Tests for organic functional groups

Tests for organic functional groups 

OCR

The qualitative analysis of organic functional groups on a test-tube scale; 

The processes and techniques needed to identify the following functional groups in an unknown compound:

(i) Alkenes by reaction with bromine 

Add bromine water (Br_2(aq)) dropwise to a testtube containing the alkene as a gas or liquid and shake vigorously.  If te testtube contains an molecule with an alkenic double bond then the bromine will decolourise.  It will go colourless (not clear). 

Bromine adds across the alkenic double bond.

—C=C—    +    Br₂(aq)      →     —CBr—CBr—

(ii) Haloalkanes by reaction with aqueous silver nitrate in ethanol

Warm the susupected haloalkane with ethanolic silver nitrate solution in atesttube.  If the suspected organic compound contains a halogen then a white, cream or yellow precipitate (This is the silver halide) will form in the otherwise colourless solution.  

C₃H₉Cl   +   AgNO₃   +   H₂O   →  C₃H₉OH  +  AgCl  +  HNO₃

AgCl is a white precipitate in this example.

(iii) Phenols by weak acidity but no reaction with CO32– 

Phenols are weak acids but much weaker than carbonates so do not liberate CO₂ after the addition of 2M sodium carbonate solution.  A confirmatory test for the phenolic —OH group is to add a few drops of neutral iron(III)chloride solution.  If a purple or dark green colouration forms then the phenolic —OH is confirmed.

(iv) carbonyl compounds by reaction with 2,4—DNP 

2,4—DNP is 2,4 dinitrophenylhydrazine which is explosive when dry but safe when wet or in solution.  The solution is golden in colour.  Warming a few drops of the DNP with your suspected carbonyl comound usually yields golden crystals of the hydrazone product.  

(v) aldehydes by reaction with Tollens’ reagent 

This sometimes referred to as the silver mirror test.  The suspected aldehyde is warmed in a water bath with a solution of ammonical silver nitrate.  The aldehyde needs to be in a pristine clean straight out of the supply box test-tube not a cleaned test-tube.  After a few minutes of the solutions in the water bath a silver mirror will coat the inside of the test-tube.

(vi) primary and secondary alcohols and aldehydes by reaction with acidified dichromate 

Sodium or potassium dichromate(VI) (K₂Cr₂O₇) can be used for this test.  If the organic is suspected of being a primary or secondary alcohol warming its aqueous solution with a solution of potassium or sodium dichromate(VI) acidified with sulphuric(VI) acid will result in the solution changing colour from the dichromate orange to the chromate(III) green.  Aldehydes will also give a positive in this test so they need to be confirmed with the silver mirror test.

The alcohol test is the basis for the Breathalyser Test to see if you are over the limit with more alcohol in your breath and blood stream than the law allows.

(vii) carboxylic acids by reaction with CO32– 

Carboxylic acids behave as any mineral acid in that they displace carbon dioxide gas (CO₂) from a carbonate such as zinc carbonate (ZnCO₃) or a solution of 2Msodium carbonate (Na₂CO₃(aq))

2CH₃COOH  +  Na₂CO₃(aq) → 2CH₃COONa  + H₂O  + CO₂