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Organic Chemistry

Topic 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)

A covalently attached halogen must first be released before an ionic halide test

Hydrolysis distinguishes reactivity; silver nitrate identifies the released halide.

Hydrolyse, then test the ions
  1. Warm with aqueous NaOH

    Comparable halogenoalkanes release halide ions at rates affected by C-X strength and mechanism. Chlorobenzene resists these ordinary conditions.

  2. Acidify with dilute HNO3

    Neutralise excess hydroxide, which would otherwise give a silver oxide interference. Do not use HCl, which adds chloride.

  3. Add aqueous AgNO3

    Cl- gives white AgCl, Br- cream AgBr and I- yellow AgI. Their dilute/concentrated ammonia behaviour can confirm the assignment.

Compare hydrolysis rates only with controlled temperature, solvent, concentrations and comparable carbon skeletons. For the same skeleton, RI usually hydrolyses faster than RBr, then RCl, because bond strength increases in the reverse order. A direct test of the original organic liquid is not a valid assumption that all covalently bound halogen is already present as X-.

Strong C-F bonds contribute to the relative chemical inertness of many fluoroalkanes and fluorohalogenoalkanes. That stability has supported uses such as refrigerants and specialised fluids; suitability also depends on physical properties. Inertness near ground level can also allow a substance to persist long enough to reach other parts of the atmosphere.

A replacement can solve one problem while retaining another
FamilyOzone issueOther environmental point
CFCsContain chlorine and can lead to stratospheric ozone destruction after high-energy UV breakdown.Persistent greenhouse gases as well as ozone-depleting substances.
HCFCsContain 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.
HFCsContain 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 requires the environmental distinctions, not the detailed CFC/HCFC ozone-depletion radical mechanism. Avoid treating lower-atmosphere inertness, ozone impact and greenhouse effect as one single property.