Alkenes- CIE iGCSE Chemistry Notes - New Syllabus
Alkenes for iGCSE Chemistry Notes
Core Syllabus
- State that the bonding in alkenes includes a double carbon–carbon covalent bond and that alkenes are unsaturated hydrocarbons
- Describe the manufacture of alkenes and hydrogen by the cracking of larger alkane molecules using a high temperature and a catalyst
- Describe the reasons for the cracking of larger alkane molecules
- Describe the test to distinguish between saturated and unsaturated hydrocarbons by their reaction with aqueous bromine
Supplement Syllabus
- State that in an addition reaction only one product is formed
- Describe the properties of alkenes in terms of addition reactions with:
(a) bromine or aqueous bromine
(b) hydrogen in the presence of a nickel catalyst
(c) steam in the presence of an acid catalyst and draw the structural or displayed formulae of the products
Unsaturated carbon and Alkenes
Bonding in Alkenes and Their Nature
Alkenes are a homologous series of hydrocarbons that contain a carbon–carbon double bond.
- The double bond consists of:
one sigma (σ) bond, formed by the head-on overlap of orbitals
one pi (π) bond, formed by the sideways overlap of p orbitals. - The presence of the π bond makes alkenes more reactive than alkanes because the π bond is weaker and easier to break during chemical reactions.
- Because alkenes have at least one double bond, they are called unsaturated hydrocarbons.
- Unsaturated means they do not contain the maximum possible number of hydrogen atoms. More hydrogen atoms can be added across the double bond in addition reactions.
- General formula of alkenes: \( \text{C}_n\text{H}_{2n} \) (for alkenes with one double bond).
- The simplest alkene is ethene (\( \text{C}_2\text{H}_4 \)).
Example
Explain why alkenes are called unsaturated hydrocarbons and give the molecular formula of the simplest alkene.
▶️Answer/Explanation
Alkenes contain a double carbon–carbon bond, which means they do not have the maximum number of hydrogen atoms that could bond to the carbon chain. This is why they are called unsaturated hydrocarbons. The simplest alkene is ethene, with the molecular formula \( \text{C}_2\text{H}_4 \).
Manufacture of Alkenes and Hydrogen by Cracking
Manufacture of Alkenes and Hydrogen by Cracking
- Alkenes are produced mainly by the process of cracking long-chain alkane molecules obtained from petroleum.
- Cracking means breaking down larger, less useful hydrocarbons into smaller, more useful ones.
- The process requires a high temperature (around 600–700 °C).
- A catalyst such as silica (\( \text{SiO}_2 \)) or alumina (\( \text{Al}_2\text{O}_3 \)) is used to speed up the reaction and reduce energy demand.
- The products of cracking are:
shorter-chain alkanes (useful as fuels) – alkenes (used in making polymers and other chemicals) – hydrogen gas (\( \text{H}_2 \)) as a valuable by-product. - This makes the process economically important because it converts surplus long-chain alkanes into useful substances.
Example
Describe the cracking reaction of hexane into an alkane, an alkene, and hydrogen.
▶️Answer/Explanation
One possible reaction is: \( \text{C}_6\text{H}_{14} \rightarrow \text{C}_4\text{H}_{10} + \text{C}_2\text{H}_4 \)
Here, hexane produces butane (alkane) and ethene (alkene). Hydrogen (\( \text{H}_2 \)) can also be formed in some cracking reactions.
Reasons for the Cracking of Larger Alkane Molecules
- Cracking is carried out because there is a higher demand for shorter-chain hydrocarbons than for long-chain hydrocarbons.
- Shorter-chain alkanes burn more easily and cleanly, so they are more useful as fuels (for example, petrol and LPG).
- Petroleum naturally contains a higher proportion of long-chain hydrocarbons, which are less useful because they are thick and have high boiling points.
- Cracking provides a way to convert surplus long-chain alkanes into more valuable short-chain hydrocarbons.
- Cracking produces alkenes, which are not found naturally in large amounts in crude oil.
- Alkenes are important feedstock chemicals used to manufacture polymers such as poly(ethene) and poly(propene).
- Hydrogen gas formed during cracking is also useful in industrial processes such as the manufacture of ammonia and margarine.
Example
Explain why cracking is an important industrial process in the petroleum industry.
▶️Answer/Explanation
Cracking is important because:
- It increases the supply of short-chain hydrocarbons which are in high demand as fuels.
- It produces alkenes, which are valuable raw materials for making plastics and other chemicals.
- It prevents the wastage of surplus long-chain hydrocarbons by converting them into useful products.
- It provides hydrogen gas that can be used in other industries.
Addition Reactions
Addition Reactions
- Alkenes undergo addition reactions because they contain a C=C double bond.
- In an addition reaction, the π bond of the double bond breaks.
- Two new atoms or groups attach to the carbon atoms of the double bond.
- Only one product is formed in an addition reaction.
- This differs from substitution reactions, where one atom is replaced and multiple products may form.
Example
Explain why the reaction of ethene with hydrogen is classified as an addition reaction.
▶️Answer/Explanation
- Ethene has a C=C double bond, which can break open.
- Hydrogen atoms add across the double bond.
- The product formed is ethane, which is a single compound.
- No other side products are formed, so it is an addition reaction.
- Equation: \( \text{CH}_2= \text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3 \)
Properties of Alkenes in Addition Reactions
(a) Reaction with bromine or aqueous bromine
- Alkenes react with bromine in an addition reaction.
- The orange colour of bromine solution decolourises.
- This is because the C=C double bond breaks and each carbon gains a Br atom.
- The product is a dibromoalkane.
Test to Distinguish Between Saturated and Unsaturated Hydrocarbons
- Bromine water is orange-brown.
- Alkenes (unsaturated, with C=C) decolourise bromine water by addition across the double bond, forming a colourless dibromo compound.
- Alkanes (saturated, no C=C) do not react, so the orange-brown colour remains.
- Thus, bromine water distinguishes alkenes from alkanes.
Example
A student adds bromine water to ethene and observes the result.
▶️Answer/Explanation
- Ethene is an alkene, so it contains a C=C double bond.
- Ethene reacts with bromine water in an addition reaction.
- The orange-brown colour of bromine water decolourises.
- The product formed is 1,2-dibromoethane, which is colourless.
- Equation:
\( \text{CH}_2= \text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}-\text{CH}_2\text{Br} \)
(b) Reaction with hydrogen in the presence of a nickel catalyst
- Alkenes react with hydrogen in an addition reaction called hydrogenation.
- The C=C double bond breaks and hydrogen atoms add to each carbon.
- The product is an alkane.
- A nickel catalyst is used to speed up the reaction.
- The reaction requires a temperature of about 150 °C.
Example
What happens when ethene reacts with hydrogen in the presence of a nickel catalyst?
▶️Answer/Explanation
- The double bond in ethene breaks.
- Hydrogen atoms add across the double bond.
- The product is ethane, a saturated hydrocarbon.
- Equation:
\( \text{CH}_2=\text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3 \)
(c) Reaction with steam in the presence of an acid catalyst
- Alkenes react with steam in an addition reaction called hydration.
- The C=C double bond breaks and -H and -OH groups add to the carbons.
- The product is an alcohol.
- A phosphoric acid catalyst is commonly used.
- The reaction requires high temperature and pressure.
Example
What happens when ethene reacts with steam in the presence of an acid catalyst?
▶️Answer/Explanation
- The double bond in ethene breaks.
- One carbon gains an H atom, the other gains an OH group.
- The product is ethanol, an alcohol.
- Equation:
\( \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{OH} \)