Acetaldehyde : properties Part 2

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]