Halogenoalkanes and competing pathways
Introduced| English | Meaning |
|---|---|
| substitution/ˌsʌbstɪˈtjuːʃn/ | A reaction in which an atom or group in a molecule is replaced by another atom or group. |
| elimination/ɪˌlɪmɪˈneɪʃn/ | A reaction in which atoms or groups are removed from a molecule, usually forming a multiple bond. |
| nucleophile/ˈnjuːklɪɒfaɪl/ | An electron-pair donor that forms a covalent bond to an electron-deficient centre. |
| hydrolysis/haɪˈdrɒləsɪs/ | Breaking chemical bonds by reaction with water. |
Why can one bromide give an alcohol or an alkene?
- Warm aqueous KOH gives substitution 取代 to an alcohol. Hot ethanolic KOH favours elimination 消去 to an alkene.
- The solvent and conditions are part of the prediction, not optional labels.
Which conditions favour ethene from bromoethane?
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Ethanolic alkali with heat favours elimination; aqueous alkali favours substitution to ethanol.
Identify the reactive carbon
- C–Br is polar: carbon is partly positive and accepts an electron pair from a nucleophile 亲核试剂.
- Primary, secondary and tertiary count the carbon groups attached to the carbon bearing the halogen.
Classify CH₃CHBrCH₃ as primary, secondary or tertiary.
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The carbon bonded to Br is attached to two other carbon groups.
Follow the electron pairs
- With OH⁻ and primary bromoethane, one arrow goes from the oxygen lone pair to the carbon bearing Br. Another goes from C–Br to Br.
- With NH₃, an alkylammonium ion forms first. A second NH₃ removes H⁺ to give the amine and NH₄⁺.
CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ with warm aqueous alkali. The carbon skeleton stays at two carbons; a cyanide route would make three.
A nucleophile donates an electron pair when forming a bond.
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The pair starts at a lone pair or existing bond and is donated to an electron-deficient site.
Check the product carbon count
- Bromoethane with alcoholic KCN gives CH₃CH₂CN. The cyanide carbon adds one carbon, making propanenitrile.
- Excess alcoholic ammonia under pressure gives ethylamine. Hot ethanolic KOH instead removes H and Br overall, giving ethene.
How many carbons are in the organic product from bromoethane with alcoholic KCN?
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The original two carbons plus the cyanide carbon give a three-carbon nitrile.
Evaluate hydrolysis 水解 evidence
- In matched aqueous silver nitrate/ethanol tests, iodoalkanes hydrolyse faster than bromoalkanes, then chloroalkanes: C–I is weaker.
- Match temperature, concentrations and volumes. Visible AgX formation is an indirect endpoint with detection/solubility limitations.
“The most polar carbon–halogen bond always hydrolyses fastest” is false. Bond strength is central to the matched chloro/bromo/iodo comparison.
Identify the nucleophile/base, conditions, electron-pair source, leaving group and carbon count.
Which two controls help compare hydrolysis rates?
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Keep variables other than the chosen substrate the same; detection limitations must still be evaluated.