Haloalkanes Flashcards
Why are haloalkanes more reactive than alkanes?
Haloalkanes are much more reactive than alkanes due to the presence of the electronegative halogens.
Means carbon-halogen bond is polar
What are the two key types of reaction haloalkanes undergo?
Nucleophilic substitution reactions
Elimination reactions
What happens during nucleophilic substitution of a haloalkane?
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
What happens during an elimination reaction of a haloalkane?
A hydrogen halide is eliminated during the reaction
The key product formed from this type of reaction is an alkene
How is an alcohol formed from a haloalkane?
Nucleophilic substitution.
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.
This is an example of a hydrolysis reaction and the product is an alcohol
What does the rate of reaction of a haloalkane to an alcohol depend on?
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
Why is NaOH a better nucleophile for converting haloalkane to alcohol than water?
The 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.
How can you measure the rate of hydrolysis of a haloalkane?
Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes.
1) Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and aqueous acidified silver nitrate. (The mixture is acidified to prevent unreacted hydroxide ions reacting with the silver ions to give a confusing precipitate. Water acts as the nucleophile)
2) 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
3) The precipitate will form as the reaction progresses and the halide ions are formed
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.
What colour do the precipitates turn when silver nitrate is added to a chloroalkane, bromoalkane and iodoalkane?
silver chloride = white precipitate
silver bromide = pale cream precipitate
silver iodide = pale yellow precipitate
How can you distinguish between the different precipitates formed? (AgCl, AgBr, AgI)
Add ammonia solution.
AgCl = precipitate dissolves in weak ammonia to give a colourless solution
AgBr = dissolves in concentrated ammonia solution to give a colourless solution
AgI = precipitate is insoluble in ammonia solution of any concentration
Haloalkane -> alcohol
+NaOH, heat
Nucleophilic substitution reaction
Haloalkane -> nitrile
+NaCN (ethanolic), heat
Nucleophilic substitution reaction
Haloalkane -> primary amine
+2NH3 (excess), heat under pressure
Nucleophilic substitution reaction
Produces primary amine + NH4Cl
Why do we use excess ammonia when converting haloalkane -> primary amine?
The amine products can undergo further reactions with haloalkanes.
By using excess ammonia, we can prevent those reactions from happeneing.
Why does bond enthalpy affect the rate of reaction of haloalkanes?
If bond enthalpy is lower, less energy required for successful collision.
So more collisions will be successful, so the rate of reaction will increase.
