4cAlkanes

Alkanes are the simplest organic family, and the fuels most of the world runs on.

The general formula

CₙH₂ₙ₊₂

Double the number of carbons, then add two.

Use the formula to check a name. C₄H₈ does not fit CₙH₂ₙ₊₂, so it cannot be butane — it is an alkene.

Saturated

Saturated means the molecule contains only single carbon-carbon bonds.

And because there are no double bonds to open, an alkane already holds the maximum possible number of hydrogen atoms. There is nowhere to put any more.

That is the whole meaning of the word. It has nothing to do with dissolving, despite the shared term in everyday use.

Every carbon forms four bonds

This is the rule that makes organic structures work. In propane, CH₃CH₂CH₃:

• The end carbons each use one bond to a carbon and three to hydrogens.
• The middle carbon uses two bonds to carbons, so it can hold only two hydrogens.

Counting bonds is the quickest way to check any structure you have written. If a carbon has three or five, something is wrong.

Why alkanes are unreactive

They have no functional group. There is no double bond to open and no reactive site, and the C–C and C–H bonds are strong.

So alkanes react in only two ways on this specification: combustion, and substitution with halogens under ultraviolet light.

That unreactivity is exactly why they make good fuels — they are stable until deliberately ignited.

Structural formulae

A structural formula shows how the atoms are joined, written on one line:

Each hydrogen is grouped with the carbon it is bonded to, so you can read the chain straight off.

A displayed formula goes further and shows every bond as a line — every C–C and every C–H drawn out.

Substitution with halogens

Alkanes react with halogens only in the presence of ultraviolet radiation. In the dark, nothing happens.

methane + chlorine → chloromethane + hydrogen chloride

CH₄ + Cl₂ → CH₃Cl + HCl

Two products. One hydrogen is replaced by a chlorine atom, and the displaced hydrogen leaves with the other chlorine as hydrogen chloride.

Forgetting the hydrogen chloride is the usual omission — but it is where the second halogen atom went, so it has to be there.

What the ultraviolet light does

It supplies the energy needed to start the reaction, breaking the halogen molecules apart.

It is not a catalyst. A catalyst is a substance, recovered unchanged; ultraviolet light is an energy source.

Mono-substitution

The specification limits this to mono-substitution — exactly one hydrogen replaced.

So ethane and bromine give bromoethane, C₂H₅Br, with one bromine atom:

C₂H₆ + Br₂ → C₂H₅Br + HBr

If your organic product has two halogen atoms, you have gone beyond what the question asked for.

Alkane or alkene?

The difference in reaction type is a direct consequence of the bonding:

When bromine meets ethene, both bromine atoms join the molecule. When it meets ethane, only one does and the other leaves as HBr. Following that second atom tells you which reaction happened.