Why Chlorinating Methane Gives a Mixture of Four Products
Name alkanes and prepare them by hydrogenation, the Wurtz reaction and decarboxylation, understand the staggered and eclipsed conformations of ethane, and follow the free radical mechanism of alkane halogenation.
Why are alkanes called saturated hydrocarbons?
Alkanes — methane in biogas, propane and butane in LPG, and the long chains in candle wax — contain only single bonds, so every carbon holds as many hydrogen atoms as it can. That makes them unreactive enough to store safely, yet they burn fiercely and swap hydrogen for halogen in light.
This lesson covers naming and preparing alkanes, the conformations of ethane, and the substitution reactions of alkanes.
This lesson covers naming and preparing alkanes, the conformations of ethane, and the substitution reactions of alkanes.
How are alkanes named and prepared, including by the Wurtz reaction and decarboxylation?
**Alkanes have the general formula and the suffix -ane, and they are prepared by hydrogenating unsaturated hydrocarbons, reducing alkyl halides, joining alkyl halides by the Wurtz reaction, or removing carbon dioxide from carboxylate salts.
Naming.** The first members are methane, ethane, propane, butane and pentane; branched alkanes are named from the longest chain, as in 2-methylpropane, .
Methods of preparation:
- Hydrogenation of alkenes and alkynes over finely divided nickel, platinum or palladium:
- Reduction of alkyl halides with zinc and dilute hydrochloric acid:
- Wurtz reaction — an alkyl halide heated with sodium in dry ether gives an alkane with double the number of carbons:
- Decarboxylation — a sodium carboxylate heated with soda lime (NaOH and CaO) gives an alkane with one carbon fewer:
- Kolbe's electrolysis of a concentrated sodium carboxylate solution gives an alkane at the anode:
Worked examples:
- Sodium propanoate, , heated with soda lime gives ethane
- Methyl bromide in the Wurtz reaction gives ethane; ethyl bromide gives butane
An everyday example. Biogas plants in Indian villages produce mostly methane, the simplest alkane, from cattle dung — made by bacteria rather than by any laboratory method.
The substance. The Wurtz reaction cannot make methane and is poor for alkanes with an odd number of carbons — two different alkyl halides give a mixture of three alkanes that is hard to separate.
Naming.** The first members are methane, ethane, propane, butane and pentane; branched alkanes are named from the longest chain, as in 2-methylpropane, .
Methods of preparation:
- Hydrogenation of alkenes and alkynes over finely divided nickel, platinum or palladium:
- Reduction of alkyl halides with zinc and dilute hydrochloric acid:
- Wurtz reaction — an alkyl halide heated with sodium in dry ether gives an alkane with double the number of carbons:
- Decarboxylation — a sodium carboxylate heated with soda lime (NaOH and CaO) gives an alkane with one carbon fewer:
- Kolbe's electrolysis of a concentrated sodium carboxylate solution gives an alkane at the anode:
Worked examples:
- Sodium propanoate, , heated with soda lime gives ethane
- Methyl bromide in the Wurtz reaction gives ethane; ethyl bromide gives butane
An everyday example. Biogas plants in Indian villages produce mostly methane, the simplest alkane, from cattle dung — made by bacteria rather than by any laboratory method.
The substance. The Wurtz reaction cannot make methane and is poor for alkanes with an odd number of carbons — two different alkyl halides give a mixture of three alkanes that is hard to separate.
What are the staggered and eclipsed conformations of ethane?
Conformations are the different arrangements of atoms produced by rotation about a carbon-carbon single bond; in ethane, the staggered form, with the hydrogens as far apart as possible, is the most stable and the eclipsed form the least stable.
How they arise. The sigma bond between the two carbons allows rotation, so the three hydrogens on one carbon can turn relative to the three on the other.
