Acetaldehyde
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1) Halogenation of Acetaldehyde—
In this reaction hydrogen atoms of alkyl group are replaced by halogen atoms.[Formaldehyde do not give this reaction]
CH3CHO + 3Cl2 —-> CCl3CHO [Trichloro acetaldehyde or chloral] + 3HCl
2) Haloform reaction –
Acetaldehyde , acetone & other methyl ketones give haloform reaction.This is a characteristic test of CH3CO- group.For this reaction NaOH +I2 or Na2CO3 +I2 is used which produces yellow solid i.e iodoform. Similarly Chloroform is formed with Cl2 +NaOH & Bromoform with Br2 +NaOH.
CH3CHO +3Cl2 +3NaOH—-> CCl3CHO +3NaCl +3H2O
CCl3CHO +NaOH—–> CHCl3 + HCOONa
Equation as a whole ,
CH3CHO +3Cl2 +4NaOH—-> CHCl3 [chloroform] +HCOONa +3NaCl +3H2O
3) Aldol Condensation –
It is a reaction in which two or more molecules of same or different carbonyl compounds containing alpha hydrogen atom, unite together in presence of dilute base such as NaOH,Ba(OH)2,K2CO3 etc.to form compounds called ALDOL.
Ex.1-Acetaldehyde undergoes aldol condensation in presence of dil NaOH or K2CO3 to form beta hydroxy butyraldehyde or aldol.
CH3CHO + HCH2CHO—–> CH3CH(OH)CH2CHO [Aldol]
On heating aldol loses a water molecule to give unsaturated aldehyde .
CH3CH(OH)CH2CHO ——-> CH3CH=CHCHO [crotonaldehyde] +H2O
Ex.2-Acetaldehyde reacts with benzaldehyde (which has no alpha hydrogen atom) in the presence of a base to form an aldol which on heating yields cinnamaldehyde (unsaturated aldehyde).
C6H5CHO +HCH2CHO —–> C6H5CH(OH)CH2CHO —–> C6H5CH=CHCHO +H2O
4) Schiff’s Test-
Schiff’s reagent is a dilute solution of rosaniline hydrochloride whose red or pink colour disappears by passing SO2. Aldehydes when treated with Schiff’s reagent, restore its pink colour.
5) Reaction with ammonia-
Except formaldehyde other aldehydes form addition product with ammonia.
RCH=O + NH3—-> RCH(OH)NH2 [aldehyde ammonia]
CH3CH=O + NH3—-> CH3CH(OH)NH2 [acetaldehyde ammonia]
