15.1 : The Chemistry Of Haloalkanes Flashcards
Define nucleophile
Electron pair donor
Give some examples of nucleophiles
Hydroxide ions , :OH-
Water molecules , H2O :
Ammonia molecules , : NH3
What is nucleophilic substitution ?
When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substation reaction. A new compound is produced containing a different functional group.
What is hydrolysis
A chemical reaction invoking water or an aqueous solution of a hydroxide that causes the breaking of a bond in molecule.
Describe the hydrolysis of a haloalkane ( example of nucleophilic substitution )
The nucleophile ( OH-) approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
This direction minimises repulsion between the nucleophile and the delta negative halogen atom.
A lone pair of electrons on the hydroxide ions is attracted and donated to the delta plus carbon atom.
A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
The carbon - halogen bond breaks by heterolytic fission.
The new organic product is an alcohol. A halide ion is also formed .
How can haloalkanes be converted to alcohols ?
Using aqueous sodium hydroxide
Heated under reflux to get good yield of product
What does the rate of hydrolysis depend on?
The strength of the carbon - halogen bond
Describe the strengths of different carbon - halogen bonds
Strongest - weakest :
C - F
C - Cl
C - Br
C - I
What can we predict from the different strengths / bond enthalpies of the different bonds ?
Iodoalkanes react faster than broom alkanes
Bromoalkanes react faster than choloroalkanes
Fluroalkanes are unreactive, as a large amount of energy is required to break the C-F bond
How can you measure the rate of hydrolysis of primary haloalkanes?
The rate of each reaction can be seen by carrying out hydrolysis in the presence of aqueous silver nitrate.
As the reaction takes place halide ions are produced to react with Ag+ ions to form a precipitate of the silver halide.
Nucleophile = water
Haloalkanes are insoluble in water so it is carried out in the presence of an ethanol solvent. This allows water and the haloalkanes to mix and produce a single solution rather than two layers.
What colour precipitate will each haloalkane produced when hydrolysed in the presence of silver nitrate?
Chlorobutane = white precipitate slowly
Bromobutane = cream precipitate faster than chlorobutane
Iodobutane = yellow precipitate rapidly
What is the experiment you would do to test the react rates of the hydrolysis of haloalkanes?
Set up three test tubes each with 2 drops of a different haloalkane and 1cm3 of ethanol
Then stand the test tubes in a water bath at 60°C
Then place a test tube containing 0.1 mol dm-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature
Add 1cm3 of the silver nitrate quickly to each of the test tubes. Start a stop clock.
Observe the test tubes for 5 minutes and record the time taken for the precipate to form.
How can the results of the experiment be explained?
The compound with this slowest rate of reaction is the one that has the strongest carbon - halogen bond and vice versa