The two extreme forms:
- Staggered — the hydrogens on the front carbon lie midway between those on the back carbon, at a dihedral angle of 60°; repulsion is least
- Eclipsed — the front hydrogens lie directly in front of the back ones, at a dihedral angle of 0°; repulsion is greatest
- Every arrangement in between is a skew conformation
Representations:
- Sawhorse projection — the C-C bond drawn as a slanted line, with three bonds on each carbon
- Newman projection — viewed along the C-C bond, with the front carbon as a point and the back carbon as a circle
Torsional strain. The repulsion between bonding electron pairs on neighbouring carbons is called torsional strain. The eclipsed form is less stable than the staggered form by only about 12.5 kJ mol.
Worked reasoning. Molecules at room temperature have enough energy to cross this small barrier constantly, so the conformations of ethane interconvert rapidly and cannot be separated.
An everyday example. Picture two three-bladed ceiling fans mounted one above the other, seen from below — when their blades line up, the arrangement is eclipsed; when each blade falls between two of the other, it is staggered.
The substance. Conformations are not isomers you can bottle — they are one molecule twisting, unlike structural isomers, which need bonds to break before one turns into another.
How they arise. The sigma bond between the two carbons allows rotation, so the three hydrogens on one carbon can turn relative to the three on the other.
The two extreme forms:
- Staggered — the hydrogens on the front carbon lie midway between those on the back carbon, at a dihedral angle of 60°; repulsion is least
- Eclipsed — the front hydrogens lie directly in front of the back ones, at a dihedral angle of 0°; repulsion is greatest
- Every arrangement in between is a skew conformation
Representations:
- Sawhorse projection — the C-C bond drawn as a slanted line, with three bonds on each carbon
- Newman projection — viewed along the C-C bond, with the front carbon as a point and the back carbon as a circle
Torsional strain. The repulsion between bonding electron pairs on neighbouring carbons is called torsional strain. The eclipsed form is less stable than the staggered form by only about 12.5 kJ mol.
Worked reasoning. Molecules at room temperature have enough energy to cross this small barrier constantly, so the conformations of ethane interconvert rapidly and cannot be separated.
An everyday example. Picture two three-bladed ceiling fans mounted one above the other, seen from below — when their blades line up, the arrangement is eclipsed; when each blade falls between two of the other, it is staggered.
The substance. Conformations are not isomers you can bottle — they are one molecule twisting, unlike structural isomers, which need bonds to break before one turns into another.
What are the substitution reactions of alkanes, and how does free radical halogenation work?
In a substitution reaction, a hydrogen atom of an alkane is replaced by another atom such as a halogen, and halogenation proceeds by a free radical chain mechanism started by light or heat.
Halogenation of methane:
With excess chlorine, substitution continues to , and — the mixture of four products.
Mechanism:
- Initiation — light splits chlorine:
- Propagation — , then ; each step regenerates a radical
- Termination — radicals combine: , and
Reactivity:
- Halogens: ; fluorination is explosive, and iodination is reversible, needing an oxidising agent such as iodic acid to remove HI
- Hydrogens: tertiary > secondary > primary, following free radical stability
Worked example. Monochlorination of propane gives two products, 1-chloropropane and 2-chloropropane, because propane has two kinds of hydrogen — six on the end carbons and two on the middle carbon.
Other reactions. Alkanes also undergo combustion, controlled oxidation, isomerisation with , and cracking of long chains into smaller molecules.
An everyday example. Dichloromethane, one product of methane chlorination, is used as a solvent in paint strippers.
The substance. Ethane among the products is evidence for the free radical mechanism — it can form only when two methyl radicals combine in the termination step.
Halogenation of methane:
With excess chlorine, substitution continues to , and — the mixture of four products.
Mechanism:
- Initiation — light splits chlorine:
- Propagation — , then ; each step regenerates a radical
- Termination — radicals combine: , and
Reactivity:
- Halogens: ; fluorination is explosive, and iodination is reversible, needing an oxidising agent such as iodic acid to remove HI
- Hydrogens: tertiary > secondary > primary, following free radical stability
Worked example. Monochlorination of propane gives two products, 1-chloropropane and 2-chloropropane, because propane has two kinds of hydrogen — six on the end carbons and two on the middle carbon.
