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