The four-question habit
- Is the halogen on or carbon?
- Is the substrate methyl, primary, secondary, tertiary, allylic or benzylic?
- Is the reagent a strong base, a good nucleophile, or neither?
- What is the solvent, and is it hot?
Statement questions
Recent Main papers use many statement pairs. Judge each statement alone before looking at the options. Watch for words such as all, always, only and every, because they often make a statement false.
Rate orders
| Asked about | Look at |
|---|---|
| Carbocation stability | |
| Crowding behind the carbon | |
| Either, with different halogens | Leaving group: |
| Substituted aryl halides | Number of ortho and para acceptors |
| Electrophilic substitution | Electron density in the ring |
Time
Named-reaction and statement questions should take about a minute each. Spend any extra time on rate orders, where drawing the cation or the crowding takes 30 seconds and saves marks.
Telling halides apart
Warm the halide with alcoholic . Silver pulls the halide away, so the speed of the precipitate follows how easily a cation forms. This is from Morrison and Boyd.
| Halide | With alcoholic |
|---|---|
| Benzylic, allylic, tertiary | Precipitate at once |
| Secondary | Precipitate on warming |
| Primary | Slow, needs longer warming |
| Vinylic, aryl | No precipitate |
The colour names the halogen: AgCl is white, AgBr pale yellow and AgI yellow.
Conversions
Conversion questions test whether you know the map. The Sim lab has a route finder: pick a start and a target, and it shows the shortest path it knows, with each reagent. Then try to write the route yourself before you look.