Substitution Reactions of Haloalkanes
Reactivity of Haloalkanes
- Haloalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
- The carbon-halogen bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge
Due to the large difference in electronegativity between the carbon and halogen atom, the C-X bond is polar
- Because of this, haloalkanes will undergo two key types of reaction
- Nucleophilic substitution reactions
- A halogen is substituted for another atom or group of atoms
- The products formed when haloalkanes undergo this type of reaction are alcohols, amines and nitriles
- Elimination reactions
- A hydrogen halide is eliminated during the reaction
- The key product formed from this type of reaction is an alkene
Formation of alcohols
- The nucleophile in this reaction is the hydroxide, OH- ion
- An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
- Halogenoalkanes are generally insoluble in water
- This means that there would be an organic layer of the halogenoalkane and an aqueous layer of sodium hydroxide
- Any reaction would only occur where the two layers meet
- The use of ethanol allows the aqueous sodium hydroxide layer and the organic halogenoalkane layer to mix, allowing the reaction to proceed
- This reaction is very slow at room temperature, so the reaction mixture is warmed
- This is an example of a hydrolysis reaction and the product is an alcohol
- The rate of this reaction depends on the type of halogen in the haloalkanes
- The stronger the C-X bond, the slower the rate of the reaction
- In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
- Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction
The halogen is replaced by the nucleophile, OH-
- This reaction could also be done with water as the nucleophile, but it is very slow
- The hydroxide ion is a better nucleophile than water as it carries a full negative charge
- In water, the oxygen atom only carries a partial charge
A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali
Measuring the rate of hydrolysis
- Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
- Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate
- Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
- Time how long it takes for the precipitates to form
- The precipitate will form as the reaction progresses and the halide ions are formed
- A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
- The yellow precipitate will form the fastest
- This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest
- The white precipitate will form the slowest
- This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest