Alkene Structures and the Bromine Test

An alkene’s double bond affects both its structure and its reactions. Start by locating the double bond and counting atoms, then use a chemical test to compare an alkene with an alkane.

Find the carbon–carbon double bond

The alkene series

The alkenes studied here are open-chain hydrocarbons with one carbon–carbon double bond, C = C. Their general formula is CₙH₂ₙ. Each carbon still forms four bonds: a double bond counts as two.

Unsaturated and saturated

An unsaturated hydrocarbon has a carbon–carbon multiple bond. In these lessons, that is a C = C double bond. Alkenes are unsaturated; alkanes have no carbon–carbon multiple bonds and are saturated.

Why the double bond matters

Atoms can add across an alkene’s double bond. The carbon–carbon connection becomes a single bond, with a new bond formed at each carbon. See Addition Reactions of Alkenes for the products and conditions.

Use formulae as a check, not a drawing

For ethene, n = 2, so 2n = 4 and its molecular formula is C₂H₄. There is no one-carbon alkene: a carbon–carbon double bond needs two carbon atoms.

A molecular formula counts atoms. It does not show their connections, the position of a double bond, or whether the carbon skeleton forms a ring. Use the general formula to check a proposed open-chain alkene, then inspect its structural formula.

Build and compare structures

Ethene and propene

Count four bonds at each carbon

Ethene has two carbons joined by a double bond, each with two hydrogens. Propene adds a CH3 group to one of those carbons; that carbon then has one hydrogen.

Scroll across the graph to read all labels.

View figure data

Atoms and bonds in each displayed structure

Alkene	Molecular formula	Carbon–carbon bonds	Hydrogen atoms
ethene	C2H4	1 double	4
propene	C3H6	1 double, 1 single	6

Atoms and bonds in each displayed structure

Alkene	Molecular formula	Carbon–carbon bonds	Hydrogen atoms
ethene	C2H4	1 double	4
propene	C3H6	1 double, 1 single	6

Change position or branching without changing atom totals

Three different carbon connections for C4H8

But-1-ene and but-2-ene have four-carbon unbranched skeletons with different double-bond positions. Methylpropene has a branched skeleton. Each contains four carbon and eight hydrogen atoms.

Scroll across the graph to read all labels.

View figure data

Compare molecular formula and carbon connections

Structure	Molecular formula	Condensed structural formula	Carbon skeleton
but-1-ene	C4H8	CH2=CHCH2CH3	Unbranched; end double bond
but-2-ene	C4H8	CH3CH=CHCH3	Unbranched; internal double bond
methylpropene	C4H8	CH2=C(CH3)2	Branched

Compare molecular formula and carbon connections

Structure	Molecular formula	Condensed structural formula	Carbon skeleton
but-1-ene	C4H8	CH2=CHCH2CH3	Unbranched; end double bond
but-2-ene	C4H8	CH3CH=CHCH3	Unbranched; internal double bond
methylpropene	C4H8	CH2=C(CH3)2	Branched

All three have molecular formula C₄H₈, but different atom connections, so they are structural isomers. In CH₂ = C(CH₃)₂, both bracketed CH₃ groups attach to the second carbon of the double bond.

Number an unbranched chain from the end nearer the double bond. Writing but-1-ene from the other end gives CH₃-CH₂-CH = CH₂, which is the same compound. It does not create a new isomer called “but-3-ene”.

Use bromine water: observation, then conclusion

In the usual test, away from UV light, an alkene reacts with aqueous bromine (bromine water) and removes its red-brown colour. An alkane does not decolourise it under these conditions.

“Colourless” describes the absence of colour. “Clear” describes transparency, so it does not tell you whether the bromine colour has disappeared. A colourless result identifies unsaturation in this comparison, not the exact alkene or its double-bond position.

Where alkenes come from

Cracking and Refinery Demand explains how hydrocarbons are changed to make smaller molecules, including alkenes and sometimes hydrogen.

