Alkenes

Q: Describe the test to distinguish between ethane and ethene using aqueous bromine. Include the observations for both gases.

Add aqueous bromine (orange/yellow) to each gas. Ethane: No reaction / solution remains orange. Ethene: The solution turns from orange to colourless.

Q: State the conditions required for the cracking of decane ($C_{10}H_{22}$) to produce octane and ethene. Write a balanced symbol equation for this reaction.

Conditions: High temperature and a catalyst (or steam). Equation: $C_{10}H_{22} rightarrow C_8H_{18} + C_2H_4$

Q: Propene ($C_3H_6$) reacts with hydrogen in the presence of a nickel catalyst. Draw the displayed formula of the product formed.

The product is propane ($C_3H_8$). Displayed Formula: $$ begin{array}{c} H quad H quad H quad quad C - C - C quad quad H quad H quad H end{array} $$

Q: Explain why alkenes are described as 'unsaturated'.

Alkenes contain at least one carbon–carbon double bond ($C=C$). This means they do not have the maximum possible number of hydrogen atoms bonded to the carbon chain.

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Structure and Bonding of Alkenes

Alkenes are a homologous series of unsaturated hydrocarbons. To understand this, we must define the terms:

Hydrocarbon: A compound containing only carbon and hydrogen atoms.
Unsaturated: This means the molecule contains at least one carbon–carbon double bond (C=C). The term 'unsaturated' implies that the carbon atoms are not bonded to the maximum possible number of hydrogen atoms; there is 'room' for more atoms to add across the double bond.
Double Bond: In alkenes, two adjacent carbon atoms share two pairs of electrons. This forms a strong covalent bond, but it is chemically reactive compared to single bonds.

General Formula:
The general formula for non-cyclic alkenes is C_nH_{2n}.

For ethene (n=2): C_2H_4
For propene (n=3): C_3H_6
For butene (n=4): C_4H_8

Notice that for every carbon atom, there are exactly two hydrogen atoms.

Addition Reaction

An addition reaction is a chemical reaction where two or more molecules combine to form a single larger product.

In the context of alkenes, this occurs because the double bond (C=C) breaks open. The carbon atoms can then form new single bonds with other atoms (such as bromine, hydrogen, or water) without losing any part of the original alkene molecule.

Key Property: In an addition reaction involving an alkene, only one product is formed. This distinguishes it from substitution reactions (typical of alkanes), where multiple products are formed.

Manufacture of Alkenes: Cracking

What is Cracking?
Cracking is the process of breaking down large, unreactive alkane molecules into smaller, more useful molecules. This process produces alkenes and often hydrogen.

Conditions Required:

High Temperature: Typically around 600^\text{--}700^\text{°C}.
Catalyst: Usually steam or a metal oxide catalyst (e.g., aluminium oxide, Al_2O_3).

Why is Cracking Performed?
Crude oil contains a mixture of hydrocarbons with different chain lengths. The demand for shorter-chain hydrocarbons (like petrol/gasoline and ethene for plastics) is much higher than the supply available from direct distillation of crude oil. Cracking allows refineries to convert long-chain alkanes (which are in surplus) into short-chain alkanes and alkenes (which are in high demand).

Example Equation:
Consider the cracking of decane (C_{10}H_{22}). It can break down into octane (a useful alkane) and ethene (an alkene):

C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4

Check the balance:

Carbon: 10 on left, 8+2=10 on right.
Hydrogen: 22 on left, 18+4=22 on right.

Another common pathway produces hydrogen gas:
C_4H_{10} \rightarrow C_2H_6 + C_2H_4
(Note: Cracking often produces a mixture of products, but you must be able to write balanced equations for specific pathways.)

Test for Unsaturation

The standard test to distinguish between saturated hydrocarbons (alkanes) and unsaturated hydrocarbons (alkenes) uses aqueous bromine (also called bromine water).

Reagent: Aqueous bromine is orange (or yellow-orange) in colour.
Observation with Alkenes: The solution rapidly turns from orange to colourless. This is known as 'decolourisation'.
Observation with Alkanes: No reaction occurs; the solution remains orange.

Reasoning: The bromine adds across the carbon–carbon double bond of the alkene, forming a colourless dibromoalkane. Since alkanes have only single bonds, they do not react with bromine under these conditions.

