3.3.3 Halogenoalkanes Flashcards
What is a halogenoalkane?
An alkane in which one or more of the hydrogen atoms have been replaced by a halogen atom (fluorine, chlorine, bromine or iodine)
What are the three types of halogenoalkane?
Primary
Secondary
tertiary
How are haloalkanes classified as primary, secondary or tertiary?
According to the position of the halogen atom in the molecule.
What is a primary haloalkane?
When there is one alkyl group attached to the carbon with the halogen atom.
What is a secondary haloalkane?
When there are two alkyl groups attached to the carbon with the halogen atom.
What is a tertiary haloalkane?
When there are three alkyl groups attached to the carbon with the halogen.
What is the important property of the carbon-halogen bond?
It is a polar bond, so has a permanent dipole.
This is because of the large difference in electronegativity, so electron density lies towards the halogen, creating a delta + and delta - region.
What happens to the boiling point of haloalkanes as they increase in size and why?
The boiling points increase as the haloalkanes increase in size (chain gets longer).
This means that there are greater van-der-walls forces between molecules which increases the boiling point.
What happens to the boiling point of haloalkanes compared to equivalent alkanes and why?
Haloalkanes have a higher boiling point than the equivalent alkanes, as the polar halogen-carbon bond means permanent dipole-dipole forces are present as well as van-der-walls forces.
What happens to the boiling point of haloalkanes as you go down group 7?
As you go down group 7, the boiling points of the haloalkanes increases, because the size of the halogen atom increases. This increases the van-der-waals forces between molecules, which increases the boiling point.
How does the c-x bond polarity determine reactivity of a halogenoalkane?
A halogenoalkane that contains a more electronegative halogen would be more reactive than those that contain a less electronegative halogen.
How does c-x bond enthalpy affect the reactivity of a halogenoalkane?
A halogenoalkane with a lower c-x bond enthalpy would be more reactive than those with a higher bond enthalpy.
What is the order of electronegativity of the halogens?
Most electronegative = fluorine
Chlorine
Bromine
Least electronegative = iodine
What is the order of bond enthalpies of the halogens when bonded with carbon?
Highest bond enthalpy = fluorine
Chlorine
Bromine
Lowest bond enthalpy = iodine
What does the reactivity of halogenoalkanes depend on?
The bond enthalpy
What are the conditions for elimination reactions of halogenoalkanes using hydroxide ions?
Warm ethanolic sodium hydroxide (source of OH- ions)
Carried out under reflux
What does elimination in halogenoalkanes using OH- ions form?
Alkene
Water
Halide ion (or metal halide depending on the source of the OH- ions)
What type of halogenoalkanes are the most reactive and why?
Tertiary halogenoalkanes are the most reactive as they form a stable intermediate which allows the nucleophile to react faster.
Describe the elimination of halogenoalkanes using OH- ions?
The OH- ions attacks the hydrogen atom on the carbon adjacent to the carbon with the halogen on.
OH- acts as a base (accepting proton/hydrogen) to form water.
The electrons in the C-H bond move to form a carbon-carbon DOUBLE bond.
The C-X (carbon halogen) bond is broken, as carbon can only form 4 bonds.
The halogen takes both electrons in the C-X bond.
This forms an alkene, a water molecule and a halide ion (or metal halide).
How can we use elimination to test the reactivity of the C-X bond?
We can react the halide ion produced with silver nitrate to form a precipitate.
We can measure how fast the precipitate forms to measure the rate of reaction and therefore reactivity.
What are the results from the elimination reaction test for the C-X bond reactivity?
Silver iodide forms fastest which shows that iodoalkanes are the most reactive halogenoalkanes.
They have the lowest bond enthalpy, which shows that this is the factor that affects halogenoalkanes reactivity.
What must we check for in the alkane produced by elimination of halogenoalkanes by OH- ions?
E-Z isomerism
What is a mechanism?
A pathway that a chemical reaction takes in order to convert the reactants into the products.
Why do halogenoalkanes undergo nucleophilic substitution?
Due to the polar C-X bond
What is nucleophilic substitution?
A reaction in which the halogen in a halogenoalkane is substituted for another atom or group of atoms.
What is a nucleophile and what are 3 examples?
An electron pair donor eg. OH-, CN-, NH3
What is a leaving group?
