4.1.2(d)-(e) - Combustion and low reactivity of alkanes

What low reactivity means

Alkanes contain only C-C and C-H single bonds. These single bonds are sigma bonds, written as σ-bonds. In the previous alkane lesson, those bonds explained the saturated structure of alkanes; here, they explain the chemical behaviour of alkanes.

When OCR says alkanes have low reactivity with many reagents, it does not mean alkanes are impossible to react. It means that, under ordinary laboratory conditions, many common reagents have no easy way to attack the alkane molecule. Alkanes do not have a C=C bond, a strongly polar bond, or a reactive functional group that gives a clear electron-rich or electron-poor reaction site.

So the important contrast is:

• alkenes are more reactive because the C=C bond includes a π-bond that can be attacked
• haloalkanes are more reactive because the C-X bond is polar
• alkanes have only strong, very low-polarity C-C and C-H σ-bonds

That is why many alkanes can be stored and handled as fuels, and why some alkanes are useful as non-polar solvents, without reacting with many substances they touch. The same low reactivity is also why alkane chemistry needs special conditions for the few reactions OCR teaches: combustion needs ignition, and radical substitution needs ultraviolet radiation in the next lesson.

Bond enthalpy and bond polarity

A high bond enthalpy means a lot of energy is needed to break a covalent bond. The C-C and C-H σ-bonds in alkanes are strong enough that many reagents cannot break them easily. This gives alkane reactions a high activation barrier.

The second reason is very low polarity. A C-C bond is non-polar because both atoms are carbon. A C-H bond is only very slightly polar because carbon and hydrogen have similar electronegativities. As a result, alkane σ-bonds do not have strong partial charges.

That matters because many organic reactions begin when a reagent is attracted to a charge difference. A nucleophile is attracted to an electron-poor atom; an electrophile is attracted to electron density. In an alkane, there is no strongly δ⁺ carbon or strongly electron-rich π-bond to guide that kind of attack.

Worked reasoning: why does aqueous sodium hydroxide not normally react with ethane?

Ethane contains only C-C and C-H σ-bonds. These bonds have high bond enthalpies, so they are not easily broken. They also have very low polarity, so hydroxide ions are not strongly attracted to any particular atom in the molecule. Therefore ethane has low reactivity with aqueous sodium hydroxide.

The exam-language chain is:

strong C-C/C-H σ-bonds → high bond enthalpy → bonds hard to break

and:

very low bond polarity → no strongly charged reaction site → little attraction for many reagents

A useful comparison is chloroethane: its C–Cl bond is polar, so its δ⁺ carbon can attract an electron-pair donor such as OH⁻. In ethane, the C–C and C–H bonds provide no comparable strongly δ⁺ carbon. This explains the contrast without needing the detailed haloalkane substitution mechanism yet.

Complete combustion of alkane fuels

Alkanes are used as fuels because combustion releases energy. Complete combustion happens when there is a plentiful supply of oxygen. In complete combustion, every carbon atom in the alkane is oxidised to carbon dioxide and the hydrogen atoms form water.

For an alkane with formula $C_nH_{2n+2}$, complete combustion follows this pattern: