source : science

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.



source : slide