Chapter 15 - Haloalkanes Flashcards
What are haloalkanes?
Compounds containing hydrogen, carbon and at least one halogen atom
When two or more halogens are present in a haloalkane, how do you name it?
Put the two halogens in alphabetical order
How do you name haloalkanes?
Add the suitable prefix onto the name of the longest chain
What is a nucleophile?
An electron pair donor
Why are haloalkanes polar?
The carbon-halogen bond is polar. The halogen generally has a much higher electronegativity than carbon, so the electrons are pulled slightly towards the halogen and the carbon is left with a slight positive charge
Give three examples of nucleophiles
Hydroxide ions, :OH-
Water molecules, H2O:
Ammonia molecules, :NH3
What happens when a nucleophile is attracted to a carbon?
It donated electrons to form a new covalent bond
Why are nucleophiles attracted to carbon?
The carbon-halogen bond is polar, so carbon is left with a slight positive charge
What happens when a nucleophile reacts with a haloalkane?
The nucleophile replaces the halogen in a substitution reaction
What is nucleophilic substitution?
When a nucleophile replaces a halogen in a haloalkane
What happens when a haloalkane is hydrolysed?
An alcohol is formed
Describe the nucleophilic substitution reaction to make an alcohol from a haloalkane
1) The nucleophile (OH- in this case) approaches the slightly positively charged carbon
2) The direction of the attack of this nucleophile reduces the repulsion between the negatively charged hydrogen atom and the OH- ion
3) The lone pair of electrons on the nucleophile is donated to the carbon atom
4) A new bond forms between the carbon and the oxygen of the OH- ion
5) The carbon-halogen bond breaks by heterolytic fission
6) An alcohol and halide ion are formed
Why is the reaction different when water is used as a nucleophile?
It is much slower but will eventually get the same result
Which bond in the haloalkane is broken during hydrolysis?
The carbon-halogen bond
What does the rate of hydrolysis in haloalkanes rely upon?
The strength of the carbon-halogen bond
Which haloalkanes have the weakest bonds and so react fastest?
Iodoalkanes
Which haloalkanes have the strongest bonds and so react the slowest?
Fluoroalkanes
Will primary, secondary or tertiary haloalkanes react fastest?
Tertiary
Why do tertiary haloalkanes react fastest?
The tertiary haloalkane is much more stable than the primary one and reacts with a two-step mechanism
What reaction would you conduct to measure the rate of hydrolysis of the haloalkanes?
1) Set up three test tubes, each containing 1cm^3 of ethanol and 2 drops of the haloalkane
2) Stand the tubes in a 60c water bath
3) Place a test tube containing 0.1 mol/dm3 silver nitrate in the same water bath, to allow everything to reach the same temperature
4) Add 1cm3 of the silver to each test tube
5) Start a timer and record your results
What would you observe when you hydrolyse iodoalkanes?
A yellow precipitate forms rapidly
What would you observe when you hydrolyse bromoalkanes?
A cream precipitate forms, more slowly than iodoalkanes but faster than chloroalkanes
What would you observe when you hydrolyse chloroalkanes?
A white precipitate forms very slowly
Why do we does the reaction between a haloalkane and silver nitrate produce a precipitate?
A halogen ion is formed, which forms bonds with the silver to produce silver halide, which is insoluble
Why is ethanol used to assist the hydrolysis of haloalkanes?
It allows the water and haloalkane to mix to produce one single solution rather than two layers
What did the Montreal Project aim to try and achieve?
Try and remove CFCs from the environment
What are brominated flame retardants?
Compounds used by the electronics industry to reduce the flammability of products
What are CFCs?
Haloalkanes
What are organohalogens?
Compounds containing at least one hydrogen attached to the carbon chain
Why are organohalogens bad?
They aren’t easily broken down
Why is UV-B radiation bad?
It is commonly linked to sunburn
Why is it bad that the ozone layer is being depleted?
More UV-B rays can reach earth, leading to an increase in genetic mutations and skin cancer
What does the ozone layer do?
Absorbs harmful UV-B rays from the sun to stop them coming to earth
Chemical formula for ozone
O3
How is ozone made?
Very high levels of Uv radiation breaks an oxygen molecule into two oxygen radicals
An equilibrium then forms between oxygen (O2) and oxygen radicals making ozone and ozone breaking down to produce the reactants
Why are CFCs very stable?
They have very strong bonds
Why are CFCs bad?
They are generally very stable until they reach the stratosphere. Here, they break down, releasing chlorine radicals that start to break down the ozone and upset the equilibrium
How does the carbon-halogen bond in CFCs break?
Sufficient energy is provided by UV radiation, which makes the bond break by homolytic fission, forming radicals
What is the equation for the photodissociation of CF2Cl2 to form chlorine radicals?
CF2Cl2 -> CF2Cl. + .Cl
What does chlorine actually do to ozone?
Once chlorine radicals are formed, they react with ozone to form an intermediary (ClO.) and an oxygen molecule. The ClO. then reacts with an oxygen atom to form Cl. + O2. Chlorine destroys the ozone and turns it into oxygen
How is nitrogen oxide?
Aircraft engines and lightning
Equations for nitrogen oxide breaking down O3
NO. + O3 -> NO2. + O
NO2. + O -> NO. + O2
How do nitrogen oxide and chlorine radicals affect the breakdown of ozone?
They catalyse it
Name three compounds that are currently being used as alternatives to CFCs
HCFCs, hydrocarbons and HFCs
Why are HFCs good?
They contain no chlorine so don’t break down the ozone
Why are HCFCs and HFCs bad?
They’re greenhouse gases and much worse than carbon dioxide
