Lesson 19 of 34
A covalently attached halogen must first be released before an ionic halide test
Hydrolysis distinguishes reactivity; silver nitrate identifies the released halide.
A-Level 9476 (2026-2027)
I can distinguish organic halogen compounds experimentally.Syllabus
Syllabus 9476, 11.5(c). Distinguish organic halogen compounds experimentally.
I can relate fluoroalkane uses to relative inertness.Syllabus
Syllabus 9476, 11.5(d). Relate fluoroalkane uses to relative inertness.
I can distinguish CFC and replacement environmental effects.Syllabus
Syllabus 9476, 11.5(e). Distinguish CFC and replacement environmental effects.
A covalently attached halogen must first be released before an ionic halide test
Hydrolysis distinguishes reactivity; silver nitrate identifies the released halide.
- Warm with aqueous NaOH
Similar halogenoalkanes release halide ions at different rates, depending on the C-X bond strength and the mechanism. Chlorobenzene does not react under these conditions.
- Acidify with dilute HNO3
This neutralises the excess hydroxide, which would otherwise form silver oxide and confuse the result. Do not use HCl, because it adds chloride.
- Add aqueous AgNO3
Cl- gives a white AgCl precipitate, Br- a cream AgBr precipitate and I- a yellow AgI precipitate. How each behaves with dilute and concentrated ammonia can confirm which halide it is.
Compare hydrolysis rates only when the temperature, solvent, concentrations and carbon chains are the same. For the same chain, RI usually hydrolyses faster than RBr, which is faster than RCl, because the bond strength rises in the opposite order. Testing the organic liquid directly does not work, because the halogen is bonded to carbon, not present as X- ions.
Strong C-F bonds help make many fluoroalkanes and fluorohalogenoalkanes unreactive. That stability made them useful as refrigerants and special fluids, together with suitable physical properties. But because they are so unreactive near the ground, they can last long enough to reach the upper atmosphere.
| Family | Ozone issue | Other environmental point |
|---|---|---|
| CFCs | Contain chlorine and can lead to stratospheric ozone destruction after high-energy UV breakdown. | Persistent greenhouse gases as well as ozone-depleting substances. |
| HCFCs | Contain H as well as Cl; more readily attacked in the lower atmosphere, but still have ozone-depleting potential. | Can still contribute significantly to greenhouse warming. |
| HFCs | Contain no chlorine, so avoid the chlorine-driven ozone-depletion mechanism. | Many are potent greenhouse gases; absence of chlorine does not mean no environmental impact. |
The syllabus asks for these environmental differences, not the detailed radical mechanism of ozone depletion by CFCs and HCFCs. Do not treat low reactivity near the ground, harm to ozone and the greenhouse effect as one single property.
Check your understanding
1-chlorobutane, 1-bromobutane and 1-iodobutane are added to separate tubes of warm aqueous ethanolic AgNO3. Which observation is correct?
Show the answer
A yellow precipitate appears first, because the C-I bond is the weakest
Water substitutes the halogen and releases X-, which Ag+ precipitates. Breaking C-X is the slow step, and bond energy falls from C-Cl to C-I, so yellow AgI appears first and white AgCl last.
Check your understanding
CFCs were replaced by HFCs. Which statement about this change is correct?
Show the answer
HFCs have no chlorine, so they do not destroy ozone, but many are strong greenhouse gases.
CFCs are inert, so they reach the stratosphere, where UV breaks C-Cl bonds and the Cl atoms destroy ozone. HFCs avoid this because they contain no chlorine, but many trap heat strongly.