Alkene Reactions And The Bromine Test

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.27 — The reaction of alkenes with bromine to produce dibromoalkanes
• 4.28 — Using bromine water to distinguish an alkane from an alkene

Specification section 4(e) Alcohols and (f) carboxylic acids — also taught in Alkenes, Alcohols, Making Ethanol, Carboxylic Acids.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The whole of this page is one reaction, seen twice: once as chemistry, and once as a test. An alkene reacts with bromine, and it does so quickly, at room temperature, without a catalyst and without any light. Compare that with the alkane on the previous pages, which needed ultraviolet light before it would react with a halogen at all.

Statement 4.27 — what happens when bromine meets an alkene

1. The bromine molecule, Br₂, approaches the carbon-to-carbon double bond.
2. One of the two bonds holding the carbons together OPENS, freeing one bond on each of those two carbon atoms.
3. The bromine molecule splits, and one bromine atom attaches to each of those two carbons.
4. The result is a single product with a bromine on each of two neighbouring carbons — a DIBROMOALKANE.
5. For ethene: C₂H₄ + Br₂ → C₂H₄Br₂, which is 1,2-dibromoethane.
6. Nothing else is formed. Two molecules have become one, which makes this an ADDITION reaction.
7. The product is now SATURATED — the double bond has gone, so the product will not react with more bromine.

Think of it like a folded deckchair being opened out

A folded deckchair takes up little room and offers nowhere to sit. Open it out and the same amount of material now has a seat. Nothing has been added and nothing removed — the structure has simply released one of the places it was holding onto itself. A double bond does the same thing. It is two carbons gripping each other twice, and when one grip is released each carbon has a hand free. The bromine atoms sit down in the two places that have just opened up.

Because the double bond disappears in this reaction and nothing else is produced, it makes an unusually clean test. Bromine has a strong orange colour in solution; the dibromoalkane formed is colourless. So the reaction announces itself simply by the colour going.

Statement 4.28 — the test, exactly as it should be described

1. Take a small sample of the unknown hydrocarbon in a test tube.
2. Add a few drops of BROMINE WATER, which is orange, and shake the tube.
3. If the hydrocarbon is an ALKENE, the bromine adds across the double bond and the orange colour is DECOLOURISED — the mixture goes colourless.
4. If the hydrocarbon is an ALKANE, there is no double bond for bromine to add across, so NO CHANGE is seen and the mixture stays orange.
5. The result is therefore positive for unsaturation: a colour that disappears means a carbon-to-carbon double bond is present.

Two points of language cost marks here every year. The first is the word DECOLOURISED. The mixture goes colourless, which is not the same as going clear — bromine water is already clear, in the sense of being transparent, before anything is added. Saying it 'goes clear' describes no change at all. The second is that the negative result must be stated as no change or remains orange, and not as 'nothing happens', which an examiner cannot credit as an observation.

It is worth being clear about why this test works when so few simple tests do. It is not that the alkene is generally more reactive in some vague sense. It is that the test reagent reacts at one specific structural feature — the double bond — and the reaction removes the very thing that was coloured. A test is only as good as the specificity of what it reacts with, and a carbon-to-carbon double bond is a very specific thing.

Working out the product of adding bromine to propene

Propene, C₃H₆, reacts with bromine. Give the molecular formula and the name of the product.

1. Propene contains one carbon-to-carbon double bond, between carbon 1 and carbon 2.
2. In an addition reaction the whole bromine molecule joins on, so add Br₂ to the formula: C₃H₆ + Br₂.
3. Nothing leaves, so the product contains all of it: C₃H₆Br₂.
4. The two bromine atoms sit on the two carbons that were double-bonded, which are carbons 1 and 2.
5. The name is therefore 1,2-dibromopropane, and the equation is C₃H₆ + Br₂ → C₃H₆Br₂.

Answer: C₃H₆Br₂, 1,2-dibromopropane. In an addition reaction the two formulae simply combine, because nothing is given off.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The test, in the words that earn marks

Reagent

Bromine water, which is orange.

Positive result

The orange colour is decolourised — the mixture goes colourless. An alkene is present.

Negative result

No change; the mixture remains orange. An alkane is present.

What is being tested for

Unsaturation — the presence of a carbon-to-carbon double bond.

C₂H₄ + Br₂ → C₂H₄Br₂

Conditions: Room temperature, no catalyst and no light needed. Ethene plus bromine gives 1,2-dibromoethane by addition.

