Post on 17-Jan-2016
description
Aldehydes
R-CHO
Physical properties
State: Gas at room temperature.
Miscibility: Miscible with water forming a 40% solution
called formalin.
Color: Colorless.
Effect on Litmus paper: Neutral.
(1) Formaldehyde (1) Formaldehyde HCHO
(1) Schiff reagent test (Fuchsin aldehyde reagent):
Procedure: In a test tube take 1 ml formaldehyde and 1 ml
of freshly prepared Schiff reagent, shake, and observe the
color at once.
Observation: Magenta color (violet color).
Chemical Reactions
(2)Tollen's reagent test (Ammonical silver nitrate)
Procedure: In a test tube take 1 mL of formaldehyde, 2 mL
of Tollen's reagent, shake well and then warm in water bath
for 5 minutes.
Observation: A grey precipitate or silver mirror.
HCHO + 2 Ag(NH3)2OH 2Ag + HCOONH4 + H2O + 3NH3w.b.
5 min.Silver mirror
(2) Fehling test:
Procedure: put 1 ml of Fehling (A) solution (CuSO4) and
1 ml of Fehling (B) solution (Rochelle salt, potassium
sodium tartarate), then add 1 ml of formaldehyde soln.,
put in water bath for 5 min.
Observation: A brick-red precipitate.
HCHO + 2CuO HCOOH + Cu2Ow.b.
5 min. Cuprous oxidered ppt.
Cupric oxide
(4) Resorcinol test:
Proceduer: take few crystals of resorcinol and 1 ml
formaldehyde soln., then add few drops of conc. sulfuric
acid drop-wise on the walls of the test tube.
Observation: a red ring is formed at the junction of the
two liquids and white precipitate is formed which turns
violet-red in color after shaking.
2
Physical properties
State: Liquid
Miscibility: Miscible with water.
Color: Colorless.
Effect on litmus paper: Neutral.
(2) Acetaldehyde (2) Acetaldehyde CH3CHO
(1) Schiff reagent test (Fuchsin aldehyde reagent):
Procedure: as formaldehyde.
Observation: Magenta color.
Chemical Reactions
(2)Tollen's reagent test (Ammonical silver nitrate)
Procedure: as formaldehyde
Observation: A grey precipitate or silver mirror.
CH3CHO + 2 Ag(NH3)2OH 2Ag + CH3COONH4 + H2O + 3NH3w.b.
5 min.Silver mirror
(3) Fehling test:
Procedure: as formaldehyde
Observation: A brick-red precipitate.
CH3CHO + 2CuO CH3COOH + Cu2Ow.b.
5 min. Cuprous oxidered ppt.
Cupric oxide
(4) Iodoform formation test:
Procedure: take 2 ml acetaldehyde, 3 ml 10% iodine
solution with shaking, then add dilute NaOH drop-wise
with shaking till a persistent straw-yellow ppt. is formed.
Observation: yellow precipitate of iodoform.
CH3CHO + 3I2 CI3CHO + 3HI iodalCI3CHO + NaOH CI3H + HCOONa iodoform
(5) Sodium nitroprusside test:
Procedure: In a test tube take 1 ml of acetaldehyde, few
drops of freshly prepared sodium nitroprusside soln., then
add dilute NaOH portion-wise with shaking.
Observation: Red color.
(3) Benzaldehyde (3) Benzaldehyde C6H5CHO
Physical properties:
State: Liquid
Miscibility: Immiscible with water.
Color: Colorless.
Effect on litmus paper: Neutral.
Chemical Reactions
(1) Schiff reagent test:
Procedure: as formaldehyde but needs strong
shaking because it’s immiscible.
Observation: Magenta color.
(2) Oxidation with potassium permanganate (KMnO4):
Procedure: take 2 ml benzaldehyde, 3 ml of potassium
permanganate with shaking, and 2 drops of dilute NaOH. Then
heat the mixture on the direct flame for five minutes, then filter
and acidify with dilute HCl.
Observation: decolorization of violet color of KMnO4 White ppt
of benzoic acid .
CHO
KMnO4
COOH
(3) Sodium bisulfite test:
Procedure: take 1 ml benzaldehyde and one drop of
sodium bisulfite with shaking.
Observation: A white precipitate.
CHO
NaHSO3
CHOH
SO3Na
Finish