Module 4 Section 2: Alcohols, Haloalkanes and Analysis Flashcards
What is IR Spectroscopy
In Infrared Spectroscopy a beam of IR radiation is passed through a sample of a chemical
The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (they vibrate more)
Bonds between different atoms absorb different frequencies of IR radiation.
Bonds in different places in a molecule absorb different frequencies too - the OH bond in an alcohol and the OH bond in a carboxylic acid absorb different frequencies
What does an infrared spectrometer show
It produces a spectrum that shows you what frequencies (wavenumber) of radiation the molecules are absorbing
This can be used to identify the functional groups in a molecule
How to tell if the functional group has changed using an infrared spectrometer
This also means that you can tell if a functional group has changed during a reaction
e.g., if an alcohol is oxidised to form an aldehyde then the OH bond will disappear from the spectrum, and a C=O absorption will appear
If it is oxidise it further to a carboxylic acid an OH peak at a slightly lower frequency than before will appear, alongside the C=O peak
Uses of IR spectroscopy
Can be used in breathalysers to workout if a driver is over the drink-drive limit
The amount of ethanol vapour in the driver’s breath is found by measuring the intensity of the peak corresponding to the C-H bond in the spectrum.
The C-H bond in particular is used because it is not affected by any water vapour in the breath
IR spectroscopy is also used to monitor the concentrations of polluting gases in the atmosphere
These include carbon monoxide (CO) and nitrogen monoxide (NO), which are both present in car emissions
The intensity of the peaks corresponding to the C=O (should be triple bond) or N=O bonds can be studied to monitor their levels
What is the finger print region?
Below 1500cm^-1
The finger print region contain unique peeks used to identify the particular molecule using a computer or comparing with a booklet of published spectra.
It is difficult to identify functional groups in this region.
What is on the X and Y axis of a IR spectrum
X - wavenumber cm^-1
Y - transmittance(%) (can sometimes be absorption)
What is a haloalkane
This is an alkane with at least one halogen atom in place of a hydrogen atom
Examples of haloalkanes
Trichloromethane
2-iodo-propane
2-bromo-2-chloro-1,1,1trifluoroethane
What bond is polar in haloalkanes
Halogens are generally much more electronegative than carbon
So the carbon-halogen bond is polar
How do nucleophiles interact with haloalkanes
The δ+ carbon is electron deficient
This means it can be attacked by a nucleophile
What is a nucleophile
This is an electron pair donor
It could be a negative ion or an atom with a lone pair of electrons
It donates an electron pair to somewhere without enough electrons
Examples of nucleophiles
OH-, CN- and NH3
Water is a nucleophile but reacts slowly
How can haloalkanes be hydrolysed and what do they make
This forms alcohols
This is a nucleophilic substitution reaction
A warm aqueous alkali e.g. sodium hydroxide (NaOH) or potassium hydroxide (KOH) must be used
General equation
R-X + OH- —(reflux)-> R-OH + X-
Reaction mechanism for hydrolysis
OH- is the nucleophile which provides a pair of electrons for the δ+ Carbon
The C-X bond breaks heterolytically - bond electrons from the bond are taken by the X-
X- falls off as OH- bonds to the carbon
How does water act as a nucleophile
Water molecule is a weak nucleophile so reaction will be slower
General equation:
R-X + H2O = R-OH + H+ + X-
What affects how quickly haloalkanes are hydrolysed
Bond enthalpy
Weaker carbon-halogen bonds break more easily so they react faster
What haloalkane hydrolysed the fastest and slowest
Iodoalkanes havw the weakest bonds
Fluoroalkanes have the strongest bonds so they hydrolyse the slowest
Mechanism of the experiment to compare the reactivity of haloalkanes
Mixing haloalkanes with water makes an alcohol
R-X + H2O = R-OH + H+ + X-
Adding silver nitrate makes a silver halide precipitate as the silver ions react with t(e halide ions as soon as they form
Ag+ + X- = AgX
Experiment to compare the reactivity of haloalkanes
Set up three test tubes each containing a different haloalkane, ethanol (as the solvent) and silver nitrate solution (this contains the water)
Haloalkanes must have all the same skeleton structure to make it a fair test
A pale yellow precipitate quickly forms with 2-iodopropane - so iodoalkanes must be the most reactive haloalkanes
Bromoalkanes react slower to from a cream ppt and chloroalkanes form a white ppt slowest
When are practical techniques used in chemistry
Used during synthesis of a product, or to purify it from unwanted by-products or unreacted reagents once it’s been made
Why reflux used
Organic reactions are slow and substances are usually flammable and volatile (low boiling points)
If they are put in a beaker and heated with a Bunsen burner they will evaporate or catch fire before they have time to react
What happens during reflux
