1) Test for an Alkene or Phenol The reaction of an alkene with Bromine water is an electrophilic addition reaction; the alkene will open up its double bond and form bonds between its carbon atoms and the electrophilic Bromine atoms. This produces dibromoalkane. The OH group on Phenol is electron releasing, because the oxygen p-orbital is delocalised, therefore the electron density of the delocalised ring is increased. It makes the substitution of Bromine with the Hydrogen atom much easier compared with, for example, Benzene. Substitution usually takes place at the 2, 4 and 6 positions.
Using a pipette add 6 drops of the unknown compound to a test tube comprised of 25cm3 Bromine water. Then shake the test tube so that the compound and the bromine water are thoroughly mixed together. Result: If the Bromine water decolourises then an alkene or Phenol is present and the electrophilic addition reaction has taken place. If no reaction follows and the Bromine water remains a brown colour then the unknown compound still has not been identified. Hazards: Bromine water can be toxic and corrosive, so gloves must be worn and care taken.
Extreme care must be taken when dealing with Phenol, direct contact with skin can cause severe burns, so gloves must be worn at all times. It is also very harmful if inhaled as it can burn the respiratory tract and may cause breathing problems, consequently the Phenol must be handled through in a fume cupboard. 2) Test for Phenol Iron(III) ions form a strongly coloured complex with phenol. Add 2 crystals of Iron(III) chloride to a test tube containing approximately 25cm3 of the solution containing either Phenol or an alkene, and stir with a glass rod.
Result: If an intense violet-purple solution is formed then the compound present is Phenol, consequently in this case, if no change occurs then it is an Alkene. Hazards: When handling Iron(III) chloride gloves must be worn, as it is highly corrosive and can cause burns to the skin. Again I am aware of the dangers of Phenol. 3) Test for a Carbonyl group Place 5 drops of Ethanol to a test tube containing 1 cm3 of the unknown compound then add methanol drop wise until the solution is clear.
Once this is achieved add 2cm3 of 2,4 DNPH, using a pipette, to the solution and agitate for 1 minute, before leaving to stand for 5 minutes in a water bath . Result: The formation of a bright orange precipitate indicates the presence of a carbonyl compound, which will be an Aldehyde or a Ketone. If no change occurs then an unknown functional group still remains. Hazards: 2, 4 DNPH is explosive and toxic, so must be handled in a fume cupboard whilst wearing gloves. Also I am aware that ethanol is highly flammable, so it will be kept away from any open flames.
4) Test for an Aldehyde Aldehydes are easily oxidised to acids, whereas Ketones cannot be oxidised, as there is no place for the oxygen from the oxidising agent to attach on to. When the silver nitrate is mixed with ammonia to form Tollen’s reagent, the complex ion [Ag (NH3) 2]+ is formed. [Ag (NH3) 2]+ + e- Ag (s) + 3NH2 This is reduced to silver during the process of oxidation. Tollen’s reagent is the oxidising agent. Into a test tube, place 2cm3 of Silver nitrate solution and add 1 drop of dilute Sodium Hydroxide (NaOH).
Add ammonia solution drop wise until the initially formed brown precipitate of silver oxide is re-dissolved. Then add 5 drops of Methanol to the solution and 5 drops of the unknown compound. Warm the test tube gently in water bath for 1 minute and leave to stand for 5 minutes. Result: If a silver mirror is formed in the test tube, this indicates the presence of an Aldehyde, if no change occurs then a Ketone is present. Hazards: Silver nitrate solution, sodium hydroxide solution and ammonia solution are all corrosive so I shall wear gloves when handling them.
Methanol is highly toxic so if ingested or inhaled it can cause a wide range of harmful effects, breathing in the vapour can also be harmful to the respiratory tract so use of Methanol will be carried out in a fume cupboard. 5) Test for a Primary Alcohol For the potential oxidation of the unknown compound, Potassium Dichromate(VI) acidified with dilute Sulphuric acid will be used. If oxidation occurs, the orange solution containing the Dichromate(VI) ions is reduced to a green solution containing Chromium(III) ions. The electron-half-equation for this reaction is
Add 3 drops of the unknown substance, using a pipette, to a test tube containing 1cm3 of Potassium Dichromate(VI) solution acidified with 1cm3 of dilute Sulphuric acid. The test tube would then be warmed in a water bath for 3 minutes. Result: A colour change to Green would highlight that an Alcohol was present, in this case a primary alcohol. No colour change would suggest that the unknown compound was an Ester or carboxylic acid. Hazards: Potassium Dichromate(VI) is toxic, so care should be taken when handling this.
It is also highly flammable, being aware of this I will keep it away from any danger of flames. Also Sulphuric acid is extremely corrosive, so gloves should be worn. 6) Test for a Tertiary alcohol The Lucas test is used to distinguish between primary, secondary and tertiary alcohols. Add 3 drops of Lucas reagent (ZnCl2/HCl) to a test tube containing 25cm3 of the solution containing the unknown alcohol. Result: A tertiary alcohol reacts rapidly and immediately, giving an insoluble white layer (alkyl chloride).
(CH3)3C-OH + HCl + ZnCl2 (CH3)3C-Cl + H2O A primary alcohol does not react at all. CH3CH2-OH + HCl + ZnCl2 Hazards: The Lucas reagent can cause pain and redness to the skin, therefore gloves must be worn during the experiment. Likewise, goggle must be worn as getting the reagent in your eye can be dangerous. 7) Test for a Carboxylic acid Using a Pipette add approximately 6 drops of the unknown compound solution and mix with 3 drops of methanol, to this then add 2 crystals of NaHCO3 and shake the test tube vigorously.
Result: Solubility will be indicated by a colour change, or the evolution of Carbon dioxide. In this case a Carboxylic acid is present. To test for the evolution of Carbon dioxide, and thus the presence of a Carboxylic acid, the limewater test can be used. Lime water is a solution of calcium hydroxide (slaked lime). If Carbon dioxide is bubbled through it, a solid precipitate of Calcium Carbonate is formed. Calcium Carbonate is chalk or limestone, it is this that makes the lime water cloudy.