Addition Reactions of Alkenes

1. Reaction with Bromine (or Aqueous Bromine)
Ethene reacts with bromine to form 1,2-dibromoethane.

Equation: C_2H_4 + Br_2 \rightarrow C_2H_4Br_2
Structural Formula of Product:
 \begin{array}{c} H \quad H \ | \quad | \ C - C \ | \quad | \ Br \quad Br \end{array}
Note: The double bond breaks, and one bromine atom attaches to each carbon.

2. Reaction with Hydrogen (Hydrogenation)
Ethene reacts with hydrogen gas in the presence of a nickel catalyst at 150^\text{°C} to form ethane.

Equation: C_2H_4 + H_2 \rightarrow C_2H_6
Product: Ethane (an alkane). This reaction is used industrially to harden vegetable oils into margarine.

3. Reaction with Steam (Hydration)
Ethene reacts with steam (H_2O) in the presence of an acid catalyst (typically phosphoric acid, H_3PO_4) at high temperature and pressure to form ethanol.

Equation: C_2H_4 + H_2O \rightarrow C_2H_5OH
Product: Ethanol (an alcohol).

Important: Propene and Isomers
When propene (CH_3-CH=CH_2) reacts with steam, the hydrogen and hydroxyl group (-OH) can attach to different carbons, forming two possible products:

Propan-1-ol: CH_3-CH_2-CH_2-OH
Propan-2-ol: CH_3-CH(OH)-CH_3

Displayed Formula of Propan-2-ol:
 \begin{array}{c} H \quad H \quad H \ | \quad | \quad | \ C - C - C \ | \quad | \quad | \ H \quad O \quad H \ \quad \quad | \ \quad \quad H \end{array} 
(Note: The oxygen is bonded to the middle carbon.)

⚠︎ Confusing Saturated vs. Unsaturated Definitions

Mistake: Stating that an alkene is unsaturated because it 'has double bonds' without specifying where the double bond is.

Correction: You must specify that the double bond is between carbon atoms. The correct definition is: 'An unsaturated hydrocarbon contains at least one carbon–carbon double bond.'

Why this matters: A molecule could have a double bond to oxygen (like in ketones or aldehydes) and still be saturated with respect to carbon-hydrogen bonding, but in the context of hydrocarbons, 'unsaturated' specifically refers to the C=C bond.

Mistake: Describing cracking as 'breaking molecules' without explaining why it is done.

Correction: When asked for the reason for cracking, you must mention demand. The correct answer is: 'To produce smaller hydrocarbon molecules that are in higher demand (e.g., petrol) or to produce hydrogen.'

Why this matters: Simply saying 'to break large molecules' describes the process, not the economic/industrial reason for it.

Describing Colour Changes in Bromine Tests

When to use: When asked to describe the observation when an alkene is added to aqueous bromine.

Correct Phrasing: You must state both the initial and final colours. Use the phrase: 'Orange to colourless' or 'Yellow-orange to colourless'.

Why examiners accept this: The markscheme requires specific colour names. Saying 'it fades' or 'it becomes clear' is often marked incorrect because it lacks precision. You must explicitly mention that the orange/yellow colour disappears.

When to use: When asked to explain why an alkene undergoes an addition reaction.

Correct Phrasing: 'The carbon–carbon double bond is reactive / weakens easily, allowing atoms to add across it.'

Why examiners accept this: This directly addresses the structural feature (the C=C bond) that defines alkenes and distinguishes their reactivity from alkanes. Do not just say 'it reacts'; explain that the double bond opens up.

Past Paper Style Questions

Describe the test to distinguish between ethane and ethene using aqueous bromine. Include the observations for both gases.

Add aqueous bromine (orange/yellow) to each gas.

Ethane: No reaction / solution remains orange.
Ethene: The solution turns from orange to colourless.

State the conditions required for the cracking of decane (C_{10}H_{22}) to produce octane and ethene. Write a balanced symbol equation for this reaction.

Conditions: High temperature and a catalyst (or steam).

Equation: C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4

Propene (C_3H_6) reacts with hydrogen in the presence of a nickel catalyst. Draw the displayed formula of the product formed.

The product is propane (C_3H_8).

Displayed Formula:
 \begin{array}{c} H \quad H \quad H \ | \quad | \quad | \ C - C - C \ | \quad | \quad | \ H \quad H \quad H \end{array}

Explain why alkenes are described as 'unsaturated'.

Alkenes contain at least one carbon–carbon double bond (C=C). This means they do not have the maximum possible number of hydrogen atoms bonded to the carbon chain.