reforming. Methane and steam over nickel at about $1273\ \text{K}$ give carbon monoxide and hydrogen: $\text{CH}_{4}\ +\ \text{H}_{2}\text{O}\ \longrightarrow \ \text{CO}\ +\ 3\text{H}_{2}$. The mixture is a source of hydrogen.
• Pyrolysis, or cracking. Heating an alkane to a high temperature in the absence of air breaks C-C bonds. A large alkane gives a smaller alkane and an alkene. Dodecane gives heptane and pentene:

Atoms are conserved: $12$ carbon atoms and $26$ hydrogen atoms on each side. Cracking is how an oil refinery gets more petrol from the heavy fractions of petroleum.

Common mistakes

• Writing chlorination as an ion reaction. It is a chain of free radicals. The bonds break evenly.
• Expecting only $\text{CH}_{3}\text{Cl}$. More hydrogen atoms are replaced when chlorine is in excess.
• Calling iodination the fastest. Fluorine is the most reactive halogen. Iodine reacts slowly and reversibly.
• Forgetting the number of hydrogen atoms. The share of a product is the number of hydrogen atoms times the rate for one.
• Balancing combustion with the wrong oxygen count. Count the oxygen atoms in the products first, then halve.

Remember these

• Alkanes are unreactive. They are halogenated by free radicals in light, burn, and crack.
• Halogenation: initiation (light), propagation (two steps), termination (radicals meet).
• Reactivity of the halogens: $\text{F}_{2} > \text{Cl}_{2} > \text{Br}_{2} > \text{I}_{2}$. Bromine is more selective than chlorine.
• Hydrogen replaced: tertiary, then secondary, then primary. Share of a product = number of hydrogen atoms times the rate for one.
• Combustion gives $\text{CO}_{2}$ and $\text{H}_{2}\text{O}$, or $\text{CO}$ and soot if the air is short. Cracking gives a smaller alkane and an alkene.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Write the two propagation steps when ethane reacts with chlorine in light.

2. Why is bromination of an alkane more selective than chlorination?

3. How many different monochloro products can 2-methylpropane give?

4. Write the balanced equation for the complete combustion of pentane.

5. Dodecane is cracked. One product is heptane. What is the other, and how can you show that it is not an alkane?

References

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons" (reactions of alkanes).
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (chemical properties of alkanes).