halogenoalkanes Flashcards
(15 cards)
General information - nucleophilic substitution reactions.
In a substitution reaction, the halogen atom is replaced by another atom/group.
Halogenoalkanes are susceptible to attack by nucleophiles (lone pair donors) such as OH-, CN- AND NH3.
This is because the C-halogen bond is polar with the C atom being delta positive.
The C-halogen bond is polar because the halogen atom is more electronegative than the carbon atom.
The rate of reaction is affected by the strength of the C-halogen bond. The weaker the bond, the faster the rate of reaction.
What is the rate of reaction order of halogenoalkanes?
fastest
iodoalkanes
bromoalkanes
chloroalkanes
fluoroalkanes
slowest
What is the C-halogen bond strength order of halogenoalkanes?
weakest bond
C-I
C-Br
C-Cl
C-F
strongest bond
nucleophilic substitution 1 outline.
reagent - NaOH
conditions - aqueous, warm
what happens - halogen atom is replaced by OH group
makes - alcohol
nucleophilic substitution 2 outline.
reagent - KCN
conditions - aqueous ethanol, warm
what happens - halogen atom is replaced by CN group
makes - nitriles
nucleophilic substitution 3 outline.
reagent - NH3
conditions - excess concentrated ammonia dissolved in ethanol at pressure in a sealed container
what happens - first molecule of NH3: the halogen atom is replaced by NH3 group. second molecule of NH3: removes H+ from added NH3.
general information - elimination reactions.
When halogenoalkanes react with OH- ions, an elimination reaction can compete with the nucleophilic substitution reaction.
Elimination is favoured if hot, ethanol KOH is used instead of warm, aqueous NaOH.
The change from NaOH to KOH is simply about solubility, with KOH being more soluble than NaOH in ethanol. The key changes are the solvent (from water to ethanol) and the higher temperature.
In elimination, an H and X are removed from adjacent C atoms giving an alkene.
This reaction can give a mixture of alkenes, either because the H can come from different adjacent C atoms, or because the alkenes formed have EZ stereoisomerism.
If there is no H on a C atom adjacent to the C-X then no elimination is possible.
In elimination, the OH- ion acts as a base. In substitution, the OH- ion acts as a nucleophile.
elimination 1 outline.
reagent - KOH
conditions - ethanol, hot
what happens - the halogen atom and one H atom from an adjacent C atom is removed giving an alkene (elimination cannot happen if there is no H on an adjacent C atom). A mixture of alkenes could be formed depending on which of the adjacent C atoms the H is lost from.
makes - alkenes
What are CFCs?
CFCs are chlorofluorocarbons.
CFCs were used as
- coolant in refrigerators
- propellant in aerosols
- degreaser for circuit boards
What is ozone?
Ozone is a form of oxygen, O3.
There is a “layer” of ozone in the stratosphere (a region where there is a higher concentration of ozone).
The ozone layer absorbs harmful UV radiation. This radiation can damage DNA and cause, for example, skin cancer.
What CFCs do to the ozone layer?
CFCs escape and rise into the stratosphere.
CFCs break down in the stratosphere to form Cl∙free radicals by the breaking of a C-Cl bond. The energy comes from UV light.
These Cl∙free radicals catalyse the destruction of ozone in a free radical chain reaction.
Cl∙+ O3 —> ClO∙+ O2
ClO∙+ O3 —> Cl∙+ 2O2
overall - 2O3 —> 3O2
One Cl∙free radical can catalyse the destruction of a huge number of ozone molecules.
Ozone depletion.
The concentration of ozone in the ozone layer fell significantly as CFCs went into large scale use.
The fall in concentration was particularly significant over the Earths poles. These areas are often referred to as “holes” in the ozone layer.
Chemists suspected that the large-scale use of CFCs would lead to depletion of the ozone layer. In 1974, Molina and Rowland published research to show how CFCs would damage the ozone layer.
In the early 1980s, evidence started to appear to show that the ozone layer was being depleted, in particular near the earths poles. Since then, skin cancer has become a much greater problem, especially in countries nearer the poles such as Australia.
In 1995 Molina and Rowland won the Nobel Prize for Chemistry for their work on ozone and CFCs.
Montreal Protocol.
In 1987, several countries (including the UK) signed the ,Montreal Protocol to ban the use of CFCs.
More countries have since banned their use and so the use of CFCs has fallen dramatically.
The banning of the use of CFCs is one of the most effective environmental actions ever.
The ozone layer since the banning of CFCs.
The concentration of ozone in the atmosphere has stopped falling in recent years.
There are still some CFCs in use, for example in old refrigerators. While the CFCs are properly removed and disposed of from many old refrigerators, this is not always the case and so some are still being emitted into the atmosphere.
It is predicted that the ozone concentration will start to rise again over time, but we have stopped its depletion.
Alternatives to CFCs.
CFCs have been replaced by chemicals that do not contain chlorine e.g.
- 1, 1, 1, 2 - tetrafluroethane as the coolants in refrigerators, CF3CH2F
- butane as the propellant in aerosols, CH3CH2CH2CH3
They cannot break down to produce chlorine free radicals as they do not contain any chlorine atoms.
All the CFC replacements are greenhouse gases (as are CFCs) and so they are not without some problems still.
