Applied Science BTEC Nationals/Chemical Laboratory Techniques/Qualitative organic analysis

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Qualitative Organic Analysis[edit]

The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.

The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.

Burning[edit]

Place a small amount of the test substance on a crucible lid on a heat resistant mat in the fume cupboard. Attempt to ignite it by playing a small Bunsen flame on it from above.


Observation Inference
Burns with non-luminous flame. Contains oxygen : alcohol; aldehyde.
Burns with luminous, slightly smoky flame, very little sooty deposit. Low % of carbon (low C:H ratio); possibly aliphatic with no more than 4-5 C atoms.
Burns with sooty flame and leaves sooty residue. High % of C : aromatic, highly unsaturated.
Residual ash. Metal salt -metal may be analysed by usual means.
Does not burn. Contains halogen atom(s).

Reaction With Alkali[edit]

To about 0.5 g or 1cm3 in a test tube, add about 5 cm3 of bench sodium hydroxide solution, a little at a time, shaking after each addition. If there is no apparent reaction in the cold, warm and eventually boil the mixture. Test for gases with litmus paper but be careful not to touch the solution with test papers.

Observation Inference
Dissolves readily, though insoluble in water; or dissolves slowly on boiling Substance is an acid or phenol
Dissolves to give bright yellow/orange solution Soluble products from the hydrolysis of : ester; acid anhydride; aromatic aldehydes (disproportionation)
Alkaline gas evolved in cold or on warming nitrophenol, ammonium salt
as above but fishy smell detected aliphatic amine salt
as above but only on boiling from hydrolysis of amide; nitrile (cyanoalkane).
Brown resin produced on boiling aliphatic aldehyde (but NOT methanal)
An "oil" separates aromatic amine salt

Reaction With Sodium Carbonate or Hydrogencarbonate[edit]

To about 0.5 g or 1 cm3 in a test tube add about 5 cm3 of sodium carbonate solution, a little at a time and with shaking. Look for evolution of gas and test for carbon dioxide with lime water.

Observation Inference
CO2 evolved as the only gas. Substance is acidic: Carboxylic acid; acid chloride; acid anhydride; substituted phenol
CO2 and fishy smell of amine Amine salt

Fehling's Solution[edit]

Mix equal volumes (about 3 cm3) of Fehling's A & B and add 1 cm3 or 1 g of the test substance. Warm, gently at first and then boil.

Observation Inference
Red or brown ppt. Reducing agent present : aliphatic aldehyde

"Neutral" Iron (III) Chloride[edit]

To 1 g or 1 cm3 of the test substance add "neutral" iron III chloride dropwise until no further colour change - if any - occurs. If in doubt perform a control or blank test using water in place of the test sample.

Observation Inference
Red colour; brown ppt. on boiling Methanoate or ethanoate: Ppt. is iron III hydroxide
Violet or blue colour. Phenol or phenol derivative
Red/violet colour 4-nitrophenol
Yellow colour Salt of aliphatic hydroxycarboxylic acid
Buff ppt. Salt of aromatic carboxylic acid

Potassium Manganate (VII) ('Permanganate')[edit]

Use acidified potassium manganate (VII) - mix equal volumes of dil. sulphuric acid and potassium permanganate solution. To about 1 cm3 or 1 g of the test sample, add a few drops of reagent. If there is no immediate reaction, warm the mixture. If reaction does take place, then by adding reagent to slight excess and boiling, it may be possible to detect the odour of the product.

Observation Inference
Test sample immiscible, rapid decolourisation. Probably an unsaturated hydrocarbon.
Sample miscible, decolourisation at room temperature. Methanoic acid or its salt; 1° or 2° alcohol; aldehyde; reducing sugar.
Sample miscible, decolourisation on warming only. Oxalic acid (ethanedioic acid) or its salt.

Note (i) A brown ppt. is sometimes observed, especially if insufficient acid is added to the manganate (VII). (ii) alkaline manganate (VII) is sometimes used as an alternative - the expected colour change is then from purple to green.

