Identification of an organic unknown.
IDENTIFICATION OF AN ORGANIC UNKNOWN.
PLANNING:
Aim:
To outline a sequence of simple chemical tests that I could use to identify each of the following groups: •an alcohol, •an aldehyde, •a ketone, •a carboxylic acid, •an ester and •a phenol.
Introduction:
I will be given one of the above organic unknowns and it is my job to investigate which one of them I have via methods of qualitative analysis. Firstly I will need to familiarise myself with each of the groups. Below is a diagram detailing how one group can be transformed into another. Such processes will need to be duly noted when planning the method for the identification of the unknown as we may unintentionally change the chemical structure of the unknown.
I must now begin to investigate tests that identify particular groups. These I will have to string together and make into a flow diagram whereby each test is dependant on the result of the previous test.
Name (and example)
Functional Group
Test for the presence of this specific group
Alcohol
(methanol)
Heat and reflux with acidified potassium dichromate (K2Cr2O7 - orange). Unless alcohol is a tertiary alcohol, solution will form Cr3+ ions - blue - and there will be a colour change.
Aldehyde
(methanal - formerly 'formaldehyde')
Aldehydes and ketones (carbonyls) will turn 2,4-dinitrophenylhydrazine (2,4-DNPH) orange in sulphuric acid and methanol.
Adding an aldehyde to a clear solution which contains ions of [Ag(NH3)2]+ in a test tube in warm water will make a shiny silver mirror form. An alternative is to use copper (II) ions in an alkaline solution instead of the silver (I). The solution should turn from blue to red-brown.
Ketone
(propanone - formerly 'acetone')
2,4-DNPH undergoes a condensation reaction with aldehydes and ketones. Adding a ketone to the above mentioned chemicals ([Ag(NH3)2]+ and complexed Cu2+) would produce no results such as a shiny mirror surface or a colour change from blue to red-brown.
Carboxylic acid
(methanoic acid - formerly 'formic acid')
The evolution of CO2 when reacted with NaHCO3 (sodium hydrogencarbonate) can distinguish between carboxylic acids and weaker acids such as phenol. Adding carboxylic acid to solid or aqueous sodium carbonate will also liberate carbon dioxide, which can be tested for later.
Ester
(ethyl methanoate)
As well as a sweet aromatic smell, esters, after being treated with H2NOH will form a hydroxamic acid. These will form blue-red complexes on addition with a few drops of iron (III) chloride.
Phenol
- or hydroxybenzene
Iron (III) chloride when reacted with phenol will produce a violet/purple colour (in fact, any colour other than yellow indicates a type of phenol is present.)
Method - logically explained:
Firstly I will need to gather groups that have the same reacting atoms. In other words, bring together the functional groups that have similar structures. The six functional groups have either a '-OH' or an '=O' element in them (or in the case of the carboxylic acid, both elements are present.) A simple test can be done to determine which group is present. By adding the alkaline metal sodium to the organic unknown, it will tell us whether or not a '-OH' group is present as sodium will react with this, liberating hydrogen gas. No hydrogen gas will be released when sodium is reacted with the '=O' group. Instead sodium will react with the water, producing sodium hydroxide and this will react with the '=O' but without the liberation of hydrogen gas.
I now have, in essence, a hydroxyl group and a carbonyl group. Each one will have to face further tests to whittle them down. I will begin with the carbonyl group.
In the carbonyl group, I have three possible functional groups that the organic unknown can be: an aldehyde, a ketone or an ester. The odd one out is obviously ester and so there is probably a test that can identify an aldehyde and a ketone from an ester. This is achieved by adding 2,4-dinitrophenylhydrazine (2,4-DNPH) to the unknown organic and recording any colour change. If the products of the reaction have turned orange, than an aldehyde or a ketone is present. Ester will not produce any colour change with addition to 2,4-DNPH. Below is the reaction between 2,4-DNPH and an aldehyde or ketone.
To further distinguish between the aldehyde and the ketone, we must use the fact that for one of the groups, the carbonyl group is at the end of the alkane chain (aldehyde), and for the other group the carbonyl group is in the middle of the alkane chain (ketone). The former is more reactive than the latter chiefly due to the "presence of two alkyl groups in ketones hinders the approach of attacking reagents to the carbonyl group. Another factor is that alkyl groups are electron-donating and reduce the partial positive charge on the carbonyl atom." (Ramsden, E. ...
