2. Qualitative ORGANIC functional group tests in various homologous series
| TEST FOR
| TEST METHOD
| TEST CHEMISTRY and comments
| ALKENE or alkyne i.e. any non-aromatic unsaturated hydrocarbon with a double or triple carbon-carbon bond.
| Bubble gas through, or add liquid to, a solution of bromine in hexane or water.
| The orange/brown bromine rapidly decolourises, as a saturated colourless organic bromo-compound is formed.
| R2C=CR2 + Br2 ==> BrR2C-CR2BrRCCR + 2Br2 ==> Br2RC-CRBr2R = H, alkyl or arylSaturated alkanes give no fast reaction with bromine.
| Hydroxy group R-OH in alcohols and phenols (in 'dry' conditions*) The first 3 tests (i) - (iii) given on the right are quite general for most alcohols.
| (i) Mix it with a few drops of ethanoyl chloride, test fumes with litmus and silver nitrate (* note ethanoyl chloride reacts with water, phenols and amines too!).(ii) Mix it with a little phosphorus(V) chloride and test as above.(iii) Warm with a little ethanoic acid and a few drops of conc. sulphuric acid. Pour into water.
| (i) Litmus turns red and a white precipitate with silver nitrate(aq) (drop on end of glass rod), if the mixture is poured into water you may detect a 'pleasant' ester odour, can test for HCl but water and amines produce it too!(ii) as for (1) but no ester smell!(iii) You should get a 'pleasant' characteristic smell of an ester.
| (i) R-OH + CH3COCl ==> CH3COOR + HClAn ester and hydrogen chloride are formed(ii) R-OH + PCl5 ==> R-Cl + POCl3 + HCla chloro compound and hydrogen chloride are formed.(i) and (ii) Ag+(aq) + Cl-(aq) ==> AgCl(s) from the hydrogen chloride fumes dissolved in water.(iii) CH3COOH + ROH ==> CH3COOR + H2O
| Primary alcoholRCH2OH, R = H, alkyl or aryl (NOT a phenol).(ii) is not a good test on its own, since so many other readily reducible organic compounds will give the same reaction, though following it up by testing for an aldehyde gives it much more validity.
| (i) Lucas test - shake a few drops with cold zinc chloride in conc. HCl(aq)(ii) Distil with potassium dichromate(VI) and mod. conc. H2SO4(aq)
| (i) Solution remains clear.(ii) If product distilled off immediately analdehyde odour can be detected and the solution colour changes from orange togreen.
| (i) Not usually reactive enough to form a primary halogenoalkane(ii) R-CH2OH + [O] => R-CHO + H2O or the full works! 3R-CH2OH + Cr2O72- + 8H+ ==> 2Cr3+ + 3R-CHO + 7H2OThe orange dichromate(VI) ion is reduced to the green chromium(III) ion. If the organic product is collected you could test for an aldehyde.
| Secondary alcoholR2CHOH, R = alkyl or aryl.(ii) is not a good test on its own, since so many other reducible organic compounds will give the same reaction, though following it up by testing for a ketone gives it much more validity.
| (i) Lucas test. (ii) Distil with K2Cr2O7/H2SO4(aq)
| (i) Solution may cloud very slowly or remains clear (hit and miss)(ii) If product distilled off immediately aketone odour can be detected and the solution colour changes from orange togreen.
| (i) May be reactive enough to slowly form an insoluble secondary halogenoalkane: R2CHOH+ HCl => R2CHCl + H2O(ii) R2CHOH + [O] => R-CO-R + H2O or the full works! 3R2CHOH + Cr2O72- + 8H+ ==> 2Cr3+ + 3R-CO-R + 7H2OThe orange dichromate(VI) ion is reduced to the green chromium(III) ion. If the organic product is collected you could test for an aldehyde.
| Tertiary alcoholR3COH, R = alkyl or aryl.
| (i) Lucas test.(ii) Distil with K2Cr2O7/H2SO4(aq)
| (i) Goes cloudy very quickly.(ii) No aldehyde or ketone readily formed
| (i) Reactive enough to immediately form an insoluble tertiary halogenoalkane R3COH + HCl => R3CCl + H2O(iii) Stable to modest oxidation.
| Phenols (OH group is attached directly to aromatic ring). R-OH, where R is aryl e.g. C6H5OH
| Add a few drops of iron(III) chloride solution to a little of the phenol in water.
| Usually gives a purple colour.
| (see also test for primary aromatic amines - use it in...
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