In this investigation I will be supplied with a compound which will contain one of the following functional groups:
* Carboxylic acid
The following is a flow diagram of the chemical tests I will use to identify the functional groups outlined above:
(1.TEST WITH UNIVERSAL INDICATOR)
TURNS RED TURNS ANY OTHER COLOUR
(2.TEST WITH BROMINE WATER) (3.TEST WITH 2, 4 DNP)
General Safety Procedures
For all experiments involving any unknown compound the wearing of goggles, lab coat and gloves is necessary because one or more of the compounds could be an irritant/corrosive/etc.
1. Test with Universal Indicator
EQUIPMENT: Universal indicator
PROCEDURE: Add several drops of the unknown substance to universal indicator inside a test tube using a pipette.
OUTCOME: A colour change to red indicates either a carboxylic acid or phenol is present. Any other colour change shows alcohol, aldehyde, ketone or ester.
EXPLANATION: Single indicators, such as litmus, are very weak acids. If the concentration of hydrogen ions is changed at a certain pH the indicator will change to a different structure.
Universal indicator is a mixed indicator i.e. it is several indicators with different pH values mixed together. When added to solutions of different pH, different indicators change colour as above. The colour you see is a result of colour mixing. The colour has been linked to a specific pH and a colour chart provided.
SAFETY: Follow general safety procedures and be aware that universal indicator can be dangerous so always make reference to the hazcards.
2. Test with Bromine Water
EQUIPMENT: Test tube
PROCEDURE: Add several drops of bromine water into a test tube which if filled with 1cmï¿½ of the unknown substance.
OUTCOME: A white precipitate formed indicates a phenol present; no change shows a carboxylic acid.
EXPLANATION: Aqueous phenol decolourises bromine water to form a white precipitate of 2, 4, 6-tribromophenol.
The presence in phenol of the OH group increases the susceptibility of the benzene ring to electrophilic attack. The oxygen in the OH group has two lone pairs of electrons, these can overlap with the delocalised ? electrons. Overall, the ? electron charge density is increased (especially at the 2, 4 and 6 positions). This is why this reaction happens only with phenol and not with the carboxylic acid.
SAFETY: Follow general safety procedures and bromine water can be toxic and corrosive so always make reference to the hazcards.
3. Test with 2, 4 DNP (2, 4-dinitrophenylhrazine)
EQUIPMENT: Test tube
2, 4 DNPH solution
1cmï¿½ of Ethanol
PROCEDURES: To 1cmï¿½ of 2, 4-dinitrophenylhydrazine solution and 1cmï¿½ of ethanol, add several drops of the test substance using a pipette and heat with a water bath.
OUTCOMES: A colour change to orange/yellow indicates the presence of a carbonyl group, which will be an aldehyde or a ketone. No colour change indicates that it is an alcohol, carboxylic acid, ester or phenol.
The equation for the reaction of ethanal with 2, 4-dinitrophenylhydrazine is:
SAFETY: Follow general safety procedures as
2,4-dinitrophenylhydrazine is toxic, also be aware that ethanol is highly flammable so make reference to the relevant hazcards.
4. Test with Tollens Reagent
EQUIPMENT: Test tube
Silver nitrate solution
Sodium hydroxide solution
PROCEDURES: Put about 1cmï¿½ of 0.05M silver nitrate solution into a very clean test tube and add 3 or 4 drops of sodium hydroxide solution. Drop by drop; add ammonia solution until the precipitate of silver oxide nearly dissolves. Add a few drops of unknown to the tube and shake gently. Place tube in a beaker of warm water, note observations and immediately rinse out test tube.
OUTCOMES: A change from clear to a silver mirror precipitate on the side of the test tube indicates the presence of an aldehyde. No change indicates a ketone.
EXPLANATION: Aldehydes are easily oxidised to make them acids. Ketones cannot be oxidised, as there is no place for the oxygen from the oxidising agent.
When the silver nitrate is mixed with ammonia to form Tollens reagent, the complex ion [Ag (NH3) 2] + is formed.
This is reduced to silver during the process; Tollens reagent is the oxidising agent.
SAFETY: Follow general safety procedures and silver nitrate solution, sodium hydroxide solution and ammonia solution are all corrosive so always make reference to the relevant hazcards.
5. Test with Acidified Potassium Dichromate
EQUIPMENT: Test tube
Potassium dichromate solution
PROCEDURES: Add 1cmï¿½ of potassium dichromate into a test tube also add 1cmï¿½ of sulphuric acid to the test tube and add a few drops of the unknown and warm using a water bath.
OUTCOMES: A colour change to green shows an alcohol is present. No colour change shows an ester.
Primary alcohols are oxidised by acidified dichromate to aldehydes and then to acids.
C2H5OH + [O] –> CH3CHO + H2O [O] is the oxygen from the oxidising agent then CH3CHO + [O] –> CH3COOH
If this is done with a secondary alcohol, only a ketone forms.
In each oxidation, the dichromate is reduced to Cr3+ (green).
No reaction occurs with a tertiary alcohol.
SAFETY: Follow general safety procedures. Potassium dichromate is toxic also sulphuric acid is extremely corrosive so make reference to the relevant hazcards.
Organic Unknown Compound A
Not Strongly Acidic
Positive: Orange Crystalline Precipitate Formed
Negative: No Silver Mirror Formed
Analysis of Results
Through my experiments I have identified the functional group of the organic unknown as that of a Ketone. I can back this conclusion up using the set of spectra provided for this substance. The Mass Spectrum shows that the compound has a mass of 72 – 73. The Infra Red spectrum shows that the compound has an aromatic C-H bond and a C=O bond (both of which are found in Ketones). The NMR spectrum shows, when used with the other spectra’s data, the structural formula for the unknown. Using the data from all three spectra I can deduce that the compound is Butanone (C4H8O). Following is the spectra provided with my notes, and the structural formula of the organic unknown on it.
My results for this investigation, I feel, are satisfactory as the spectra vouch for them.
The chemical techniques I used only gave the functional group of the organic unknown and I cannot find out exactly what the organic unknown is solely through using these techniques. To get a more specific answer I had to use the physical techniques as well.
With all chemical techniques there is always a possibility of an error occurring through the contamination of equipment, chemicals or the unknown compound itself. This would result in an incorrect result so therefore I had to pay particular care when conducting all parts of the chemical tests.
None of the spectra are particularly useful independently and only really come in to good use when one or more of them are used together.
The mass spectrum gives the relative atomic mass of the unknown this can be used to give a general idea of the molecular formula. However the atomic mass could be appropriate to more than one functional group so I would not be able to use this technique on its own to find the identity of any organic unknown substance.
The infrared spectrum doesn’t simply make a distinction between an aldehyde and ketone so other tests would be required to make this distinction if this situation occurred.
The NMR spectrum was very useful as it gave most information needed to identify an unknown.
Another technique I could use to identify an unknown which I didn’t use would be finding the melting and boiling points. There would however be difficulties if the compound contained any impurities so I would have to make sure the organic unknown was pure.