1 CHEMICAL TESTS FOR CARBONYL COMPOUNDS
Observation: Yellow/ orange precipitate formed 2 ❖Brady’s reagent : 2,4-dinitrophenylhydrazine ❖2,4-dinitrophenylhydrazine will produce a yellow / orange precipitate with all carbonyl compounds. ❖ It is used to determine the presence of a carbonyl functional group. BRADY’S TEST
3 General equation : + H2O R C H + O H2 NHN NO2 NO2 NO2 N NO2 H C N H R H or R Orange or yellow precipitate
4 H3 C C H O + N NH H H NO2 NO2 C N NH NO2 H3 C NO2 H + H2 O + N NH H H NO2 C NO2 + H2 O H O C N H NO2 NO2 H3 C C CH3 O + N NH H H NO2 NO2 C N NH NO2 H3 C NO2 CH3 + H2 O ETHANAL BENZALDEHYDE PROPANONE EXAMPLES:
❖Mixing aqueous silver nitrate with aqueous ammonia, [Ag(NH3 )2 ] + produces a weak oxidizing agent known as Tollens’ reagent. ❖ It will oxidize all aldehydes to carboxylate ions and reduce Ag+ to Ag. ❖The formation of metallic silver on the wall of the test tube or a grey precipitate indicates a positive result for the aldehyde. ❖This test is used to differentiate aldehydes from ketones. TOLLENS’ TEST
General equation : R C H O R C O Ag - O + [Ag(NH3 )2 ] + + OH− SILVER ALIPHATIC MIRROR OR AROMATIC ALDEHYDES Carboxylate ion
H3 C C H O + H3 C C + O - [Ag(NH3 ) 2 ] O + OHAg ETHANAL + [Ag(NH3 ) 2 ] + + C H O C O - O Ag OHBENZALDEHYDE EXAMPLES: SILVER MIRROR SILVER MIRROR Carboxylate ion Carboxylate ion
Silver mirror forms on the test tube wall No silver mirror forms 8
FEHLING’S TEST ❖Fehling’s reagent: Cu2+ in OH− ❖This test is used to differentiate an aliphatic aldehyde from aromatic aldehyde and ketone. ❖Aliphatic aldehydes give positive results when reacted with Fehling’s solution by forming a brick-red precipitate of Cu2O.
10 R C H O OHR C O - O + Cu + Cu2+ 2 O General equation : Aliphatic aldehydes Carboxylate ion brick red precipitate EXAMPLES: H3 C C H O + + Cu Cu 2 O 2+ H3 C C O - O OH− Carboxylate ion
IODOFORM TEST 11 ❖Iodoform reagent, alkaline solution of iodine (I2 ,OH− ) will react positively with aldehyde and ketone compound that have the following group; O ║ CH3 -Cmethyl carbonyl group ❖The light yellow precipitate formed is CHI3 (triiodomethane or iodoform)
R C CH3 O CHI R C O 3 - O + I + 2 OH− Yellow precipitate R or H General equation : Carboxylate ion
H3 C C H O + + H C O - O I 2 OHCHI3 ETHANAL H3 C C CH3 O + + H3 C C O - O I 2 OHCHI3 PROPANONE YELLOW PRECIPITATE YELLOW PRECIPITATE EXAMPLES: Carboxylate ion Carboxylate ion
14 CHEMICAL TESTS FOR CARBONYL COMPOUNDS FEHLING’S TEST IODOFORM TEST TOLLENS’ TEST BRADY’S TEST To identify C═O in a compound To differentiate aldehyde from ketone To differentiate aliphatic aldehyde from aromatic aldehyde & ketone To detect methyl carbonyl group – C – CH3 O Observation: Orange precipitate Observation: Silver mirror Observation: Brick-red precipitate Observation: Light yellow precipitate