Saturated Hydrocarbon Reactions (HSC SSCE Chemistry): Revision Notes

Saturated Hydrocarbon Reactions

Introduction to saturated hydrocarbons

Saturated hydrocarbons, also called alkanes, are hydrocarbon molecules containing only single bonds between carbon atoms. The term 'saturated' refers to the fact that each carbon atom is bonded to the maximum number of hydrogen atoms possible.

Unlike alkenes and alkynes, alkanes show relatively low reactivity with most chemical substances. This is because they lack reactive functional groups such as double or triple bonds. The absence of these reactive sites means alkanes are quite stable under normal conditions.

Despite their general unreactivity, alkanes do undergo two important types of chemical reactions that are crucial for their use as fuels and in chemical synthesis:

• Combustion reactions (burning in oxygen)
• Substitution reactions (replacing hydrogen atoms with halogen atoms)

Combustion of alkanes

Complete combustion

When alkanes burn in a plentiful supply of oxygen, they undergo complete combustion. This reaction produces carbon dioxide and water as the only products, along with releasing large amounts of heat energy. This high energy output makes alkanes ideal for use as fuels in heating, cooking, and transportation.

The general pattern for complete combustion is:

Alkane + Excess oxygen → Carbon dioxide + Water + Heat energy

For example, the complete combustion of propane:

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

And for octane, a component of petrol:

$$2\text{C}_8\text{H}_{18}(l) + 25\text{O}_2(g) \rightarrow 16\text{CO}_2(g) + 18\text{H}_2\text{O}(l)$$

In both cases, notice that all the carbon atoms are converted to carbon dioxide ($\text{CO}_2$) and all the hydrogen atoms form water ($\text{H}_2\text{O}$). This is what makes the combustion 'complete'.

Incomplete combustion

In many real-world situations, such as furnaces and car engines, oxygen is not present in excess amounts. When alkanes burn with limited oxygen supply, incomplete combustion occurs. This produces different products including carbon monoxide ($\text{CO}$) and solid carbon particles, also known as soot.

The products formed during incomplete combustion depend on how much oxygen is available. Less oxygen generally leads to less oxidised products. The equations below show hexane burning with decreasing amounts of oxygen:

To maximise efficiency, car engines are designed to allow as much air as possible into the combustion chamber. Regular vehicle servicing includes checking that air intake systems are working properly to encourage complete combustion.

Health and environmental impacts

The products of incomplete combustion pose serious health and environmental concerns beyond just wasting fuel.

Carbon monoxide effects:

At lower levels ($50-100$ ppm, typical of city traffic during peak hours), carbon monoxide causes:

• Impaired judgement
• Headaches
• Dizziness
• Altered visual perception

At higher concentrations above $250$ ppm, carbon monoxide can cause loss of consciousness and death.

Soot particle effects:

Carbon particles produced during incomplete combustion are known as soot. These are tiny crystalline carbon particles small enough to be inhaled deep into the lungs. Once inside the respiratory system, soot particles cause several problems:

• They coat the lungs, reducing the surface area available for gas exchange
• This decreases the body's ability to take in oxygen, impairing respiration
• The particles have molecules of unburnt fuel and other toxic substances bonded to their surface
• These toxins enter the body when inhaled along with the soot particles

Substitution reactions with halogens

Conditions and mechanism

In a substitution reaction, an atom of one element replaces (substitutes for) an atom in a molecule. For alkanes, substitution reactions involve replacing a hydrogen atom with a halogen atom (usually chlorine or bromine).

The need for UV light is a key difference between alkane and alkene reactivity. Alkenes react readily with halogens at room temperature, whilst alkanes need the extra energy from UV light to initiate the reaction.

Chlorination and bromination

When methane is exposed to chlorine gas in the presence of UV light, one hydrogen atom is replaced by a chlorine atom. This forms chloromethane and hydrogen chloride as products:

$$\text{CH}_4(g) + \text{Cl}_2(g) \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl}(g) + \text{HCl}(g)$$

Similarly, methane reacts with bromine to produce bromomethane and hydrogen bromide:

$$\text{CH}_4(g) + \text{Br}_2(aq) \xrightarrow{\text{UV light}} \text{CH}_3\text{Br}(aq) + \text{HBr}(g)$$

These substitution reactions produce molecules called haloalkanes (in this case, chloromethane and bromomethane). Haloalkanes are important in organic synthesis and industrial chemistry.

Multiple substitutions

Substitution reactions can continue beyond the first hydrogen replacement. Under continued exposure to UV light and halogen, additional hydrogen atoms can be substituted one at a time until all hydrogens have been replaced.

The same stepwise process occurs with bromine, eventually forming tetrabromomethane ($\text{CBr}_4$).

Comparing reactivity of alkanes and alkenes

Alkanes and alkenes show markedly different reactivity patterns with halogens. This difference can be demonstrated through simple laboratory tests.

When bromine water (aqueous bromine solution) is added to:

• An alkene (e.g., cyclohexene): The orange-brown colour of bromine disappears rapidly at room temperature due to an addition reaction
• An alkane (e.g., cyclohexane): No colour change occurs at room temperature; reaction only proceeds under UV light

Investigation 10.4 in the textbook demonstrates this comparison experimentally, testing the reactivity of hexane, cyclohexane (both alkanes), and cyclohexene (an alkene) with acidified potassium permanganate and bromine water. Such experiments clearly show that alkanes require much more forcing conditions (UV light, heat) to react compared to alkenes.

Remember!