The mixture is heated in a flask fitted with a vertical Liebig condenser - this continuous boils, evaporates and condenses the vapours and recycles them back into the flask, giving them time to react
Why are heating elements electrical in reflux
Hot plates, heating mantles or electrically controlled water baths are normally used
This avoids naked flames that might ignite the compounds
When is distillation used
Used to separate substances with different boiling points
How does distillation work
A mixture is gently heated in a distillation apparatus
The substances will evaporate out of the mixture in order of increasing boiling point
The thermometer shows the boiling points of the substances that is evaporating at any given time
What if you know the boiling point of the pure product in distillation
You can use the thermometer to tell you when it’s evaporating and therefore when it’s condensing
What happens if the product of a reaction has a lower boiling point than the starting materials in distillation
The reaction mixture can be heated so that the product evaporates from the reaction mixture as it forms
What happens if the starting material has a higher boiling point than the product
Aa long as the temperature is controlled, it won’t evaporate out from the reaction mixture
What happens when a product is left in the reaction mixture
It can go on to react further
E.g. when a primary alcohol is oxidised to an aldehyde and then oxidised to a Carboxylic acid
If the desired product is the aldehyde, the reaction can be done in the distillation equipment
The aldehyde product has a lower boiling point than the alcohol, so will distil out of the reaction mixture as soon as it forms
It is then collected in a separate container
How can volatile liquids be purified
Redistillation
How does redistillation work
If a product and the impurities have different boiling points, regular distillation equipment is used to heat an impure product, instead of a reaction mixture
When the desired liquid boils (when thermometer is at the boiling point of the liquid), place a flask at the open end of the condenser to collect your product
When the thermometer shows the temperature is changing, put another flask at the end of the condenser because a different liquid will be delivered
When to use separation
If a product is insoluble in water then separation is used to remove any impurities that do dissolve in water such as salts or water soluble organic compounds ( e.g. alcohols )
Process of separation
Once the reaction to form the product is completed, pour the mixture into a separating funnel, and add water
Shake the funnel and allow it to settle
The organic layer and the aqueous layer (which contains any water soluble impurities) are immiscible ( don’t mix ), so separate out into two distinct layers
You can then open the tap and run each layer off into a separate container
What usually happens when separation is used to purify a product
The organic layer will end up container trace amounts of water, so it has to be dried
How to dry out the organic layer after separation
Add anhydrous salt (e.g. magnesium sulfate or calcium chloride )
The salt is used as a drying agent - it binds to any water present to become hydrated
You can filter the mixture to remove the solid drying agent
What may happen when you first add the salt to the organic layer after separation
It may become lumpy
This means more must be added
You know that all the water has been removed when you can swirl the mixture and it looks like a snow globe
What are CFCs
Chlorofluorocarbons
Contain only chlorine fluorine and carbon - all hydrogens have been replaced
Properties of CFCs
Stable, volatile, non flammable and non toxic
Where are CFCs used
Fridges, aerosol cans, dry cleaning and air conditioning
Why were CFCs stopped
Scientists realised they were destroying the ozone layer
What is the chemical formula for ozone
O3
What is the purpose of ozone
Acts as a chemical blocker for UV radiation
Absorbs a lot of UV radiation which can cause sunburn or even skin cancer
How is ozone formed naturally
When an oxygen molecule is broken down into two free radicals by UV radiation
The free radicals attack other oxygen molecules forming ozone
O2 -UV light—> O + O ——> O2 + O —> O3
Why are holes in the ozone layer bad for earth
They allow more harmful UV radiation to reach the earth
This can cause an increase in skin cancer
When was the ozone layer found to be thinning
1970s and 80s was when scientists found the ozone layer above Antarctica and the Arctic was getting thinner
How are the holes in the ozone layer formed
CFCs in the upper atmosphere absorb UV radiation and split homolytically to form chlorine radicals
These free radicals catalyse the destruction of ozone - they destroy ozone molecules and are then regenerated to destroy more ozone
Initiation equations for how chlorine radicals catalyse the destruction of ozone
Cl• radicals are formed when C-Cl bonds in CFCs are broken down by UV radiation
CF2Cl2 -UV-> •CF2Cl + Cl•
Intermediate/ propagation equations for how chlorine radicals catalyse the destruction of ozone
Cl• + O3 -> O2 + ClO•
ClO• + O -> O2 + Cl•