Investigate the structural formulae, properties and reactions of alkanes, alkenes and alkynes, including combustion and addition reactions of alkenes

What this dot point is asking

NESA wants you to distinguish alkanes, alkenes and alkynes by structure, predict and write equations for their characteristic reactions (combustion, addition, substitution), and explain why a $C=C$ bond makes alkenes much more reactive than alkanes. The dot point also covers the trends in physical properties down each series and the test that distinguishes saturated from unsaturated hydrocarbons.

The answer

Structural comparison

A double bond is one $\sigma$ plus one $\pi$ bond; a triple bond is one $\sigma$ plus two $\pi$ bonds. The $\pi$ electrons are loosely held and are the site of attack in addition reactions.

Physical properties

Across all three series, increasing chain length raises both melting point and boiling point because the dispersion forces between molecules grow with molecular size. C1 to C4 hydrocarbons are gases at room temperature, C5 to about C16 are liquids, and longer chains are waxy solids. All hydrocarbons are non-polar, immiscible with water, and float on water because their density is below 1 g/mL.

Comparing series at the same carbon number: an alkane and the corresponding alkene or alkyne have very similar boiling points, because the molecules differ only in two or four hydrogens. The double bond does not introduce significant polarity.

Combustion (all hydrocarbons)

Complete combustion (excess $O_2$) gives carbon dioxide and water. The general equation for any $C_xH_y$:

Combustion is highly exothermic and is the basis for using hydrocarbons as fuels. For methane:

Incomplete combustion (limited $O_2$) gives carbon monoxide $CO$ or soot $C$ plus water. Alkenes and alkynes burn with a sootier flame than alkanes because they have a higher carbon to hydrogen ratio, so less oxygen reaches the inner part of the flame.

Substitution reactions of alkanes

Alkanes are unreactive towards most reagents at room temperature. With halogens (chlorine or bromine) in UV light, they undergo free radical substitution:

The mechanism has three steps: initiation ($Cl_2 \rightarrow 2Cl \cdot$ under UV), propagation ($Cl \cdot + CH_4 \rightarrow HCl + CH_3 \cdot$, then $CH_3 \cdot + Cl_2 \rightarrow CH_3Cl + Cl \cdot$), and termination (radicals combine).

Addition reactions of alkenes

Addition reactions break the weaker $\pi$ bond and add two new groups across the former double bond, leaving a saturated product.

1. Hydrogenation (H₂, Ni catalyst, heat). Alkene plus hydrogen gives the corresponding alkane:

2. Halogenation ($Br_2$ or $Cl_2$, room temperature, no catalyst). Alkene decolourises bromine water (orange-brown to clear) instantly. This is the standard test for unsaturation:

3. Hydrohalogenation (HX, e.g. HCl or HBr). Alkene plus a hydrogen halide gives a haloalkane. Asymmetric alkenes follow Markovnikov's rule: H adds to the carbon already carrying more hydrogens, X adds to the more substituted carbon.

4. Hydration ($H_2O$, dilute $H_2SO_4$ catalyst, heat). Alkene plus water gives an alcohol. Markovnikov also applies.

This is industrially how ethanol is made from ethene.

Reactions of alkynes

Alkynes undergo combustion and addition like alkenes, but each $\pi$ bond can be added across in turn. So ethyne plus excess bromine gives 1,1,2,2-tetrabromoethane:

Ethyne $C_2H_2$ burns at very high temperatures with oxygen, which is why oxyacetylene torches are used for welding and metal cutting.

The bromine water test

To distinguish a saturated hydrocarbon (alkane) from an unsaturated one (alkene or alkyne), add a few drops of bromine water and shake.

• Alkene or alkyne: orange-brown colour rapidly disappears (clear/colourless) due to addition.
• Alkane: colour persists, unless exposed to UV light in which case it fades slowly with HBr fumes (substitution).

A second confirmatory test is acidified $KMnO_4$: alkenes and alkynes decolourise purple permanganate at room temperature; alkanes do not react.

Common traps

Confusing addition with substitution. Alkenes add (no atoms are lost); alkanes substitute (an H is replaced and HX is a byproduct). Different mechanisms, different conditions.

Forgetting the catalyst or conditions. Hydrogenation needs $Ni$ or $Pd$ catalyst and heat. Hydration needs dilute sulfuric acid and heat. Bromination of alkenes needs neither, only room temperature.

Markovnikov direction wrong. The H adds to the carbon with more Hs already. Think "the rich get richer" for H atoms.

Soot from incomplete combustion of alkanes. Alkanes generally burn cleanly; alkenes and alkynes are sootier. If asked to compare flames, mention the C:H ratio.

Writing a hydration product with the OH on the terminal carbon of a propene. Markovnikov puts OH on C2 of propene (giving propan-2-ol), not C1.

In one sentence

Alkanes are unreactive saturated $C_nH_{2n+2}$ molecules that only substitute under UV, alkenes ($C_nH_{2n}$) and alkynes ($C_nH_{2n-2}$) have reactive $\pi$ bonds that undergo addition with $H_2$, halogens, $HX$ and water, and all three series combust to $CO_2$ and $H_2O$ in excess oxygen.