Acetaldehyde : properties Part 3

Acetaldehyde

source : science madness.wiki-wikia

1) Oxidation –

Aldehydes are easily oxidised & thus act as reducing agents . Aldehyde on oxidation form corresponding acid containing same number of carbon atoms.

RCHO +[O]—–>  RCOOH {acid}

CH₃CHO + [O] —–>  CH₃COOH [acetic acid]

Oxidising agents may be strong such as acidified K₂Cr₂O₇ or acidified KMnO₄ or mild such as Tollen’s reagent or Fehling solution or Benedict solution.

Tollen’s Reagent-

It is an ammonical solution of Silver nitrate. When aldehyde is added to tollen’s reagent, Ag₂O is reduced to silver which deposits as black ppt. or silver mirror.

RCHO + Ag₂O—->  RCOOH + 2Ag (silver mirror)

CH₃CHO + Ag₂O—->  CH₃COOH + 2Ag (silver mirror)

Fehling solution-

It is an alkaline solution of cupric ion complexed with sodium potassium tartarate . Aldehyde reduces cupric ion (blue ) to cuprous oxide (red).

Fehling solution I – dilute solution of CuSO₄
Fehling solution II – Sodium potassium tartarate & NaOH

RCHO + 2 CuO —–>   RCOOH + Cu₂O(red ppt. of cuprous oxide)

Benedict’s solution- It is a solution of CuSO₄, Sodium citrate & Na₂CO₃. When heated with an aldehyde , a reddish brown ppt. appears.

2) Reaction of Acetaldehyde with alkali –

Aldehydes having alpha hydrogen atom on heating with alkali (Except Formaldehyde) give a brown resin like substance of characteristic odour.

CH₃CH=O + H₂CHCH=O +H₂CHCH=O —–>  CH₃CH=CHCH=CHCH——— (Brown resin)

3) Tischenko’s Reaction-

This is a modified form of Cannizarro’s reaction . All aldehydes undergo Cannizarro’s reaction in Aluminium ethoxide. The acid & alcohol formed react together to give an  ester.

2CH₃CHO —–>   [CH₃COOH + C₂H₅OH] ——> CH₃COOC₂H₅ (ester)

4) Reaction with alcohols-

Aldehydes  combine with alcohol in presence of dry HCl gas forming hemiacetal then acetal.

CH₃CH=O + HOC₂H₅——>  CH₃CH(OH)OC₂H₅ [hemiacetal] —-> CH₃CH(OC₂H₅)₂[acetal]

Uses :

1. As an antiseptic.

2. In the preparation of paraldehyde and metaldehyde.

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