Alkanes and alkenes, free-radical substitution, electrophilic addition, Markovnikov addition, addition polymerisation, and combustion.

What this dot point is asking

WJEC wants you to describe the reactions of alkanes and alkenes: free-radical substitution of alkanes, electrophilic addition to alkenes (with Markovnikov's rule), addition polymerisation, and combustion, including the mechanisms and conditions.

The answer

Alkanes: free-radical substitution

Alkanes are otherwise unreactive because their $\text{C}-\text{C}$ and $\text{C}-\text{H}$ bonds are strong and non-polar.

Alkenes: electrophilic addition

The $\text{C}=\text{C}$ double bond is a region of high electron density that attracts electrophiles. Adding $\text{HBr}$, $\text{Br}_2$ or steam (with acid catalyst) opens the double bond.

Addition polymerisation and combustion

Many alkene monomers join to form a saturated polymer chain, for example $n\,\text{CH}_2=\text{CH}_2 \rightarrow (\text{CH}_2\text{CH}_2)_n$ (poly(ethene)). Complete combustion gives $\text{CO}_2$ and $\text{H}_2\text{O}$; incomplete combustion in limited oxygen gives $\text{CO}$ or soot.

Why Markovnikov works: carbocation stability

The regiochemistry of electrophilic addition comes from the stability of the carbocation intermediate. Alkyl groups are electron-releasing (a positive inductive effect), so they spread out and reduce the positive charge on the carbocation carbon. A tertiary carbocation (three alkyl groups) is therefore more stable than a secondary one, which is more stable than a primary one. When $\text{HBr}$ adds to propene, the hydrogen adds to the terminal carbon so the positive charge lands on the more substituted (secondary) carbon, the more stable cation, giving 2-bromopropane as the major product. This stability argument is exactly what an Explain question wants.

Addition polymers and their properties

In addition polymerisation, the $\text{C}=\text{C}$ double bonds of many alkene monomers open and join into a long saturated chain with no small molecule lost. Poly(ethene), poly(propene) and poly(chloroethene) (PVC) are made this way. Because the backbone is saturated and non-polar, these polymers are unreactive and non-biodegradable, which makes them durable but creates a waste problem. The side group (chlorine in PVC, methyl in poly(propene)) tunes the properties, linking polymer structure to use.

Examples in context

Cracking and fuels. Long-chain alkanes from crude oil are cracked into smaller alkanes and alkenes; the alkanes become petrol while the alkenes feed polymer manufacture. Poly(ethene) and plastics. Addition polymerisation of ethene produces the millions of tonnes of poly(ethene) used in packaging, while incomplete combustion concerns (carbon monoxide) link to the wider environmental impact studied later in Unit 2.

Try this

Q1. State the reagent and condition needed to convert ethene to ethane. [1 mark]

• Cue. Hydrogen with a nickel catalyst (and heat).

Q2. Write the equation for the complete combustion of propane. [1 mark]

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

Q3. Name the type of mechanism by which bromine adds to ethene. [1 mark]

• Cue. Electrophilic addition.

Q4. Explain why a secondary carbocation is more stable than a primary one. [1 mark]

• Cue. It has more electron-releasing alkyl groups, which reduce the positive charge on the carbon.

Q5. State why addition polymers such as poly(ethene) are non-biodegradable. [1 mark]

• Cue. Their saturated, non-polar carbon backbone is unreactive, so microorganisms cannot break it down.