Chapter 15 Haloalkanes Flashcards
How to name of haloalkanes
a prefix is added to the name of the longest chain.
Reactivity of haloalkanes
halogen atoms are more electronegative than carbon atoms. The electron pair in the carbon halogen bond is closer to the halogen atom than the carbon atom.
the carbon atom has a slightly positive charge and can attract species containing a lone pair of electrons. Species that donate a lone pair of electrons are know as neutrophiles.
Neutrophiles
Species that donate a lone pair of electrons are know as nucleophile.
Nucleophilic substitution
When a haloalkanes react with a nucleophile, the nucleophile replaces the halogen in a substitution reaction. A new compound is produced containing a different functional group. The reaction mechanism.
Hydrolysis in nucleophilic substitution in the alkanes
In the hydrolysis of a haloalkane, the halogen atom is replaced by an -OH group.
- the nucleophile, OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
- This direction of attach by the OH- ion minimises repulsion between the nucleophile and the slightly negative halogen atoms.
- A lone pair of electrons on the hydroxide ion is attracted and donated to the slightly positive carbon atom.
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
- The carbon-halogen bond breaks by heterolytic fission.
- The new organic product is an alcohol. A halide ion is also formed.
How can haloalkanes be converted to alcohols
Using sodium hydroxide. The reaction is very slow at room temperature so teh mixture is heated under reflux to obtain a good yield of product.
Hydrolysis and carbon-halogen bond strength
In hydrolysis, the carbon- halogen bodn is broken and the -OH group replaces the halogen in the haloalkane. The rate of hydrolysis depends upon the strength of the carbon-halogen bonds. The strength decreases as you go from C-F to C-I.
This means iodoalkanes react faster tha bromoalkanes etc.
Measuring the rate of hydrolysis of primary haloalkanes
You will carry out the reaction in the presence of silver nitrate. As the reaction takes place the halide ions are produced which will react with Ag+ ion to form a precipitate of the silver halide.
The haloalkanes are insoluble in water, and the reaction is carried out in the presence of an ethanol solvent. Ethanol allows water and haloalkane to mix and produce a single solution rather than two layers.
How does the rate of hydrolysis change with carbon-halogen bond strength
The rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
Uses of organhalogen compounds
they are molecules that contain at least one halogen atom joined to a carbon chain. They are used in pesticides, solvent, dry cleaning solvents, making polyments, flame retardants, refrigerents.
Ozone layer
found on the outer edge of the stratosphere, at about 10km - 40km.
only a time portion of the gas is ozone but is enough to absorb most ultraviolet radiation.
Types of UV radiation and how ozone is formed from oxygen
UV-B is most commonly linked to sunburn
Initially UV-B breaks down the oxygen molecule into two oxygen radicals. Other oxygen molecules then react with radicals which will create O3.
CFCs and the ozone layer
CFCs are used in aerosol propellants, air-conditioning units, and refrigerants. CFCs are very stable becuase of the strength of the carbon-halogen bonds within their molecules. They remain stable until they reach the atmosphere where they begin to break down. These are thought to catalyse the breakdown of the ozone layer.
How to CFCs deplete the ozone layer
CFCs have long residence time in the troposphere. It may take them many years to reach the stratosphere. Once in the startosphere UV radiation provides sufficient energy to break a carbon-halogen bond in CFCs by homolytic fission to form radicals. The C-Cl bond has the lowest bond enthalpy and so is the bond that breaks.
CF2Cl2 -> CF2CL. + Cl.
The chlorine radical formed is very reactive so it can react with the ozone molecule breaking down the ozone into oxygen.
CHECK PAGE 236 for mechanism steps
Are CFCs responsible for ALL ozone depleting reactions
No. other radicals also catalyse the breakdown of ozone. nitrogen oxide radicals are formed naturally during lightening strikes, and also as a result fo aircraft travell in the stratosphere.
This has the same overall equation.
