4.5Core Organic Chemistry
Haloalkanes
5 min read 10 parts Lesson 19 of 29
Lesson summary
Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Haloalkanes Contents Reactions of Haloalkanes Nucleophilic Substitution of Haloalkanes Hydrolysis of Primary Haloalkanes Environmental Concerns of Organohaloge…
Section 1 of 10Reactivity of haloalkanes
Reactivity of haloalkanes
Polar bond
- Polar bond haloalkanes are much more reactive than alkanes because of the electronegative halogen
- The large electronegativity difference makes the C–X bond polar, with C δ+ and X δ−.
Nucleophilic substitution
- Nucleophilic substitution the halogen is substituted by another atom or group
- The products include alcohols, amines and nitriles.
Elimination
- Elimination a hydrogen halide is removed, giving an alkene as the key product
Section 2 of 10Nucleophiles and the mechanism
Nucleophiles and the mechanism
Nucleophile
Nucleophile an electron-rich species that donates a pair of electrons
- The name means "nucleus-loving" because it is attracted to positive charge.
Nucleophilic substitution
Nucleophilic substitution a reaction in which a nucleophile attacks a carbon carrying a partial positive charge and replaces the halogen
- The polar C–X bond makes haloalkanes undergo this reaction.
Strength of nucleophile
- Strength of nucleophile hydroxide is a better nucleophile than water
- OH− has a full formal negative charge but the oxygen in water has only a partial negative charge.
Section 3 of 10Hydrolysis to form alcohols
Reagent
- Reagent aqueous sodium or potassium hydroxide, with ethanol
- Haloalkanes are insoluble in water and would form a separate organic layer, reacting only where the layers meet.
- Ethanol allows the aqueous and organic layers to mix.
Conditions
- Conditions the reaction is very slow at room temperature, so the mixture is warmed
Reaction
- Reaction a hydrolysis in which OH− replaces the halogen to give an alcohol
- R–X + OH− → R–OH + X−.
- Water can also be the nucleophile, but the reaction is much slower.
Mechanism
- Mechanism the hydroxide lone pair attacks the δ+ carbon and the C–X bond breaks heterolytically with the pair going to the halogen
Section 4 of 10Rate and bond enthalpy
Bond strength order
- Bond strength order bond enthalpy C–F > C–Cl > C–Br > C–I
- Substitution involves breaking the C–halogen bond.
Rate
- Rate the stronger the C–X bond, the slower the reaction
- C–I is the weakest, so iodoalkanes react very fast.
- C–F is the strongest, so fluoroalkanes do not react.
Data
- Data bond enthalpy values vary between sources because they are mean values from different substances, so use the data given
Section 5 of 10Measuring hydrolysis rate with silver nitrate
Measuring hydrolysis rate with silver nitrate
Set up three test tubes in a 50 °C water bath, each with ethanol and acidified silver nitrate.
Add a few drops of a chloroalkane, a bromoalkane and an iodoalkane, one to each tube, and start a stopwatch.
Time how long each precipitate takes to appear.
| Haloalkane | Precipitate | Speed |
|---|---|---|
| Chloroalkane | White AgCl | Slowest, C–Cl strongest |
| Bromoalkane | Cream AgBr | Intermediate |
| Iodoalkane | Yellow AgI | Fastest, C–I weakest |
- Principle
- the precipitate forms as halide ions are released by hydrolysis
- The faster the C–X bond breaks, the faster the precipitate forms.
Section 6 of 10CFCs and HFCs
CFCs
CFCs chlorofluorocarbons, containing carbon with chlorine and fluorine, e.g. CCl3F and CCl2F2
- They are chemically inert, non-flammable and non-toxic.
- Uses include refrigerants, aerosol propellants and dry-cleaning solvents.
HFCs
HFCs hydrofluorocarbons, containing carbon with hydrogen and fluorine only, e.g. CH2F–CF3
- They are inert and used widely.
- They contain no chlorine, so they do not damage the ozone layer and are an alternative to CFCs.
Section 7 of 10Ozone depletion by chlorine radicals
Cause
- CFCs absorb UV in the upper atmosphere and break down, forming chlorine radicals
Catalytic chain reaction
- Propagation 1: Cl• + O3 → ClO• + O2.
- Propagation 2: ClO• + O → Cl• + O2.
- OverallO3 + O → 2O2.
Catalyst
- Cl• is regenerated in propagation 2, so it can destroy more ozone
- Only the propagation steps involving a single oxygen atom are credited;
- 2O3 → 3O2 is the overall breakdown, not the radical steps.
Section 8 of 10Nitrogen oxide radicals
1Sources
- NO forms naturally in lightning strikes,
- from air travel in the stratosphere and by direct combination of nitrogen and oxygen at high temperature in combustion engines
- NO is a radical because it has an odd number of electrons, i.e. an unpaired electron.
2Catalytic cycle
- NO• + O3 → NO2• + O2.
- NO2• + O → NO• + O2.
- OverallO3 + O → 2O2.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Nucleophile | An electron pair donor attracted to positive centres |
| Nucleophilic substitution | A nucleophile replaces an atom or group by attacking a δ+ carbon |
| Hydrolysis | A reaction in which water or hydroxide breaks a bond |
| CFC | A compound of carbon, chlorine and fluorine that damages the ozone layer |
| Radical catalyst | A radical regenerated in a chain, such as Cl• in ozone breakdown |
Section 10 of 10Exam technique
1
Saying iodoalkanes react slowly because the C–I bond is strong — it is the weakest, so they react fastest
2
Omitting ethanol from the hydrolysis conditions — it mixes the organic and aqueous layers
3
Writing the overall ozone equation as the propagation steps
4
Saying the C–X bond breaks homolytically in hydrolysis — it breaks heterolytically