Summary — What This Chapter Covers
- Halogenoalkanes come from alkanes (free radical), alkenes (electrophilic addition) or alcohols (substitution).
- Primary, secondary and tertiary halogenoalkanes.
- Nucleophilic substitution with NaOH(aq), KCN in ethanol, NH3 in ethanol and AgNO3(aq) in ethanol.
- Elimination with NaOH in ethanol gives an alkene.
- SN1 and SN2 mechanisms; reactivity RI > RBr > RCl, set by C–X bond strength.
1. Halogenoalkanes
15.1
Making halogenoalkanes
- From an alkane: Cl2 or Br2 in UV light (free-radical substitution), e.g. C2H6 + Br2 → C2H5Br + HBr.
- From an alkene: X2 or HX(g) at room temperature (electrophilic addition).
- From an alcohol (substitution of –OH):
- HX(g), or KCl with concentrated H2SO4 or H3PO4 (makes HCl in the flask): ROH + HCl → RCl + H2O
- PCl3 and heat: 3ROH + PCl3 → 3RCl + H3PO3
- PCl5 at room temperature: ROH + PCl5 → RCl + POCl3 + HCl (steamy fumes)
- SOCl2: ROH + SOCl2 → RCl + SO2 + HCl (both by-products are gases, so the product is easy to separate)
Classification
Count the carbon atoms bonded to the carbon carrying the halogen: one (or none) = primary, e.g. 1-bromobutane; two = secondary, e.g. 2-bromobutane; three = tertiary, e.g. 2-bromo-2-methylpropane.
15.1
Nucleophilic substitution and elimination
| Reagent and conditions | Product (from bromoethane) | Type |
|---|
| NaOH(aq), heat under reflux | CH3CH2OH, ethanol | nucleophilic substitution |
| KCN in ethanol, heat under reflux | CH3CH2CN, propanenitrile (chain gains one C) | nucleophilic substitution |
| excess NH3 in ethanol, heated under pressure (sealed tube) | CH3CH2NH2, ethylamine | nucleophilic substitution |
| AgNO3(aq) in ethanol, warm | ethanol + Br−, then AgBr(s) | nucleophilic substitution (hydrolysis by water) |
| NaOH in ethanol, heat | CH2=CH2, ethene | elimination |
Key equations
CH3CH2Br + OH− → CH3CH2OH + Br−
CH3CH2Br + CN− → CH3CH2CN + Br−
CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br
CH3CH2Br + NaOH → CH2=CH2 + NaBr + H2O (ethanolic)
Identifying the halogen. Warm the halogenoalkane with aqueous silver nitrate in ethanol (ethanol lets the two mix). Water slowly substitutes the halogen and the halide ion released forms a precipitate: AgCl white, AgBr cream, AgI yellow.
15.1
SN2 and SN1 mechanisms
SN2 (one step, two species in the rate-determining step). The nucleophile’s lone pair attacks the δ+ carbon from the side opposite the halogen; at the same moment the C–X bond breaks heterolytically and X leaves as X−. The transition state has the nucleophile and the halogen both partly bonded to carbon, with a negative charge spread over it, drawn in square brackets.
SN1 (two steps, one species in the rate-determining step). Step 1 (slow): the C–X bond breaks heterolytically to give a carbocation and X−. Step 2 (fast): the nucleophile attacks the planar carbocation.
- Primary
SN2
- Tertiary
SN1
N
- Secondary
Memorise