Saturated versus Unsaturated

Introduction

Ethane and ethene are both colourless gases, and hexane and hex-1-ene are both colourless liquids. You cannot tell them apart by looking. Yet one of each pair is saturated and the other unsaturated , and that difference in bonding changes their chemistry completely. A simple colour test with bromine water reveals which is which in seconds. Understanding why the test works is a neat introduction to how alkenes react.

Core explanation

The difference in bonding.

- A saturated compound contains only single carbon–carbon bonds. Every carbon already has four single bonds, so no more atoms can be added. Alkanes are saturated. - An unsaturated compound contains at least one carbon–carbon double (or triple) bond. The multiple bond can open up to take on extra atoms. Alkenes are unsaturated.

The bromine water test. Bromine water is a dilute solution of bromine in water and is orange (yellow-orange when very dilute). When it is shaken with a hydrocarbon:

Substance tested Observation Conclusion --- --- --- alkane, e.g. hexane stays orange saturated — no C=C alkene, e.g. hex-1-ene turns from orange to colourless unsaturated — C=C present

Why the colour fades. The orange colour comes from bromine molecules, Br₂. An alkene reacts with bromine in an addition reaction : the double bond opens and one bromine atom attaches to each of the two carbons that were doubly bonded. For ethene:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

The product, 1,2-dibromoethane, is colourless. As the bromine is used up, the orange colour disappears. Alkanes have no double bond to open, so at room temperature, in the dark, they do not react with bromine and the colour remains.

Only the carbon–carbon multiple bond matters. The test detects C=C and C≡C bonds anywhere in a molecule. That makes it useful beyond simple hydrocarbons: vegetable oils, which contain C=C bonds in their chains, also decolourise bromine water, while saturated animal fats react much less.

Safe practice. Bromine is toxic and corrosive, and its vapour irritates the eyes and lungs. Schools use only very dilute bromine water, in small volumes, with eye protection and good ventilation, and many use a safer alternative reagent with the same colour change. The chemistry is the same whichever reagent is used.

Step-by-step reasoning

To decide from a test result whether a hydrocarbon is saturated:

1. Note the colour of the bromine water before mixing: orange. 2. Note the colour after shaking with the hydrocarbon. 3. If it has become colourless, bromine has added across a C=C bond — the compound is unsaturated. 4. If it stays orange, there is no C=C bond — the compound is saturated.

Visual explanation

Imagine two test tubes side by side, each with an orange layer of bromine water. After shaking, the left tube (hexane) is still orange; the right tube (hexene) is clear. In SIM-ORG-001, the ethene model shows a Br₂ molecule approaching the double bond and the two bromine atoms attaching one to each carbon as the double bond becomes single.

Real-world analogy

An unsaturated molecule is like a hotel with empty rooms: new guests (bromine atoms) check in straight away and the queue outside (orange colour) disappears. A saturated molecule is a fully booked hotel — the queue stays where it is.

Real-world example

Food labels list "saturated" and "unsaturated" fats. Olive oil, rich in unsaturated fats, is liquid at room temperature, while butter, rich in saturated fats, is solid. Food scientists measure unsaturation with a related halogen test called the iodine value: the more halogen a fat absorbs, the more C=C bonds it contains.

Why?

Why does the bromine add on rather than replace hydrogen atoms? Opening one part of the double bond lets each carbon form a new single bond without losing anything. Addition is therefore quick and easy, while substitution in alkanes needs UV light to break bonds first.

Common misconception

"Bromine water turns clear because it is diluted by the hydrocarbon." Diluting orange bromine water with a colourless liquid would only make it paler; complete loss of colour with an alkene happens because bromine molecules are chemically used up in an addition reaction. Also note that the result is "colourless", not "clear" — bromine water is already clear.

Worked example

Question: Three liquids, A, B and C, are each shaken with bromine water. A and C stay orange; B becomes colourless. Which could be cyclohexane, hexane and hex-1-ene?

Reasoning: Only a compound with a C=C bond decolourises bromine water. Hex-1-ene has one; hexane and cyclohexane are both saturated.

Answer: B is hex-1-ene; A and C are hexane and cyclohexane (the test cannot tell those two apart).

Quick check

1. What colour change is seen when an alkene is shaken with bromine water? Answer: Orange to colourless.

Exam focus

Write the observation precisely: "the bromine water changes from orange to colourless". Never write "clear" instead of colourless, and do not write "goes red" or "goes brown". For alkanes, write "stays orange" or "no change". Be ready to write the addition equation for ethene with bromine.

Advanced insight

In aqueous bromine, water molecules compete with bromide ions to attack the intermediate formed when bromine adds to the double bond. As a result the main product with bromine water is actually a bromo-alcohol, such as 2-bromoethanol from ethene, rather than the dibromo compound formed with pure bromine. Either way, bromine is consumed and the colour disappears.

Summary

Saturated compounds contain only C–C single bonds; unsaturated compounds contain C=C or C≡C bonds. Bromine water, which is orange, turns colourless when shaken with an alkene because bromine adds across the double bond, forming a colourless product. Alkanes do not react, so the bromine water stays orange. Bromine is hazardous, so only very dilute solutions are used, with suitable precautions.

Practice questions

1. Define the term saturated. Answer: Containing only single carbon–carbon bonds. 2. Describe the results of the bromine water test for propane and for propene. Answer: With propane the bromine water stays orange; with propene it turns from orange to colourless. 3. Write an equation for the reaction of ethene with bromine and name the product. Answer: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br; 1,2-dibromoethane. 4. Explain why the orange colour disappears when bromine water is shaken with an alkene. Answer: Bromine molecules, which cause the orange colour, add across the C=C bond to form a colourless compound, so the bromine is used up.