Other reactions. Alkanes also undergo combustion, controlled oxidation, isomerisation with , and cracking of long chains into smaller molecules.
An everyday example. Dichloromethane, one product of methane chlorination, is used as a solvent in paint strippers.
The substance. Ethane among the products is evidence for the free radical mechanism — it can form only when two methyl radicals combine in the termination step.
Exam tip
What earns full marks on alkanes?
For every preparation question, count the carbons in the reactant and the product — the Wurtz reaction doubles them, and decarboxylation removes one.
- Wurtz: in dry ether
- Decarboxylation: , with CaO
- Ethane: staggered more stable than eclipsed, by about 12.5 kJ mol
- Halogenation: initiation, propagation and termination
The trap. Writing ethane as the decarboxylation product of sodium ethanoate. Sodium ethanoate has two carbons, so it gives methane.
- Wurtz: in dry ether
- Decarboxylation: , with CaO
- Ethane: staggered more stable than eclipsed, by about 12.5 kJ mol
- Halogenation: initiation, propagation and termination
The trap. Writing ethane as the decarboxylation product of sodium ethanoate. Sodium ethanoate has two carbons, so it gives methane.
Did you know
Why can you smell an LPG leak if its gases have no smell?
Propane and butane, the alkanes in LPG, are colourless and have no smell of their own, so a leak could otherwise go unnoticed.
To make leaks detectable, a tiny amount of a strong-smelling sulphur compound, ethyl mercaptan, is added to the gas. The unpleasant smell near a leaking cylinder comes entirely from that additive, not from the fuel.
It is a simple safety step built on the fact that alkanes themselves are odourless.
To make leaks detectable, a tiny amount of a strong-smelling sulphur compound, ethyl mercaptan, is added to the gas. The unpleasant smell near a leaking cylinder comes entirely from that additive, not from the fuel.
It is a simple safety step built on the fact that alkanes themselves are odourless.
Exam relevance
How do JEE Main and NEET test alkanes, conformations and halogenation?
Hydrocarbons is a recurring chapter in both JEE Main and NEET, and alkanes are its starting point.
What gets asked. Products of the Wurtz reaction, decarboxylation and Kolbe's electrolysis, Newman projections and the relative stability of conformations, steps of the free radical halogenation mechanism, and the number of monochlorinated products of a given alkane.
Question types. Mostly single-correct questions, with numerical-value questions in JEE Main on counting substitution products.
Why it matters later. Free radical mechanisms return in Haloalkanes and Haloarenes, and alkane preparations are chained into multi-step conversion questions across organic chemistry.
The trap that costs marks. Treating every hydrogen as different when counting monochlorinated products — equivalent hydrogens give the same product, so 2-methylbutane gives only four structural isomers.
What gets asked. Products of the Wurtz reaction, decarboxylation and Kolbe's electrolysis, Newman projections and the relative stability of conformations, steps of the free radical halogenation mechanism, and the number of monochlorinated products of a given alkane.
Question types. Mostly single-correct questions, with numerical-value questions in JEE Main on counting substitution products.
Why it matters later. Free radical mechanisms return in Haloalkanes and Haloarenes, and alkane preparations are chained into multi-step conversion questions across organic chemistry.
The trap that costs marks. Treating every hydrogen as different when counting monochlorinated products — equivalent hydrogens give the same product, so 2-methylbutane gives only four structural isomers.
Key takeaways
What must you be able to do from this lesson?
- Preparation: hydrogenation, reduction of alkyl halides, the Wurtz reaction doubling carbons, and decarboxylation removing one carbon
- Conformations of ethane: staggered and eclipsed forms, Sawhorse and Newman projections, and a small barrier to rotation
- Substitution: free radical halogenation through initiation, propagation and termination
Which alkane forms when sodium butanoate is heated with soda lime, and how many carbons does it have?
- Conformations of ethane: staggered and eclipsed forms, Sawhorse and Newman projections, and a small barrier to rotation
- Substitution: free radical halogenation through initiation, propagation and termination
Which alkane forms when sodium butanoate is heated with soda lime, and how many carbons does it have?