Q.Write a test to detect the presence of double bond in a molecule.

Concept: Structural Isomerism — specifically, functional group tests based on chemical reactivity.

The presence of a carbon-carbon double bond ($\ce{C=C}$) is detected using the bromine water test or the Baeyer’s test (alkaline $\ce{KMnO4}$).

Reasoning:

1. A double bond is electron-rich and undergoes addition reactions readily.
2. Bromine water test: Bromine ($\ce{Br2}$) adds across the double bond, decolourising the reddish-brown bromine water. …

Why this approach works

A double bond consists of one sigma ($\sigma$) bond and one pi ($\pi$) bond. The pi bond is weaker and more exposed than a sigma bond, making it susceptible to electrophilic attack. In the bromine water test, bromine ($\text{Br}_2$) acts as an electrophile — it adds across the double bond in a characteristic addition reaction. This consumes the bromine, removing its colour.

The reaction is:

The reddish-brown colour of bromine water comes from molecular bromine. When bromine adds to the double bond, it is no longer present as free $\text{Br}_2$, so the colour disappears. This visual change is the basis of the test.

Step-by-step procedure

1. Prepare the sample — Take a small amount of the organic compound (about 0.5 mL or a few crystals) in a clean, dry test tube. If the compound is a solid, dissolve it in a few drops of a non-reactive solvent like carbon tetrachloride ($\text{CCl}_4$) or chloroform. Do not use water if the compound is insoluble in water — the test works best in a homogeneous solution.

2. Add bromine water — Add 2–3 drops of bromine water (a dilute, reddish-brown solution of $\text{Br}_2$ in water) to the test tube. Shake gently to mix.

3. Observe the colour change — Look for the disappearance of the reddish-brown colour. If the colour fades or vanishes completely within a few seconds to a minute, a double bond is likely present. If the colour persists, the compound is saturated (no double bond).

4. Confirm with a control — Always run a blank test with the solvent alone (no compound) to ensure the bromine water itself is not decolourised by impurities or light. Also test a known saturated compound (e.g., hexane) — it should not decolourise bromine water.

Why the colour disappears — the chemistry

Bromine water is reddish-brown because of the presence of $\text{Br}_2$ molecules. When a double bond is present, the following addition reaction occurs: …

Concept: Unsaturation Test (Bromine Water / Baeyer’s Test)

A double bond (alkene) is an unsaturated functional group. It reacts readily with certain reagents by addition reaction, while a saturated compound (alkane) does not. This difference in reactivity is used to detect the double bond.

Method: Bromine Water Test

Principle

Bromine ($\text{Br}_2$) in water has a reddish-brown colour. An alkene adds bromine across its double bond to form a colourless dibromo compound. The disappearance of the colour indicates unsaturation.

(Colourless)

Steps

1. Prepare the sample

   Take about 2 mL of the organic compound (liquid or solution in a non-polar solvent like $\text{CCl}_4$) in a clean test tube.

2. Add bromine water

   Add 2–3 drops of bromine water (or bromine in $\text{CCl}_4$) dropwise to the sample.

3. Shake and observe

   Shake the test tube gently.

   • Positive test (double bond present): The reddish-brown colour of bromine disappears immediately (decolourisation).
   • Negative test (no double bond): The reddish-brown colour persists.

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Common Mistakes: Testing for a Double Bond (Unsaturation)

Students often lose marks on this seemingly simple question. Here are the most frequent errors and how to avoid each.

Mistake 1: Naming the Wrong Reagent

The error: Students write "bromine water test" but then describe using chlorine water, potassium permanganate, or even iodine solution.

Why it happens: Confusion between halogen tests (Br₂, Cl₂, I₂) and oxidation tests (KMnO₄).

How to avoid: Memorise the two standard tests for unsaturation:

• Bromine water test — decolourisation of reddish-brown Br₂
• Baeyer's test — decolourisation of purple alkaline KMnO₄

Key fact: Only bromine water is the specific test for a double bond. Baeyer's test is for unsaturation generally (double or triple bonds).

Mistake 2: Describing the Wrong Colour Change

The error: "Bromine water turns from brown to colourless" — but bromine water is reddish-brown, not brown.

The error: "KMnO₄ turns from purple to colourless" — but in Baeyer's test, KMnO₄ turns from purple to brown (MnO₂ precipitate), not colourless.

How to avoid: Learn the exact colours:

Mistake 3: Forgetting to Mention the Observation

The error: Students write "Add bromine water" and stop — no observation, no conclusion.

How to avoid: Always write the full sequence:

1. Procedure: Add a few drops of bromine water to the compound.
2. Observation: The reddish-brown colour of bromine water is discharged (decolourised).
3. Conclusion: This indicates the presence of a double bond (addition reaction occurs).

Mistake 4: Confusing Addition with Substitution

The error: "Bromine water decolourises because bromine substitutes hydrogen atoms."

Why it happens: Students mix up alkene addition (C=C + Br₂ → dibromo compound) with alkane substitution (C-H + Br₂ → C-Br + HBr).

How to avoid: Remember the mechanism:

• Double bond: Addition — Br₂ adds across the π-bond → decolourisation is immediate
• Saturated compound: Substitution — requires UV light or heat → decolourisation is slow or absent

Exam tip: If a question asks "test for unsaturation", the answer is always about addition, not substitution.

Mistake 5: Writing an Incomplete Chemical Equation

The error: Writing only R-CH=CH-R + Br₂ → without showing the product.

How to avoid: Write the complete addition reaction: …