This is a preview of the whole essay
To further distinguish between the aldehyde and the ketone, we must use the fact that for one of the groups, the carbonyl group is at the end of the alkane chain (aldehyde), and for the other group the carbonyl group is in the middle of the alkane chain (ketone). The former is more reactive than the latter chiefly due to the "presence of two alkyl groups in ketones hinders the approach of attacking reagents to the carbonyl group. Another factor is that alkyl groups are electron-donating and reduce the partial positive charge on the carbonyl atom." (Ramsden, E. N; A-Level Chemistry, (Stanley Thornes Ltd. page 647)
There are therefore a number of reagents that will react with and also oxidise aldehydes with ease but not ketones. One such reagent is Tollens' reagent. Another is warm Fehlings' solution that works on the same bases as Tollens' reagent but with different consequences. The change from reactants to products by using Tollen's reagent is more noticeable than the change from using warm Fehlings' solution. Furthermore I have used Tollens' reagent before in preliminary tests and therefore have seen the change myself by using this reagent. Tollens' reagent will form the bases of my next test.
Tollens' reagent works on the bases that its silver(I) ion is reduced by the aldehyde to silver(0). The silver ion needs to be in an alkaline solution for the test to work but by adding hydroxide ions to Ag+, they cause a precipitate of silver oxide, Ag2O. The ion therefore needs to be complexed and is done so with addition of the ligand ammonia (NH3). Ammonia can donate its pair of electrons on the N atom to the vacant orbitals in the silver ion. The overall molecule is now [Ag(NH3)2]. If an aldehyde is added to this solution, a shiny silver mirror will form (if the test tube is clean). A ketone will produce no such results.
From my original group of three carbonyls, I have found ways to identify two of them. The one left is an ester. Esters have a sweet, pleasant smell and also, on reaction with H2NOH, the solution it produces has a hydroxamic acid in it, which forms red-blue complexes on addition with iron (III) chloride (FeCl3).
Now that I have found ways to identify all the carbonyl functional groups, I will focus attention on isolating the three hydroxyl functional groups: the alcohol, phenol and carboxylic acid. All of which produce hydrogen gas when reacted with sodium, as explained before.
Carboxylic acids liberate carbon dioxide when reacted with sodium hydrogencarbonate (NaHCO3) whereas other weaker acids (such as phenol) do not. My next test will therefore be to add sodium hydrogencarbonate to the organic unknown and test for carbon dioxide gas being given out. This will be done by bubbling the gas through limewater and seeing if a milky precipitate is formed.
I now need to distinguish between a phenol and an alcohol. The electron rich benzene ring in phenols give rise to many reactions which phenol can undergo with dramatic changes and an alcohol or a carboxylic acid cannot. Furthermore, the '-OH' group "increases the susceptibility of the benzene ring to electrophilic attack." (Ratcliff, Brian; Chemistry 2, (Cambridge University Press) page 18) As the '-OH' group has a positive inductive effect, it pushes electrons towards the benzene ring thereby increasing electron density (particularly in the 2,4 and 6 carbon positions).
There are therefore a number of reactions that phenol can undergo and other '-OH' functional groups, i.e. alcohols, cannot. Phenols cannot be oxidised directly as alcohols can and no elimination reaction is possible. But perhaps the most important reaction from a qualitative analysis point of view that phenol can undergo but which alcohols cannot is the reaction with neutral iron (III) chloride. As mentioned in the first table, phenols turn a purple colour with addition of a few drops of FeCl3. In fact any colour other than yellow indicates that a phenol is present.
My final test will be to confirm that I do indeed have an alcohol (if the above tests are positive). To do this I will heat and reflux with acidified potassium dichromate (K2Cr2O7 - orange) and unless the alcohol is a tertiary alcohol, the solution will form Cr3+ ions - blue - and there will be a colour change.
Now that I have explained and carefully thought out my flow diagram, I will begin to piece it together, beginning with the addition of sodium to the organic unknown.
Method - step by step.
Safety:
As always, a standard risk assessment sheet will need to be filled out before any experiment can take place.
Standard laboratory safety procedures should be observed for all off the methods following: •wear goggles •tie hair back •do not run •do not work on the edge of the work surface •turn Bunsen burner on yellow safety flame when not in immediate use •tuck stools under tables etc. Any safety procedures specific to the method will be mentioned after each method. What makes this experiment much more unsafe is the use of an unknown substance. Therefore extra care will need to be taken. Below is a table of the simplest organic compounds the unknowns can be with their risk and safety phrases. As all compounds in a homologous series behave similarly, I can deduce the risk and safety factors from one of those in the group and apply it to any one of them compounds.
