Swap one hydrogen in an alkane for a halogen and the molecule becomes a chemical crossroads. The polar C–X bond lets nucleophiles in, so halogenoalkanes turn into alcohols, nitriles, amines and alkenes.
Halogenoalkanes contain a polar Cδ+–Xδ− bond, so the carbon is attacked by nucleophiles. They are classed as primary, secondary or tertiary by how many carbons are attached to the C–X carbon. With aqueous NaOH they give alcohols; with KCN in ethanol, nitriles (one extra carbon); with ammonia in ethanol under pressure, amines: all nucleophilic substitution. With NaOH in ethanol they undergo elimination to alkenes. The rate of hydrolysis is governed by bond strength, not polarity: C–I is the weakest bond, so iodoalkanes react fastest.
Sort halogenoalkanes into primary, secondary and tertiary in the simulator by counting the carbons on the C–X carbon.
| From | Reagent and conditions | Example |
|---|---|---|
| alkane | X2, UV light (free radical substitution) | CH4 + Cl2 → CH3Cl + HCl |
| alkene | HX or X2, room temperature | C2H4 + HBr → C2H5Br |
| alcohol | HX (e.g. KBr + concentrated H2SO4), heat | C2H5OH + HBr → C2H5Br + H2O |
| alcohol | PCl5, room temperature | ROH + PCl5 → RCl + POCl3 + HCl |
| alcohol | SOCl2 | ROH + SOCl2 → RCl + SO2 + HCl |
| alcohol | PCl3 (or red P + I2), heat | 3ROH + PCl3 → 3RCl + H3PO3 |
Run each reaction of bromoethane in the simulator. The same reagent, NaOH, gives different products depending on the solvent.
| Reagent | Conditions | Product | Type |
|---|---|---|---|
| NaOH(aq) | heat under reflux | ethanol | nucleophilic substitution (hydrolysis) |
| KCN | in ethanol, heat under reflux | propanenitrile, CH3CH2CN | nucleophilic substitution; chain grows by one C |
| NH3 | excess, in ethanol, heat in a sealed tube | ethylamine, CH3CH2NH2 | nucleophilic substitution |
| NaOH | in ethanol, heat | ethene | elimination of HBr |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the product of heating 1-bromopropane with aqueous sodium hydroxide?
Which reagent converts a halogenoalkane into an alkene?
2-bromo-2-methylpropane is classified as:
Which reaction increases the number of carbon atoms in the chain?
Warm 1-chlorobutane, 1-bromobutane and 1-iodobutane with aqueous silver nitrate in ethanol in the simulator. The halide ion released forms a precipitate; time how long each takes.
| Bond | Bond energy / kJ mol−1 | Precipitate | Speed |
|---|---|---|---|
| C–Cl | 340 | white AgCl | slowest |
| C–Br | 280 | cream AgBr | medium |
| C–I | 240 | yellow AgI | fastest |
C–Cl is the most polar bond, which would suggest it reacts fastest, but the experiment shows the opposite: breaking the bond is the deciding factor. CFCs are so unreactive because C–F and C–Cl bonds are strong; in the upper atmosphere, UV light breaks C–Cl to release Cl radicals that destroy ozone.
Aqueous NaOH gives substitution (an alcohol); ethanolic NaOH gives elimination (an alkene). For the rate of hydrolysis, bond strength wins over bond polarity: iodoalkanes react fastest. KCN is the reagent that lengthens the carbon chain.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
1-chlorobutane, 1-bromobutane and 1-iodobutane are warmed with aqueous AgNO3 in ethanol. Which forms a precipitate first, and why?
What is the organic product of bromoethane with KCN in ethanol?
Which reagent converts ethanol into chloroethane at room temperature?
Why are CFCs harmful in the stratosphere even though they are very unreactive?
This skill is now recorded on your profile. Keep the chain going.