Unsaturated Hydrocarbon Reactions (HSC SSCE Chemistry): Revision Notes

Unsaturated Hydrocarbon Reactions

Introduction to unsaturated hydrocarbons

Unsaturated hydrocarbons include alkenes (containing carbon-carbon double bonds) and alkynes (containing carbon-carbon triple bonds). These compounds are significantly more reactive than their saturated counterparts, alkanes.

Combustion reactions

Combustion reactions occur when substances burn in the presence of oxygen, releasing large amounts of energy. This energy has practical applications including:

• Powering internal combustion engines
• Industrial furnaces
• Domestic heating and cooking

Both alkenes and alkynes undergo combustion. When sufficient oxygen is available, complete combustion occurs, producing carbon dioxide and water as the only products.

Addition reactions

An addition reaction is a type of chemical reaction where atoms are added to an unsaturated compound by breaking the double or triple bond.

Unsaturated hydrocarbons undergo addition reactions with:

• Hydrogen ($\text{H}_2$)
• Halogens ($\text{X}_2$ such as $\text{Cl}_2$ or $\text{Br}_2$)
• Hydrogen halides ($\text{HX}$ such as $\text{HCl}$ or $\text{HBr}$)
• Water ($\text{H}_2\text{O}$)

Addition of hydrogen (hydrogenation)

Hydrogenation is the process of adding hydrogen to an unsaturated hydrocarbon. This is one of the most important addition reactions both industrially and in organic synthesis.

When an alkene undergoes hydrogenation, it converts to an alkane. For example, ethene forms ethane:

$\text{CH}_2=\text{CH}_2(\text{g}) + \text{H}_2(\text{g}) \rightarrow \text{CH}_3\text{CH}_3(\text{g})$

The role of catalysts

Hydrogenation reactions are quite slow without assistance, so a metal catalyst is necessary. Common catalysts include nickel (Ni), platinum (Pt), palladium (Pd), or rhodium (Rh). The catalyst works through a three-step mechanism:

1. The hydrogen molecule reacts with the metal catalyst, breaking the H—H bond and forming two weak metal–hydrogen bonds
2. The alkene molecule reacts with the metal catalyst, breaking one bond in the double bond and forming weak carbon–metal bonds
3. The metal catalyst brings the hydrogen atoms and carbon atoms together, creating the final alkane molecule

Industrial application: Margarine production

To make margarine, liquid oils with double bonds react with hydrogen gas in the presence of heat and a nickel catalyst. Enough hydrogen is added to make the oil solid at room temperature, but not enough to completely saturate all double bonds in the molecule.

Converting alkynes to alkenes

Alkynes can also be hydrogenated. With careful control, an alkyne converts to an alkene rather than continuing to an alkane.

In this reaction, 2-butyne converts to 2-butene. To prevent the alkene from reacting further to form an alkane, a special Lindlar catalyst is used. This heterogeneous catalyst consists of palladium deposited on calcium carbonate. The catalyst acts as an inhibitor, stopping the reaction at the alkene stage.

Addition of halogens (halogenation)

Halogenation involves adding halogen molecules (such as chlorine or bromine) across a double or triple bond. The mechanism is similar to hydrogenation, but the high reactivity of halogens means no catalyst is needed.

Figure (a) shows propene reacting with bromine to form 1,2-dibromopropane. Both bromine atoms add to the carbons that were originally part of the double bond.

Figure (b) shows propyne reacting with bromine to form 1,2-dibromopropene. This product still contains a double bond. If more bromine is available, the double bond can react again, breaking to form 1,1,2,2-tetrabromopropane (a fully saturated compound).

Addition of hydrogen halides

Hydrogen halides are molecules containing one hydrogen atom bonded to one halogen atom. Common examples include hydrogen chloride (HCl) and hydrogen bromide (HBr).

When a hydrogen halide adds to an unsaturated hydrocarbon, the hydrogen atom attaches to one carbon from the multiple bond, whilst the halogen attaches to the other carbon.

Addition of water (hydration)

Adding water across a double bond requires a dilute sulfuric acid ($\text{H}_2\text{SO}_4$) catalyst. When water adds to an alkene, one hydrogen from water attaches to one carbon, whilst the —OH group attaches to the other carbon.

Hydration of alkenes

The hydration of alkenes produces alcohols. For example, ethene reacts with water to form ethanol:

$\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{Dilute H}_2\text{SO}_4} \text{CH}_3\text{CH}_2\text{OH}$

Industrial importance of ethanol production

Hydration of alkynes

The hydration of alkynes requires different catalysts: mercury(II) compounds combined with sulfuric acid. Addition of water to an alkyne produces a ketone rather than an alcohol.

General notes on addition reactions

Understanding the following patterns will help you predict products of addition reactions:

Symmetrical reagents

When a symmetrical reagent (such as $\text{H}_2$ or $\text{Cl}_2$) adds to an alkene, the same group adds to each carbon. This produces only one possible product.

Asymmetrical reagents with symmetrical alkenes

When an asymmetrical reagent (such as $\text{H}_2\text{O}$ or HBr) adds to a symmetrical alkene (like ethene), there is only one possible product because the molecule is symmetrical.

Asymmetrical reagents with asymmetrical alkenes

When an asymmetrical reagent adds to an asymmetrical alkene (like propene), two different products are theoretically possible. However, one product predominates over the other.

Markovnikov's rule

Polymerisation

Alkenes and alkynes are extensively used in addition polymerisation reactions, where multiple small molecules (monomers) join together to form long chain molecules called polymers. You will learn more about polymerisation reactions in Chapter 13.

Investigation 10.2: Modelling reactions of unsaturated hydrocarbons

Aim: To model the reactions of alkenes and alkynes using molecular model kits.

Materials:

• Organic model kits (alternatively, use modelling clay in various colours for different atoms and toothpicks for bonds)
• Digital camera

Risk assessment:

What are the risks?	How can you manage these risks?
Possible risk of slipping on model kit components if they are on the floor	Keep all components within containers when not being used

Method:

Part A: Modelling hydrogenation

1. Build models of ethene and hydrogen molecules
2. Take photos of the reactants
3. Combine the models to make the product
4. Take photos of the product
5. Repeat for two other simple alkenes

Part B: Modelling addition of hydrogen halides, halogens and water

1. Repeat the above process to model the addition of hydrogen halides, halogens, and water
2. Use at least two simple alkenes for each type of reaction
3. Take photos of reactants and products

Discussion questions:

Part A:

1. Write equations for each reaction you modelled, naming all reactants and products
2. Identify a benefit of using modelling to improve your understanding
3. Identify a specific limitation of modelling reactions (What doesn't it show properly?)
4. Explain what you would do differently to model addition of hydrogen to an alkyne

Part B:

1. Identify two addition reactions you modelled and use your photos to explain what is meant by an addition reaction
2. Using a model of propene reacting with water, explain why two different products are possible from some addition reactions

Summary