Potassium Dichromate (VI)[edit]

Experimental details are similar to the manganate (VII) test. The expected colour change is orange to blue/green.

Nitrous Acid (Nitric (III) Acid)[edit]

Into a test tube cooled in a beaker of crushed ice, place 1 g sodium nitrite (nitrate III) and dissolve it in 5 cm3 water. Add 5 cm3 dil. hydrochloric acid dropwise slowly. Dissolve 0.5 g or 0.5 cm3 of the test sample in a similarly cooled tube. Add the nitrous acid solution to this.

Observation Inference
Effervescence - nitrogen evolved - do not confuse this with the gases produced by the decomposition of the nitrous acid.
Yellow oil separates Secondary amine present
When the sample is mixed with alkaline β-naphthol (naphthalene-2-ol) and nitrous acid, an orange/red ppt. is produced. Sample contains an aromatic primary amine and an azo dye is produced.

Iodine Or Potassium Iodide[edit]

To 0.5 cm3 sample add 3 cm3 iodine solution (or potassium iodide) and 1 cm3 sodium hypochlorite (chlorate I). Add aqueous sodium hydroxide until the brown colour just fades.

Observation Inference
Yellow ppt., sometimes only on gentle warming; possible antiseptic smell. Iodoform (triiodomethane) ppt: Sample is ethanal; ethanol; a secondary -2-ol; a -2-one (methyl ketone)

Phosphorus (V) Chloride[edit]

Place a small amount of the sample in a dry tube and add the phosphorus V chloride carefully - "spitting" may occur.

Observation Inference
Hydrogen chloride evolved- detected as misty fumes & by (a) silver nitrate (b) ammonia Sample contains -OH group : alcohol; carboxylic acid; phenol. NB - Water gives this reaction
Dense white fumes Sample contains an -NH2 group

Bromine Water[edit]

NB - bromine is sometimes used in organic solvents such as tetrachloromethane - the following observations may be different.

Place 1 cm3 of the sample in a tube and add the aqueous solution of bromine dropwise with shaking. If decolourisation occurs continue to add it until it is in excess.

Observation Inference
White ppt., possible white fumes Aromatic amine -NH2 attached to benzene ring
White ppt. no fumes Aromatic amine -NH2 attached to benzene ring
Decolourisation, separate layers, no ppt. Unsaturation - alkene or alkyne
Slow decolourisation, catalysed by a few drops of sulphuric acid. aldehyde or ketone (possibly a primary or secondary alcohol)
Heavy white fumes, no ppt. aliphatic amine

2,4-Dinitrophenylhydrazine (Brady's Reagent)[edit]

To 1 cm3 of reagent add several drops of the sample. Dissolve solid samples in the minimum volume of methanol.

Observation Inference
Yellow or orange ppt. Carbonyl compound -aldehyde or ketone.

Schiff's Reagent[edit]

Observation Inference
Immediate purple colour. Aliphatic aldehyde.
Purple colour slowly forms. Aromatic aldehyde.

Tollen's Reagent[edit]

To 5 cm3 of silver nitrate solution, add 2 drops sodium hydroxide to ppt. silver oxide. Decant as much of the supernatant liquid as possible. Add aqueous ammonia dropwise until the silver oxide just dissolves. To this solution add a few drops of the sample, or an aqueous solution of it, and place the tube in a beaker of hot water for a few minutes. Use a clean narrow test tube for best results.

NB Tollen's reagent must never be stored and must be disposed with copious amounts of water immediately.

Observation Inference
Silver mirror or black ppt. Reducing agent e.g. aldehyde - aromatic aldehydes produce mirror more slowly; methanoic acid or salt.

Chlorate (I) - Hypochlorite[edit]

A very sensitive and specific test for aniline - blue or purple colouration produced.

References[edit]

Davies, DG & Kelly, TVG (1974) Organic Reactions at Advanced Level published by Mills & Boon ISBN 0 263 05495 1