Reactions of alkanes with halogens

The most important reaction of alkanes is combustion — burning in oxygen. This releases large amounts of heat, which is why alkanes (natural gas, petrol, diesel) are our main fuels.

When there is plenty of oxygen (excess O₂), combustion is complete: every carbon atom is turned into carbon dioxide and every hydrogen into water.

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

$\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$

How to balance a complete-combustion equation for an alkane CₓHᵧ:

• Carbons: x carbon atoms → x CO₂.
• Hydrogens: y hydrogen atoms → y/2 H₂O.
• Count the oxygen atoms in the products, then halve to get the number of O₂ molecules.
• If you get a fraction (e.g. 3.5 O₂), multiply the whole equation by 2 to clear it.

Worked check for C₃H₈:

• Oxygen atoms in products = (3 × 2 from CO₂) + (4 × 1 from H₂O) = 10 → 5 O₂. ✓

Features of complete combustion (e.g. a Bunsen burner with the air hole open):

• A clean, blue flame.
• Maximum heat released.
• Only CO₂ + H₂O as products — no soot, no toxic carbon monoxide.

When there is not enough oxygen (a limited supply of O₂), combustion is incomplete. The carbon is only partly oxidised, so as well as some CO₂ you also get:

• Carbon monoxide (CO) — carbon partly oxidised.
• Carbon (C) — soot, carbon not oxidised at all.
• Water (H₂O) — hydrogen always ends up as water.

Example equations (you may be asked to write either):

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \quad \text{(carbon monoxide formed)}$

$\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O} \quad \text{(soot formed)}$

Features of incomplete combustion (e.g. a Bunsen with the air hole closed):

• A smoky, yellow flame.
• Less heat released (less efficient — fuel is wasted).
• Soot deposited on cool surfaces.
• Produces toxic carbon monoxide.

Why carbon monoxide is dangerous:

• It is colourless and odourless — you cannot detect it without a CO alarm.
• It binds strongly to haemoglobin in red blood cells, stopping them carrying oxygen.
• This causes oxygen starvation of the body — headaches, drowsiness and, in high amounts, death.

Alkanes also react with halogens (chlorine, Cl₂, or bromine, Br₂) in a substitution reaction. In a substitution reaction, one hydrogen atom of the alkane is replaced by a halogen atom.

Essential condition: ultraviolet (UV) light. Without UV light (or strong sunlight) the reaction does not happen at room temperature.

General word equation:

$\text{alkane} + \text{halogen} \xrightarrow{\text{UV light}} \text{halogenoalkane} + \text{hydrogen halide}$

Examples (note how just one H is swapped for a halogen, and HX is always a product):

$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV}} \text{CH}_3\text{Cl} + \text{HCl}$

$\text{CH}_4 + \text{Br}_2 \xrightarrow{\text{UV}} \text{CH}_3\text{Br} + \text{HBr}$

$\text{C}_2\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{UV}} \text{C}_2\text{H}_5\text{Cl} + \text{HCl}$

Why UV light is needed. UV light carries enough energy to break the halogen–halogen bond (the Cl–Cl or Br–Br bond). This splits the halogen molecule into reactive atoms, which then attack the alkane and start the reaction. (At Double Award you only need: UV light breaks the halogen–halogen bond to start the reaction — the detailed mechanism is not required.)

A common exam question asks for the difference between substitution and addition. The same halogen can react with an alkane and an alkene in completely different ways:

	Substitution (alkanes)	Addition (alkenes)
What happens	one H atom is replaced by a halogen atom	the halogen adds across the C=C double bond
Number of halogen atoms	one halogen atom added (one H removed)	two halogen atoms added
Hydrogen halide (HX)?	yes, HCl or HBr is formed	no HX is formed
Condition needed	UV light	none — happens at room temperature, no UV

Why the difference?

• Alkanes are saturated — they have no C=C double bond, so nothing can add across; a hydrogen must be swapped out instead (substitution).
• Alkenes are unsaturated — the C=C double bond opens up and the halogen atoms add on (addition).

This is also why a bromine water test can tell an alkene from an alkane:

• An alkene turns bromine water from orange-brown to colourless (rapid addition).
• An alkane leaves bromine water orange-brown (no reaction — there is no UV light and no C=C).