Organic unknown
Risk phrases
Safety phrases
Alcohol (methyl alcohol)
• Highly flammable
• Toxic by inhalation
• Toxic in contact with skin
• Toxic if swallowed
• Keep container tightly closed
• Keep away from sources of ignition
• Avoid contact with eyes and skin
Aldehyde (formaldehyde, 37% solution)
• Flammable
• Very toxic by inhalation
• Very toxic in contact with skin
• Very toxic if swallowed
• Causes burns
• Possible risk of irreversible effects
• Risk of serious damage to the eyes
• May cause sensitisation by skin contact.
• Wear safety glasses
• Wear gloves
• Keep room well ventilated.
Ketone (propanone)
• Highly flammable
• Irritating to eyes
• Irritating to respiratory system
• Irritation to skin
• Repeated exposure may cause skin dryness or cracking
• Vapours may cause drowsiness and dizziness
• Keep container in a well-ventilated place
• Keep away from sources of ignition
• Do not breathe vapour
• In case of contact with eyes, rinse immediately with plenty of water and seek medical advice
• Take precautionary measures against static discharges.
Carboxylic acid (formic acid)
• Causes severe burns
• Do not breathe vapour
• In case of contact with eyes, rinse immediately with plenty of water and seek medical advice
• In case of accident or if you feel unwell, seek medical advice immediately (show the label whenever possible)
Ester
-
-
Phenol
• Toxic in contact with skin
• Toxic if swallowed
• Causes burns.
• After contact with skin, wash immediately with plenty of soap-suds
• In case of accident or if you feel unwell, seek medical advice immediately (show the label whenever possible)
(http://ptcl.chem.ox.ac.uk/MSDS)
Test 1: add sodium to the organic unknown.
Apparatus: •two 10ml measuring cylinders (±1ml)
•roll of cling film
•tray
•test tube with matching bung and delivery tube (and plasticine)
•5ml of the organic unknown
•small amount of sodium (1-2 grams)
•forceps
) fill a 10ml measuring cylinder full with water and place a piece of cling film over it
2) turn the measuring cylinder upside down in a tray full of water and take the cling film off
3) prepare a test tube with its bung by placing the open end of the bung in the inverted, water-tight measuring cylinder and making sure the actual bung fits the test tube
4) measure out 5ml of the organic unknown in another 10ml measuring cylinder
5) pour the unknown from the measuring cylinder into the test tube
6) add about 5-10mm cubed of sodium with care using the forceps to the test tube and quickly stopper the tube with the bung
7) wait for the reaction to complete for about 3-4 minutes, stirring occasionally
8) during the reaction, make sure that no gas is escaping through any unseen holes in the delivery tube or bung if there is, quickly seal with plasticine
9) carefully place another piece of cling film over the measuring cylinder and take the measuring cylinder out of the water tray
Safety:
Sodium or the organic unknown should not make contact with the skin at any time. Sodium is harmful by ingestion, inhalation and skin contact. Do not use a water fire extinguisher on sodium fires.
Test 2: pop test.
Apparatus: •splint
•bunsen burner with heat mat and matches
•gas that was collected from test 1
) turn bunsen burner on blue flame
2) light the splint
3) take off the cling film from the measuring cylinder containing the gas and quickly place the burning splint at the top of the measuring cylinder.
4) record any sounds which come from the cylinder
Safety:
Be sure to turn the bunsen burner off after use and have a heat mat ready for the burning splint to be rested on if the splint burns all the way. As mentioned above, tie long hair back.
Test 3 and 4: add sodium hydrogen carbonate to the unknown and limewater test
Apparatus: •two 10ml measuring cylinders (±1ml)
•2 test tubes with 1 matching bung and delivery tube (and plasticine)
•5ml of the organic unknown
•small amount of sodium hydrogencarbonate (2-3 grams)
•calcium hydroxide solution (limewater) (10ml)
) fill a 10ml measuring cylinder with 10ml of limewater and pour into a test tube
2) prepare another test tube with its bung by placing the open end of the bung in the bottom of the test tube with the limewater in it and make sure the actual bung fits the test tube
3) measure out 5ml of the organic unknown in another 10ml measuring cylinder
4) pour the unknown from the measuring cylinder into the test tube
5) add about 2-3 grams of sodium hydrogencarbonate with care to the test tube and quickly stopper the tube with the bung
6) wait for the reaction to complete for about 3-4 minutes, stirring occasionally
7) during the reaction, make sure that no gas is escaping through any unseen holes in the delivery tube or bung if there is, quickly seal with plasticine
8) note any colour change in the limewater.
Safety:
Make sure the water tray does not spill onto the floor and if it does, quickly mop away as floor may become slippery.