Burning ethene

Find the equation and product states in the combustion section.

Predicting addition products

The reaction lesson follows the same double bond through several changes:

Hydrogenation

Adding hydrogen converts ethene to ethane.

Hydration

Adding steam converts ethene to ethanol.

Addition polymerisation

Joining alkene monomers makes a long chain.

Vegetable oils and hydrogenation

The food application explains polyunsaturation and margarine manufacture.

Check bonds before naming

• Count a double bond as two bonds when adding hydrogen to a displayed structure.
• Include every branch carbon in the molecular formula.
• A reversed drawing has the same connectivity; it is not another structural isomer.
• A molecular formula alone does not prove where a double bond lies.

Make your reasoning visible

When asked to draw a structure, show the required bonds and atoms rather than only the molecular formula. When asked for a test, state the reagent, each sample’s observation and the conclusion supported by that observation.

Check a formula and plan a comparison

Modelled example 1

General formula check

Core

Problem

Can C₅H₁₀ be an alkene containing one C = C bond? Show the general-formula check.

Study the worked solution

1. Choose the alkene formula

   Method

   Use CₙH₂ₙ for an open-chain alkene with one double bond.

   Reason

   The formula relates the hydrogen subscript to the number of carbon atoms.

   Working

   For C₅H₁₀, n = 5.

2. Substitute and compare

   Method

   Calculate the required number of hydrogen atoms.

   Reason

   2n = 2(5) = 10, matching the given formula.

   Working

   C₅H₁₀ fits CₙH₂ₙ, so it can be an alkene.

Guided practice 2

Bromine water test (distinguish alkane vs alkene)

About 5 min

Problem

You have ethane and ethene. Describe a chemical test and give the expected observation for each substance.

Choose the reagent and observations

Reagent

Bromine waterLimewaterLitmus solution

Ethene observation

Red-brown to colourlessNo colour change

Ethane observation

Remains red-brownTurns colourless

Check my attempt

Hints

Hint 1: bond difference

Ethene contains C = C; ethane contains only C-C.

Hint 2: observation wording

State both the starting and final colour, and give a separate observation for the alkane.

View solution step by step

1. Add the test reagent

   Method

   Add small amounts of red-brown bromine water to separate samples, away from UV light.

   Reason

   Bromine undergoes addition across an alkene’s C = C bond.

   Working

   Use equal small samples so their observations can be compared.

2. Distinguish the samples

   Method

   Identify ethene by decolourisation and ethane by no change.

   Reason

   Ethene is unsaturated; ethane has no double bond and does not react under normal conditions.

   Working

   Ethene: red-brown → colourless. Ethane: remains red-brown.

Correct a repeating unit

The repeating-unit example is beside the polymerisation explanation.

Explain hydration conditions

The hydration example is beside the ethanol equation.

Deduce a cracking product

The atom-conservation example is beside the cracking model.

Try without prompts

Mind stretcher 1: What can the evidence distinguish?Extension

A is CH₂ = CHCH₂CH₃, B is CH₃CH = CHCH₃ and C is CH₃CH₂CH₂CH₃. Work out their molecular formulae. Identify a structural-isomer pair and predict each bromine-water observation away from UV light. Can this test distinguish A from B? Explain.

Show answer

A and B are both C₄H₈ and have different double-bond positions, so they are structural isomers. C is C₄H₁₀ and is not their isomer. A and B decolourise red-brown bromine water; C leaves it red-brown. The test cannot distinguish A from B because both contain C = C; it does not locate the double bond.

Mind stretcher 2: Check the direction of a colour changeExtension

A learner writes: “Alkenes turn bromine water from colourless to red-brown.” Explain the error and correct it.

Show answer

The direction is reversed. Bromine water starts red-brown and becomes colourless as the alkene reacts with it.

Compare cracking with separation

Try the cracking-versus-distillation question.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.