11.5 Alkenes - Revision Notes

1. Overview

Alkenes are a homologous series of unsaturated hydrocarbons that are significantly more reactive than alkanes due to the presence of a carbon-carbon double bond ($C=C$). They are essential raw materials in the chemical industry, used primarily to produce plastics (polymers) and alcohols.

Key Definitions

Alkene: A homologous series of unsaturated hydrocarbons containing at least one carbon-carbon double bond.
Unsaturated: A molecule containing one or more double (or triple) covalent bonds between carbon atoms.
Hydrocarbon: A compound consisting only of carbon and hydrogen atoms.
Cracking: The thermal decomposition of long-chain alkanes into shorter, more useful alkanes and alkenes.
Addition Reaction: A chemical reaction where two or more substances combine to form a single product.

Core Content

Bonding and Structure

Alkenes contain a double covalent bond between two carbon atoms ($C=C$).
They follow the general formula: $C_nH_{2n}$.
Because they contain a double bond, they are described as unsaturated (they do not have the maximum possible number of hydrogen atoms).

Manufacture of Alkenes: Cracking

Large alkane molecules obtained from crude oil are often in low demand. Cracking breaks these into smaller, high-demand molecules.

Conditions: High temperature (approx. 600–700°C) and a catalyst (silica/silicon dioxide, $SiO_2$, or alumina/aluminum oxide, $Al_2O_3$).
Products: A shorter-chain alkane + an alkene (or sometimes an alkene + hydrogen).

Example: Cracking of Decane

Word Equation: Decane → Octane + Ethene
Symbol Equation: $C_{10}H_{22}(l) \rightarrow C_8H_{18}(l) + C_2H_4(g)$

Reasons for Cracking

To match supply with demand: Crude oil provides more long-chain alkanes than the market needs, and not enough short-chain alkanes (like gasoline/petrol).
To produce alkenes: Alkenes are needed to make plastics (polyethene) and other chemicals.
To produce hydrogen: Hydrogen gas is used in the Haber process to make ammonia.

Test for Unsaturation

To distinguish between an alkane (saturated) and an alkene (unsaturated), use aqueous bromine (bromine water).

Procedure: Add bromine water to the sample and shake.
Result for Alkenes: The orange/brown bromine water turns colorless (decolorizes).
Result for Alkanes: The bromine water remains orange/brown.

Extended Content (Extended Only)

Addition Reactions

In an addition reaction, the $C=C$ double bond "opens up" to form a single $C-C$ bond, allowing new atoms to bond to the carbon atoms. Only one product is formed.

(a) Reaction with Bromine ($Br_2$)

Word Equation: Ethene + Bromine → 1,2-dibromoethane
Symbol Equation: $C_2H_4(g) + Br_2(aq) \rightarrow C_2H_4Br_2(l)$
Structural Formula of Product: H H | | H–C – C–H | | Br Br

(b) Reaction with Hydrogen (Hydrogenation)

Conditions: Nickel catalyst, temperature of 150°C.
Word Equation: Ethene + Hydrogen → Ethane
Symbol Equation: $C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$
Structural Formula of Product: H H | | H–C – C–H | | H H

(c) Reaction with Steam (Hydration)

Conditions: Phosphoric acid ($H_3PO_4$) catalyst, 300°C, 60 atm pressure.
Word Equation: Ethene + Steam → Ethanol
Symbol Equation: $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$
Structural Formula of Product: H H | | H–C – C–O–H | | H H

Key Equations

Process	Equation	Key Symbols
General Formula	$C_nH_{2n}$	$n$ = number of carbon atoms
Cracking	$C_{15}H_{32}(l) \rightarrow C_{13}H_{28}(l) + C_2H_4(g)$	Alkane → Alkane + Alkene
Bromination	$C_2H_4(g) + Br_2(aq) \rightarrow C_2H_4Br_2(l)$	1,2-dibromoethane (colorless)
Hydrogenation	$C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$	Nickel catalyst required
Hydration	$C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$	$H_3PO_4$ catalyst required

Common Mistakes to Avoid

❌ Wrong: Describing the bromine water test result as "clear."
✅ Right: Use the word "colorless." (A solution can be clear but still have a color, like tea).
❌ Wrong: Forgetting to show the double bond when drawing an alkene.
✅ Right: Always ensure the $C=C$ bond is visible in alkene structural formulas.
❌ Wrong: Keeping the double bond in the product of an addition reaction.
✅ Right: The $C=C$ bond always becomes a $C-C$ single bond after an addition reaction.

Exam Tips

Command Words: When asked to "describe" the test for an alkene, you must give the reagent (bromine water) AND the observation (orange to colorless).
Unknowns: In questions involving an "unknown hydrocarbon," look for the mention of bromine water. If it decolorizes, the unknown is an alkene.
Real-world Contexts: Cracking is frequently tested in the context of the oil industry and the economic need to produce petrol for cars and plastics for packaging.
Displaying Structures: When drawing the product of hydration (ethanol), ensure the bond goes from the Carbon to the Oxygen (C—O—H), not the Hydrogen (C—H—O).