Test 5: add 'neutral' iron (III) chloride to the unknown
Apparatus: •test tube
•measuring cylinder
•5ml 'neutral' iron (III) chloride (10%)
•pipette
•few drops of organic unknown
) measure out 5 ml of 'neutral' 10% iron (III) chloride in a measuring cylinder and pour this into a test tube
2) add a few drops of the organic unknown using the pipette into the test tube
3) note any colour changes
Safety:
Iron (III) chloride will cause burns and is corrosive. Suitable eye protection must be worn.
Test 6: add acidified potassium dichromate to the unknown
Apparatus: •bunsen burner with heat mat
•pear shaped flask with stopper
•stand with cork lined clamp
•10 ml of acidified orange potassium dichromate (acidified with sulphuric acid)
•5 ml of organic unknown
•measuring cylinder
•pipette
) set-up the clamp and stand to the correct height just so that the tip of the bunsun burners blue flame can reach the pear shaped flask.
2) clamp the pear shaped flask and pour in 10 ml of acidified potassium dichromate
3) add the organic unknown drop by drop to the acidified potassium dichromate using the pipette
4) slowly begin to boil the acidified potassium dichromate
5) note any colour change in the pear shaped flask
Safety:
Make sure the potassium dichromate does not make contact with skin and if so wash of with soapsuds.
Test 7: 2,4-dinitrophenylhydrazine test
Apparatus: •pipette
•10ml methanol
•3m3 of Brady's reagent (2g 2,4-DNPH in 4cm3 concentrated sulphuric acid, 30cm3 methanol and 10cm3 water)
•test tube
•2-3 drops if the organic unknown
) measure 3cm3 of Brady's reagent into a measuring cylinder and pour this into a test tube
2) using a pipette, drop 2-3 drops of the organic unknown into the test tube
3) note any colour changes
Safety:
Make sure room is well ventilated. Take care to make sure that 2,4-DNPH does not come in contact with skin. "If swallowed, if conscious, immediately induce vomiting. If inhaled, remove to fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. In case of contact, immediately flush eyes with plenty of water for at least 15 minutes. Flush skin with water" (http://physchem.ox.ac.uk/MSDS/DI/2,4-dinitrophenylhydrazine)
Test 8: Tollens test
Apparatus: •5cm3 Tollens reagent (ammonia solution added drop wise with shaking to silver nitrate)
•2-4 drops of the organic unknown
•pipette
•test tube
•water bath
) prepare 5cm3 of Tollens reagent in a test tube
2) to this drop 2-3 drops of the organic unknown using the pipette
3) if there is no reaction, place test tube in a water bath in warm water
4) observe any physical changes in the appearance of the test tube
Safety:
Ensure the water in the water bath does not exceed temperatures over 30oC. The test tube must be washed thoroughly after use as if left to stand, can form explosive substances including the detonating material silver azide. A little concentrated nitric acid may be used to clean the test tube after use.
Test 9: Add H2NOH to the unknown
Apparatus: •5ml H2NOH in solution
•5ml 'neutral' iron (III) chloride (10%)
•5ml organic unknown
•2 measuring cylinders (10ml)
) add 5ml of H2NOH in solution into a test tube
2) to this, add 5ml of the organic unknown
3) in this mixture, add a few drops of 'neutral' iron (III) chloride
4) notice any colour changes
Safety:
Be sure to work away from the edge of the work surface. Wear goggles as iron (III) chloride can damage eyes.
Test 10: smell the organic unknown
Apparatus: •10 ml organic unknown
•small conical flask
) pour out 10ml of the organic unknown into a conical flask
2) whilst swirling the unknown, waft over vapours from the unknown towards your nose using your hands
3) notice any smells encountered
Safety:
Never directly smell unknown organics.
Below is a table for which to record my results onto. I will place a tick for a successful test, a cross for an unsuccessful one and a dash of the test need not be completed.
Tests
2
3
4
5
6
7
8
9
0
tick (?)/cross (?)/dash (?)
3150 WORDS
REFERENCES
Rob Ritchie AS Revision Notes Chemistry (Letts - Endorsed by OCR)
Ramsden, E. N; A-Level Chemistry, (Stanley Thornes Ltd.)
Lewis, Eric and Berry, Martyn AS and A Level Chemistry (Longman)
Microsoft, Encarta Encyclopaedia, (1999)
http://ptcl.chem.ox.ac.uk/MSDS
APPROVED CHEMISTRY: AS PRACTICAL TASK (SKILLS P, A), Identification of un organic unknown.
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