Halogenoalkanes and nucleophilic substitution
Introduced| English | Meaning |
|---|---|
| halogenoalkane/ˈheɪləʊdʒnəʊlkeɪn/ | An alkane derivative in which one or more hydrogen atoms have been replaced by halogen atoms. |
| nucleophilic substitution/ˌnjuːklɪəˈfɪlɪk ˌsʌbstɪˈtjuːʃn/ | A reaction in which an electron-pair donor replaces a leaving group in a molecule. |
| nitrile/ˈnaɪtraɪl/ | An organic compound containing a carbon atom triple-bonded to a nitrogen atom. |
| amine/ˈæmaɪn/ | An organic compound derived from ammonia by replacing one or more hydrogen atoms with carbon-containing groups. |
| reflux/ˈriːflʌks/ | Heating a reaction mixture while condensing its vapour and returning the condensate to the same vessel. |
Swapping out a halogen
- A halogenoalkane 卤代烷 is an alkane with a halogen in place of hydrogen (a C–X bond).
- The C–X bond is polar, which makes these compounds react.
- They come in three classes.
A halogenoalkane is:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A halogenoalkane has a polar C–X bond, which is the reactive site.
Halogenoalkanes react with aqueous hydroxide ions by:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
OH⁻ is a nucleophile replacing the halogen.
Three classes
- primary, secondary or tertiary, set by how many carbons are joined to the carbon holding the halogen (one, two or three).
- Made by: free-radical substitution of an alkane, electrophilic addition to an alkene, or substitution of an alcohol.
The solvent decides whether a halogenoalkane substitutes or eliminates
Primary, secondary and tertiary halogenoalkanes
Nucleophilic substitution
A nucleophile swaps in for the leaving group on a halogenoalkane.
A tertiary halogenoalkane has the halogen on a carbon joined to:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Primary = 1 carbon attached, secondary = 2, tertiary = 3, on the carbon bearing the halogen.
Match each halogenoalkane idea.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Each item links the term to its correct meaning.
In nucleophilic substitution, OH⁻ attacks the δ+ carbon and the halogen leaves as a halide ion.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The C–X bond is polar, so the carbon is δ+ — exactly where an electron-pair donor attacks.
Nucleophilic substitution 亲核取代
- The C–X carbon is slightly positive, so a nucleophile (lone-pair species) attacks and replaces the halogen.
| Reagent | Product |
|---|---|
| $\text{NaOH(aq)}$, heat | an alcohol |
| $\text{KCN}$ in ethanol, heat | a nitrile 腈 (adds a carbon) |
| $\text{NH}_3$ in ethanol, heat + pressure | an amine 胺 |
Many halogenoalkane reactions are carried out by heating under reflux 回流.
Warming a halogenoalkane with aqueous NaOH produces:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
NaOH(aq) substitutes the halogen for –OH, giving an alcohol.
Reaction with KCN in ethanol gives a nitrile, which is useful because it:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The CN group adds a carbon, lengthening the chain; NH₃ instead gives an amine.
Identifying the halogen
- Warm with silver nitrate in ethanol: a silver halide precipitate forms — white AgCl, cream AgBr, yellow AgI.
Volatile halogenoalkanes were once widely used as refrigerants and aerosol propellants.
You've got it
- a halogenoalkane has a polar C–X bond; classes are primary/secondary/tertiary
- nucleophilic substitution: NaOH(aq) → alcohol; KCN → nitrile (+1 carbon); NH₃ → amine
- identify the halogen with silver nitrate (white/cream/yellow precipitate)