Esters also contain a C=O bond but here the carbon is attached to another oxygen, which is bonded with another carbon. There is no simple test to carry out for identifying an ester, however esters have a distinct fruity smell. Hydrolysis and then testing for a carboxylic acid can test for the presence of an ester because esters hydrolyse to form carboxylic acids.
Phenols are compounds containing a hydroxyl group attached to an aromatic ring, for example 2-methylphenol. Phenol produces a purple complex when mixed with iron (III) chloride in which the phenol is acting as a ligand. This is often used as a test for a phenol.
On the following page I have constructed a flow diagram to show how I will test for the unknown organic compound.
1.Add a solution of 2,4 dinitrophenylhydrazine
+ve
No precipitate Orange precipitate
Formed
2.Add Fehlings reagent (blue)
3.Add Sodium Hydrogen Carbonate
CO2 -ve +ve
No CO2 Liberated No Change Turns to
Red/brown
4. Add FeCl3
-ve +ve
Turns black/purple
No colour
Change
5. Add bromine water
-ve +ve
No turns colourless
change
6. Add potassium dichromate
-ve +ve
no change Turns green
7.Add sodium
Effervescence 8. To confirm presence of ester, it should be
Hydrolysed and tested for carboxylic acid.
If positive then it is an ester.
Hypotheses
General safety procedures:
Always wear gloves, goggles and lab coat when handling any unknown because some materials could be corrosive or an irritant. Handle glass equipment with care for example put the test tube in the test tube rack. Be careful when disposing organic materials ensure there is an organic waste beaker or bucket available. If any chemicals are ingested, contact the teacher without delay.
Note down all observations occurring for each test.
- Test for carbonyl group
Apparatus:
- Pipette
- Measuring cylinder
- Test tube
- 2,4 Dinitrophenylhydrazine solution
- Water bath
Procedure: To 1cm3 of 2,4 dinitrophenylhydrazine add several drops of the unknown substance using a pipette. Heat with a water bath if necessary.
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 suggests that it is an alkene, ester, phenol, alcohol or carboxylic acid. I will also add 4 drops of 2,4 DNP to a test tube of water, this will be used as a control and no reaction will occur, this is to show that it only reacts with carbonyl compounds.
Safety: follow general safety procedures. Careful when handling 2,4- DNP as it is toxic.
- Test for aldehyde or ketone
Apparatus:
- Test tube
- Pipette
- Measuring cylinder
- Fehlings solution
- Water bath
Procedure: add several drops of the unknown organic to a 1cm3 of Fehlings solution in a test tube; use a water bath if necessary. I will also add water to the Fehlings solution in a separate test tube and use it as a control to show it only reacts with aldehydes.
Outcomes: a colour change to red/ brown precipitate indicates presence of aldehyde, no colour change suggests it is a ketone. Refer to hypotheses table.
Justification: only aldehydes are easily oxidised to acids, ketones are not. Fehlings solution is an alkaline copper (Cu2+) solution, which becomes reduced to (Cu+), a red/ brown precipitate of Cu2O, and the aldehyde becomes oxidised to RCOO-
Safety: follow general safety procedures.
- Test for carboxylic acid
Apparatus:
- Sodium hydrogen carbonate
- Pipette
- 2 test tubes
- Limewater
- Clamp
- Rubber tube
Procedure: add sodium hydrogen carbonate solid to 2cm3 of the unknown in a test tube. If effervescence occurs then add bubbles to limewater in test tube attached to a clamp, should be bubbled by using a rubber tube.
Outcomes: if effervescence occurs then the bubbles should be added to limewater. If the limewater turns milky/ cloudy this shows that carbon dioxide gas is being released. If there is no effervescence and limewater does not turn cloudy then therefore the unknown must either be ester, alkene, phenol or alcohol.
Justification: This shows that it is a carboxylic acid because it is the only compound in the unknown sample, which liberates carbon dioxide. The hydrogen in the carboxylic acid is replaced by sodium and water and carbon dioxide are also formed.
Safety: follow general safety procedures. Be careful with apparatus to avoid breakages.
- Test for phenol
Apparatus:
-
FeCl3 crystals
- Water
- Test tube
Procedures: dissolve crystals of FeCl3 in 2cm3 of water and add to 2cm3 of the unknown organic. Also add crystals to a test tube of water to use as a control to show it doesn’t react with other groups and compare results.
Outcome: a colour change to purple/ violet suggests that it is a phenol however if there is no change then it is an ester, alkene or alcohol.
Justification:
The colour change should be from yellow to a black/ purple if phenol is present.
Safety: follow general safety procedures.
Negative Positive
- Test for alkene
Apparatus:
- Bromine water
- Test tube
- Pipette
Procedure: add several drops of the unknown organic to 1cm3 of bromine water.
Outcome: if the bromine water decolourises then this suggests that it is an alkene but if there is no colour change then it is either an alcohol or ester. I will also add bromine water to a test tube of water and use it as a control to show colour difference. I will carry out the test for alkene using bromine water after testing for a phenol because phenol also reacts with bromine water and forms white precipitate however alkene doesn’t react with Iron (III) Chloride.
Justification: the orange bromine colour decolourises if the C=C is present.
Safety: Follow general safety procedures. Careful when handling Bromine water as acts as irritant and can stain clothing.
- Test for primary alcohol
Apparatus:
- Potassium dichromate (VI) solution
- Dilute sulphuric acid
- Test tube
- Water bath
Procedure: to 2cm3 of potassium dichromate solution add 3 drops of dilute sulphuric acid then add several drops of the unknown organic in a test tube, use a water bath if necessary.
Outcomes: if the colour change goes from orange to green then this suggests it is a primary alcohol. If there is no colour change then it must be either an ester or a tertiary alcohol.
