EQUITMENT: universal indicator,
Pipette,
Test tube.
SAFTETY: always follow general safety procedures, and be aware that universal indicator is dangerous.
PROCEDURE: Add several drops of the unknown substance to universal indicator inside a test tube using a pipette.
OBSERVATIONS: A colour change to red indicates either a carboxylic acid or phenol is present. Any other colour change shows alcohol, aldehyde, ketone or ester.
3. Test unknown with bromine water.
THEROY: Aqueous phenol decolourises bromine water to form a white precipitate of 2,4,6-tribromophenol, the reaction takes place as follows
The bromine water acts as an electrophile and attacks the delocalised pi electrons of the benzene ring (electrophilic substitution). This electrophilic attack will not take place with the carboxylic acid.
EQUITMENT: Test tube,
Pipette,
Bromine water.
SAFTETY: Care should be taken when using bromine water as it can be toxic and corrosive, as always follow general safety procedures.
PROCEDURE: Add several drops of bromine water into a test tube containing the carboxylic acid or phenol unknown.
OBSERVATIONS: A white precipitate formed indicates a phenol present; no change shows a carboxylic acid.
4. Test unknown with 2,4-DNPH
THEROY: The equation for the reaction of ethanal with 2,4-dinitrophenylhydrazine is:
The reaction follows the same principals for all aldehydes and ketones
EQUITMENT: Test tube
Pipettes
2,4 DNPH solution
1cm³ of Ethanol
Water Bath (if necessary).
SAFETY: always follow general safety procedures, 2,4-DNPH is explosive and toxic and ethanol is extremely flammable, so take extra care when appropriate and ensure Bunsen burners are switched off in the presence of any flammable chemical.
PROCEDURE: To 1cm³ of 2,4-dinitrophenylhydrazine solution and 1cm³ of ethanol, add several drops of the carbonyl, ester or alcohol unknown via pipette, Heat with a water bath if necessary.
OBSERVATIONS: 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, or ester.
5. Test unknown with tollens reagent
THEROY: Aldehydes are easily oxidised to acids. Ketones cannot be oxidised, as there is no place for the oxygen from the oxidising agent. Tollens reagent consists of silver nitrate and ammonia to form the complex ion {Ag (NH3)2} this is reduced to silver in the reaction with the aldehyde forming a silver mirror.
EQUIPMENT: Test tube
Pipette
Silver nitrate solution
Sodium hydroxide solution
Ammonia solution
Water Bath
SAFETY: Follow general safety procedures and silver nitrate solution, sodium hydroxide solution and ammonia solution are all corrosive so handle with care.
PROCEDURE: Put about 1cm³ 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.
OBSERVATIONS: 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.
6. Test unknown with acidified potassium dichromate
THEROY: Primary alcohols are oxidised by acidified dichromate to aldehydes. C2H5OH + [O] --> CH3CHO + H2O.
In the oxidation, the dichromate is reduced to Cr3+ causing the colour change to green.
No reaction occurs with a tertiary alcohol as there is nowhere for the oxygen from the oxidising agent to bond.
EQUIPMENT: Test tube
Pipette
Potassium dichromate solution
Sulphuric acid
Water bath
SAFETY: follow general safety procedures and potassium dichromate is toxic and sulphuric acid is corrosive so take care when handling.
PROCEDURE: acidify the potassium dichromate by adding equal amounts of sulphuric acid to a test tube then simple add the alcohol or ester unknowns and warm with a water bath.
OBSERVATIONS: a colour change to green shows an alcohol present; no colour change and sweet smell indicates an ester.
7. Test unknown with Lucas reagent
THEROY: lucas reagent consists of ZnCl2 + HCL the test is pacifically designed to distinguish between primary, secondary and tertiary alcohols. The equation is as follows; C4H10O + HCL C4H9Cl
EQUIPMENT: test tube
Pipette
HCL
ZnCl2
SAFETY: follow general safety procedures and be careful when using HCL as it is corrosive and can become and irritant.
PROCEDURE: add equal amounts of HCL and ZnCl2 to a test tube then add several drops of the alcohol and ester unknown and shake gently.
OBSERVATIONS: no change indicates an ester; cloudy very quickly indicates a tertiary alcohol.
8. Confirmation of carboxylic acid with addition of base
THEROY: Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equation.
RCO2H + NaHCO3 RCO2 (–) Na (+) + CO2 + H2O
EQUITMENT: Pipette
test tube
NaHCO3
SAFTY: Take care when handling acids and bases as irritation to the skin can be caused, always follow general safety procedures.
PROCEDURE: Add equal amounts of unknown with the NaHCO3 into a test tube using a pipette and shake gently.
OBSERVATIONS: after a few seconds and salt precipitate should start to form, confirming the unknown to be a carboxylic acid.
9. Confirmation of ester by hydrolysis
THEROY: Hydrolysis is simply a reaction with water, the hydrolysis of an ester produces an alcohol and a carboxylic acid the reaction is in equilibrium as shown below
EQUITMENT: Test tube
Pipette
HCl
Water
SAFETY: follow general safety procedure, handle chemicals with care.
PROCEDURE: reflux the ester with excess acid and water
OBSERVATIONS: The sweet smelling liquid will form 2 layers, alcohol and acid
References
Chemistry 2 chapters 2, 3, 4
http://www.studyzones.com/questionzone/exchange/detail.cfm?objectid=5B88E323-B0D0-49C8-DFA3892AB43050EF
http://www.chemguide.co.uk/organicprops/alkenes/kmno4.html
http://en.wikipedia.org/wiki/Lucas_reagent
http://www.chemguide.co.uk/organicprops/esters/hydrolysis.html