14. Haloalkanes Flashcards
Give some examples of synthetic compounds which haloalkanes are the basis of.
Teflon - non-stick coating on pans
Anaesthetics
Solvents
Do haloalkanes occur naturally?
Not many haloalkanes occur naturally, they are the basis of many synthetic compounds.
What is the general formula of a haloalkane?
CnH2n+1X
Often shortened to R-X
Haloalkanes have what kind of bond?
Haloalkanes have a polar bond because halogens are more electronegative than carbon.
(C(^d+) - X(^d-))
Are haloalkanes soluble in water?
The C-X bond is not polar enough to make the haloalkanes soluble in water.
Which are the main intermolecular forces of attraction?
Dipole-dipole attractions and van der Waals forces.
Can haloalkanes mix with hydrocarbons? What application does this have?
Haloalkanes mix with hydrocarbons so they can be used to remove oily stains.
What are the two factors which affect boiling point in haloalkanes?
Boiling point increases with chain length
Boiling point increases as we go down the halogen group (there are caused by increased van der Waals forces because the larger the molecules, the greater the number of electrons)
Boiling point decreases with increased branching (less surface area to attach)
Do haloalkanes have a higher boiling point than alkanes of similar chain lengths?
Yes because
- They have higher relative molecular masses
- They are more polar
When haloalkanes which bond usually breaks?
It is almost always the C-X bond that breaks.
Which two factors determine how readily the C-X bond breaks?
The C-X polarity
The C-X bond enthalpy
Describe how bond polarity effects the reactivity of the alkanes.
As the C-X bond is polar the C will be electron deficient. It can be attacked by reagents that are electron rich or have electron rich areas. These are called nucleophiles.
This would dictate that the C-F would be most reactive and the C-I bond least reactive as it is least polar.
Describe how bond enthalpy effects the reactivity of the alkanes.
The bonds get weaker as you go down the group.
As we go down the group the shared electrons in the C-X bond get further away from the halogen nucleus and so the bonds get weaker.
C-I are therefore the weakest and C-F the strongest.
Describe the reactivity of the alkanes.
The reactivity increases as we go down the group so bond enthalpy is a more important factor than bond polarity.
Name the two different types of mechanisms you could be asked to show in the haloalkanes topic.
Nucleophilic substitution
Elimination
Describe nucleophilic addition.
A nucleophiles (-:OH, :NH3 or -:CN) replaces the halogen in the haloalkane. They attack and form bonds with positive carbons.
(Two curly arrows:
- From lone pair on nucleophile to positive carbon by the halogen group
- From C-X bond to X )
Special case for ammonia
What are the special conditions for nucleophilic substitution with aqueous sodium/potassium hydroxide?
Room temperature
(Ethanol is used as solvent so haloalkanes mix with WATER)
(Hydrolysis reaction)
What is the product of a nucleophilic substitution involving cyanide ions?
You produce a nitrile (functional group -C(triple bond)N)
The new extra carbon is counted as part of the root when naming
How is ammonia a nucleophile when it is not an ion?
It has a lone pair of electrons that is can donate and the N had a d- charge.
What are the necessary conditions for a nucleophilic substitution with ammonia?
Excess ammonia
Ethanol solvent
Under pressure.
Why are nucleophilic substitutions useful?
They are a way of introducing new functional groups into organic compounds.
What are produced in nucleophilic substitutions involving ammonia?
Amines
Methyl/ethyl/propyl-amine for example.
Describe an elimination reaction.
An OH- ion acts as a base, removing an H+ ion from the haloalkane. In this process a hydrogen halide is removed leaving an alkene (double bond).
Three arrows:
- From the lone pair on the nucleophile to the H next to X
- From H-C to C-C
- From C-X to X
In an elimination, what are your products?
H2O, an alkene and an X ion leaving group.
For eliminations, the sodium/potassium hydroxide is dissolved in ethanol and mixed with the haloalkane. What is DEFINITELY NOT present?
Water!
The mixture is heated
Describe conditions where OH- favours substitution.
Room temperature
Dissolved in water (aqueous)
Primary haloalkanes
Describe conditions where OH- favours elimination.
High temperature
Dissolved in ethanol
Tertiary haloalkanes
What is the reaction called between alkanes and halogens?
Free radical substitution.
What does free-radical substitution start off?
A chain reaction
What are the three major steps of a chain reaction?
Initiation, propagation and termination
Give an example of a reaction between a halogen and an alkane (free radical substitution).
CH4 (g) + Cl2 (g) —> CH3Cl (g) + HCl (g)
Takes a Cl, takes an H
What happens in initiation?
The chlorine molecule absorbs a single quantum of ultraviolet light and the bond breaks homolytically (one electron going to each chlorine atom) results in two free radicals which are highly reactive.
(The C-H bond needs more energy to break than is available in the quantum of light so it doesn’t break).
Describe what happens in propagation.
- Free radical + alkane —> alkane (minus one H) free radical + original free radical atom H compound
- New alkane (minus one H) free radical + halogen molecule —> halogen free radical + new alkane (minus H plus halide)
Describe termination.
Two free radicals react to form a stable compound with no unpaired electrons.
Why are chain reactions not very useful?
They produce such a mixture of products.
They also occur without light at high temperatures
What does too much ozone at ground level cause?
Lung irritation and the degradation of paints and plastics.
What does ozone high in the atmosphere protect us from?
UV rays
Causes skin cancer in people by damaging DNA
Protects plankton
Chlorine free radicals are formed from CFCs because the C-Cl breaks homolytically. Give the equations of the reactions between chlorine and oxygen where chlorine is a catalyst in breaking down ozone.
- Cl* + O3 —> ClO* + O2
- ClO* + O3 —> 2O2 +Cl*
Overall: 2O3 —> 3O2
What are CFCs?
Haloalkanes containing both chlorine and fluorine atoms but no hydrogen
Short chain CFCs are usually… And were used for…
Gases
Aerosols and refridgerants
(Long chain one are used as dry-cleaning solvents)