Justification: Primary alcohols undergo oxidation when reacted with a mixture of potassium dichromate (VI). In the reaction the orange dichromate (VI) ions are reduced to green chromium (III) ions. Tertiary alcohols resist oxidation. I can test for a tertiary alcohol by adding sodium however I tested for a primary alcohols first because only primary and secondary alcohols react with potassium dichromate and tertiary alcohols do not whereas all alcohols react with sodium.
2Cr2O72 –(aq) + 3C2H5OH(l) + 16H+(aq) 4Cr3+(aq) + 3CH3CO2H(aq) + 11H2O(l)
Safety: follow general safety procedures
- Test for tertiary alcohol
Apparatus:
- One piece of sodium
- Test tube
- Splint
- Matches
Procedure: to 2cm3 of the unknown organic add 1 piece of sodium. If gas is released, test for hydrogen by using a lighted splint.
Outcomes: if there is effervescence and when the lighted splint is put in the test tube and a ‘pop’ sound is heard then this shows that it is a tertiary alcohol. If there is no effervescence and no hydrogen gas I given off then it is therefore an ester.
Justification: all alcohols react with sodium but we have already tested for a primary alcohols therefore must be a tertiary alcohol.
Safety: follow general safety procedures and take car when using matches and when lighting the splint.
- Testing for an ester
Apparatus:
- Dilute sulphuric acid
- Test tube
- Bunsen burner
- Heat mat and tripod/gauze
Procedure: hydrolyse the ester by refluxing it with the dilute acid. The dilute acid provides both the acid catalyst and water. Repeat test three to check for presence of carboxylic acid.
OR
Esters have a distinct fruity smell. So you have to heat the organic unknown by using a Bunsen burner and a fruity smell will be present. Use tripod and gauze for safety.
Justification: If refluxing with an acid hydrolyses the ester then a carboxylic acid and alcohol is formed. Test for a carboxylic acid, if this is positive this confirms the presence of the ester. OR by heating the organic unknown we can distinguish whether it is an ester by its distinct fruity smell. OR I can determine it is an ester by negative results from previous tests.
To make my experiments fair, I will try to use same amounts of the unknown in each test and will carry out all tests in the same lab under the same confounding variables, e.g. light, temperature. I will also leave each experiment to stand for 2 minutes each to show whether a colour change has occurred. I will use a control for each test and add reagent to an unreactive compound, for example water or any other compound as some reagents may react with the water. However in this investigation there are many variables, which are impossible to measure because all tests are different.
The precision of the results is not a huge important factor because we are only trying to find the functional group and does not have any numerical value and no graphs have to be created. However precision reliability is very important because it will determine whether the results are trustworthy and correct as small mistakes could lead to wrong results. It is more reliable to use a measuring cylinder than a syringe because syringe could have bubbles and is more likely to have a higher margin of error and is less precise than a measuring cylinder. The reliability of the results is entirely dependant on observations therefore must take into account differences between different people. Also it would be more reliable to use a colorimeter to test the colour of certain results however it was not available therefore it could increase the chance of human error. The equipment that will be used is basic because the wet tests show definite change/ observations and time is a limitation therefore no need for detailed and complicated equipment. I feel the reliability and accuracy of my equipment and method is enough to prove my results valid, as I will be using a good technique for each test and sufficient equipment. Also small errors should not have a great affect on results as it is not quantitative and the only error, which could lead to wrong results, is adding of wrong reagent and contamination therefore I will use a separate clean test tube for each test. The reason I chose to use Fehling reagent instead of Tollen’s was because Fehling solution would show a more specific colour change whereas with Tollen’s it would be difficult as in previous experiments it was very hard to tell whether silver ions were formed. I reason I chose that I would use Iron (III) Chloride test before using bromine water is because phenol and alkene both react with bromine water whereas only phenol reacts with the Iron (III) Chloride and not alkene. I will also use sodium after testing for Primary alcohol because all alcohols react with sodium however tertiary alcohols do not react with oxidising agents and if previous test is negative then it can only test for tertiary alcohols as you can see in the flow chart. My flow chart I feel is very reliable and precise because each test is dependant on previous result and the reagents will not react with other functional groups as set out in my flow chart. I can confidently say my methods and equipment are reliable and accurate to prove my results valid even though certain variables cannot be controlled.
After identifying the functional group present, infra red spectra and mass spectrum can be used to confirm results and identify structure.
Precautions must be taken when handling:
- 2, 4-dinitrophenylhydrazine as it toxic by skin contact
- Eye protection must be worn at all times in the lab.
- Be certain that there are no flammables in the area when lighting a flame.
- Be alert for hazards in the lab. Do not proceed with any experiment until you understand the hazards involved.
- Report all injuries to your instructor, even if they appear to be minor.
- Do not come into contact with any of the solvents or reagents used in this lab other than soap and water. Remove gloves and wash hands before leaving lab.
- Clean up all chemical spills immediately, this includes drips on bottles.
- Do not leave any bottle uncapped.
- Dispose of all wastes as directed. (Ask before you dump.)
Cleaning Up:
1. Discard wastes for each test after consulting the teacher
2. Return all standard samples and test reagents to their original sources.
- Wipe down your work area of the lab bench with a damp sponge/ tissue.
- Place all equipment and apparatus in their correct location.
Bibliography:
quicklearn on alcohols
information on phenols and chemical procedures.
OCR book- Chemistry 2- 2001- Brian Ratcliff and Helen Eccles. Pages 31 and 32- information on esters.
Collins Instant Revision- AS Chemistry- 2002- Anthony Ellison of Queens’s gate School. Pages 84, 85, 90 and 91- information on alkenes and alcohols.