The molecule replaced by a nucleophile eg. Halogen
What are the conditions for nucleophilic substitution of halogenoalkanes using hydroxide ions?
Warm aqueous sodium hydroxide (source of OH- ions)
Carried out under reflux
What are the conditions for nucleophilic substitution of halogenoalkanes using cyanide ions?
Warm ethanolic potassium cyanide (source of CN- ions)
Carried out under reflux
What are the conditions for nucleophilic substitution of halogenoalkanes using ammonia?
Heat with ethanolic ammonia
Must be excess ammonia
What is produced in nucleophilic substitution of halogenoalkanes using hydroxide ions?
An alcohol
A halogen free radical
What is produced in nucleophilic substitution of halogenoalkanes using cyanide ions?
A nitrile
A halogen free radical
What is produced in nucleophilic substitution of halogenoalkanes using ammonia?
An amine
An ammonium ion
A halogen free radical
Describe what happens in nucleophilic substitution of halogenoalkanes using hydroxide ions?
An OH- ions attacks the delta positive carbon to form a bond with it.
This replaces the halogen, so the C-X bond is broken and both electrons move to the halogen.
This forms an alcohol and a halogen free radical.
Describe what happens in nucleophilic substitution of halogenoalkanes using cyanide?
The CN- ion attacks the delta positive carbon to form a bond with it.
This replaces the halogen, so the C-X bond breaks, and both electrons in the bond move to the halogen.
This forms a nitrile and a halogen free radical.
Describe what happens in nucleophilic substitution of halogenoalkanes using ammonia?
Ammonia attacks the delta positive carbon to form a bond with it.
This replaces the halogen, so the C-X bond breaks, and both electrons move to the halogen.
This forms a halogen free radical, and an unstable intermediate (as nitrogen can only form 3 bonds but has 4).
Another NH3 molecule acts as a base by reacting with hydrogen.
This breaks the N-H bond as hydrogen can only bond once, and electrons move to the nitrogen atom.
This forms a stable amine and an ammonium ion.
What plays a role in determining whether elimination or substitution take place?
The solvent used
What two ways can we make different products when reacting sodium hydroxide with halogenoalkanes?
We can use ethanol solvent so an elimination reaction will take place, and an alkene will be formed.
We can use water solvent so a substitution reaction will take place, and an alcohol will be formed.
What does OH- act as when in a water solvent?
A nucleophile
What does OH- act as when in an ethanol solvent?
A base
What are CFCs?
Chlorofluoroalkanes
Halogenoalkanes that contain both chlorine and fluorine but no hydrogen.
Why were CFCs developed?
For use in industry as refrigerator coolants and propellants.
What happens to CFCs when exposed to UV?
They break down to form free radicals.
Why do chlorine bonds in CFCs break rather than fluorine bonds?
C-CL bonds have the lowest bond enthalpy.
What do the radicals produced when CFCs are broken down by UV do?
Attack and break down ozone
What does ozone do?
It prevents harmful UV radiation from entering the earths atmosphere.
What is ozone?
O3
Part of the stratosphere that is formed naturally.
What bond is formed between O2 and O to form ozone and why?
Dative covalent bond
The oxygen atom from the molecule donates a pair of electrons to share with the oxygen atom, as this is the only way to achieve a stable structure for all 3 oxygen atoms, since the original O2 molecule was already stable.
What is the overall equation for ozone depletion?
2O3 -> 3O2
Describe how ozone depletion takes place ?
Chlorine free radical formed from a CFC using UV radiation.
Chlorine radical reacts with ozone to form chlorine monoxide intermediate and oxygen.
Chlorine monoxide reacts with more ozone to form original chlorine free radicals and more oxygen.
What is the role of chlorine free radicals in ozone depletion?
They act as a CATALYST as it is reformed (so not used up).
Why do the risks of CFCs outweigh the benefits?
Despite being stable, unreactive and non-toxic, they damaged the ozone layer, which means UV radiation would not be effectively absorbed.
What safer alternatives to CFCs are used now and why are they safer molecules?
HFCs - hydrofluorocarbons
Hydrocarbons
They are much safer as they do not contain chlorine.
What is ozone formed from?
Oxygen molecule and an oxygen free radical
