4.2.4 : Haloalkanes Flashcards
(30 cards)
Use electronegativity of the halogens to describe their reactivity
- Electronegativity of halogens decreases going down the group
- So bond polarity decreases going down the group
- This means bonds become weaker
- So reactivity increases going down the group
Use bond enthalpy of the halogens to describe their reactivity
- Greater Mr of halogen in bond = lower bond enthalpy
- So bond enthalpy decreases going down the group
- So bonds become easier to break
- So reactivity increases going down the group
Describe the substitution reaction of a haloalkane to produce an alcohol
- React with an aqueous alkali to produce alcohols
- This is a nucleophilic substitution reaction
- The hydroxide ion acts as a nucleophile
Describe the substitution reaction of a haloalkane to produce an alkene
- React with ethanoic potassium hydroxide (KOH) to produce alkenes
- This is an elimination reaction
- The hydroxide ion acts as a base
Describe what takes place in the hydrolysis of a haloalkane
- Haloalkanes can be broken down when they react with aqueous silver nitrate in the presence of ethanol
- The water in the solution acts as a nucleophile which leads to formation of the alcohol + releases the halide ions into the solution
- The halide ions then react with the silver ions from the silver nitrate to form silver precipitates, which can be used to identify the halide ion present
What is hydrolysis?
A chemical reaction in which water is a reactant
Why can the silver nitrate test not be used to determine the presence of fluoride ions?
- Fluoride ions react with silver nitrate to make silver fluoride
- Silver fluoride is very soluble so no precipitate is formed
What is a nucleophile?
Nucleophiles are electron pair donors - they contain a lone electron pair that is attracted to positive regions of molecules
What are some examples of nucleophiles?
CN:-
:NH3
:OH-
What is nucleophilic substitution?
A chemical reaction in which an atom/group of atoms is exchanged for a nucleophile
What nucleophile is used to produce:
1. alcohols?
2. nitriles?
3. amines?
- aqueous potassium hydroxide
- potassium cyanide
- ammonia
Which haloalkanes can nucleophilic substitution reactions occur for?
Primary and secondary
What are CFCs?
Chlorofluorocarbons are haloalkanes containing carbon, chlorine and fluorine atoms only
Explain why haloalkanes can be attacked by nucleophiles
- Haloalkanes contain a polar bond (since halogen atom is more electronegative than C atom)
- This creates a bond with a slight positive charge on the C atom
- This slight positive charge attracts nucleophiles
Briefly outline a method for monitoring the rate of hydrolysis of haloalkanes
- Add aqueous silver nitrate to each haloalkane in the presence of an ethanol solvent ( after using a water bath to ensure they are all at the same temp.)
- Measure using a stopwatch how long it takes for a precipitate to form
Why is ozone depletion bad for the environment?
Ozone absorbs harmful UV radiation in the stratosphere, stopping it from reaching earth and causing damage
Why are CFCs frequently used in refrigerants and air conditioning units?
C-X bonds are very strong so molecules are stable and unreactive
CFCs are also non-toxic and non-flammable
How are haloalkanes broken down in the atmosphere?
CFCs absorb UV radiation which causes them to dissociate and form radicals
Give the equation for the formation of chlorine radicals from CF3Cl (initiation reaction)
CF3Cl -> .CF3 + .Cl
Write free radical substitution equations to show how Cl free radicals catalyse the decomposition of ozone
Cl2 -> 2Cl. (in the presence of UV light)
Cl. + O3 -> ClO. + O2
ClO. + O3 -> Cl. + 2O2
What else can cause the break down of ozone?
Radicals of nitrogen oxides
Write the equations which show how the .NO radical breaks down ozone
.NO + O3 -> .NO2 + O2
.NO2 + O -> .NO + O2
Overall : O3 + O -> 2O2
Give the overall equation for the break down of ozone
2O3 -> 3O2
What is the role of the radicals in the break down of ozone?
Catalysts