C₃H₆ + Br₂ → C₃H₆Br₂

Conditions: Propene plus bromine gives 1,2-dibromopropane. Nothing is given off, so the two formulae simply combine.

The bromine-water test, step by step

1. Place a small sample of the hydrocarbon in a test tube.
2. Add a few drops of orange bromine water.
3. Stopper the tube and shake it.
4. Observe the colour: decolourised means an alkene, unchanged orange means an alkane.

Why the two families behave differently

Model answer [4 marks]

Describe how you would use bromine water to distinguish between two colourless gases, one an alkane and one an alkene. Give the reagent, both observations and your conclusion. [4]

Bubble each gas separately through a small volume of orange bromine water in a test tube, or shake the gas with it in a stoppered tube. With the alkene the orange colour is decolourised and the mixture becomes colourless, because bromine adds across the carbon-to-carbon double bond to form a colourless dibromoalkane. With the alkane there is no change and the mixture stays orange, because the molecule is saturated and has no double bond for the bromine to add across. The gas that decolourises the bromine water is therefore the alkene.

Model answer [3 marks]

Explain why the product formed when ethene reacts with bromine does not itself decolourise more bromine water. [3]

In the addition reaction the carbon-to-carbon double bond opens, and one bromine atom attaches to each of the two carbon atoms that were joined by it. The product, 1,2-dibromoethane, therefore contains only single carbon-to-carbon bonds and is saturated. Since decolourising bromine water depends on bromine adding across a double bond, and there is no longer any double bond present, no further reaction can occur and the orange colour would remain.

Not this: The bromine water "goes clear", which shows a reaction has happened.

This: Bromine water is already clear in the sense of transparent — it is a solution, not a suspension. The observation is that it is DECOLOURISED: the orange colour disappears and the mixture becomes colourless. Writing "goes clear" describes no change at all.

Mark-losing trap. Decolourised, not "goes clear". The colour disappears; the transparency was there all along.

Mark-losing trap. State the negative result as "no change, stays orange". "Nothing happens" is not an observation.

Mark-losing trap. The product of adding bromine is SATURATED, so it will not decolourise any further bromine water.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is seen when bromine water is shaken with an alkene?
2. Grade 7 · Deduce [2 marks] — Give the molecular formula of the product when but-1-ene, C₄H₈, reacts with bromine.
3. Grade 8 · Explain [3 marks] — Explain why the reaction between an alkene and bromine is classified as addition, while the reaction between an alkane and bromine is classified as substitution.
4. Grade 9 · Describe [4 marks] — Select every statement that belongs in a full answer describing how bromine water distinguishes ethane from ethene.
5. 9+ · Evaluate [6 marks] — A student shakes a hydrocarbon with bromine water. The orange colour disappears, so they shake the resulting mixture with fresh bromine water and the orange colour now remains. Explain both results.

The people behind this science

Two ways into the same idea — the one who invented the idea of identifying a substance by a colour change, and the one who turned an opening double bond into a material you can wear. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who invented the idea of identifying a substance by a colour change

The bromine-water test belongs to a kind of chemistry Boyle effectively invented. In the 1660s he began using plant extracts — syrup of violets, and later litmus — to distinguish acids from alkalis by the colour they produced, and argued that such tests were how a substance should be identified: by what it DOES to a known reagent, rather than by where it was found or what it looked like. Every test on this course descends from that idea, and he is the right person to ask what makes a colour change trustworthy evidence rather than a curiosity.

• “How can a change of colour prove what a substance is?”
• “What makes a chemical test trustworthy rather than a curiosity?”
• “Why test what a substance does instead of describing it?”
• “How would you check that a test cannot give a false result?”
• “What did chemists identify substances by before your tests?”

Wallace Carothers — the one who turned an opening double bond into a material you can wear

This page shows a double bond opening once, to take on two bromine atoms. Carothers' work rests on what happens when the same thing occurs millions of times over and the bond opens onto another molecule of the same alkene instead of onto a bromine. He established that the giant molecules formed this way are genuine single molecules rather than clumps, and he separated this kind of joining, in which nothing is lost, from the kind in which a small molecule is expelled. He is the right figure to ask what a reaction that has been demonstrated in a test tube can become at industrial scale.

• “What happens if a double bond opens onto another alkene instead?”
• “How did you prove a giant molecule was really one molecule?”
• “Why does nothing get given off when a double bond opens?”
• “What decides how long such a chain grows before it stops?”
• “How far is it from a test-tube reaction to a material?”

Carry on through the course

• Previous lesson: Alkenes
• Next lesson: Alcohols
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification