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Aldehydes are oxidized not only by the same reagents which oxidizes primary and secondary alcohols (such as acidified KMnO4 and K2Cr2O7) but also by mild oxidizing agents such as Tollen’s reagent, Fehling’s solution, and Benedict’s solution. Aldehydes are very easily oxidized and thus are very powerful reducing agents.
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Description: 49210.png
Description: 49219.png (Fehling’s and Benedict’s test)
Name of compounds
Fehling’s test
Tollen’s test
Glucose, fructose
α-Hydroxy ketone
α-Hydroxy aldehyde
Glyoxal (OHC . CHO)
Benzaldehyde and other aromatic aldehydes
Formic acid
Glyoxylic acid (OHC . CO2H)
Succinaldehyde (OHCCH2CH2CHO)
Pyruvaldehyde (CH3COCHO)
Aldehydes restore the magenta color of Schiff’s reagent (rosaniline hydrochloride is dissolved in H2O and SO2 is passed till the magenta color is decolorized). Ketones do not restore the color of Schiff’s reagent except acetone, which restores the color very slowly.
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Nucleophilic addition reactions

  • Addition of Grignard reagents
    Description: 51599.png
  • Addition of hydrogen cyanide: Aldehydes and ketones add on hydrogen cyanide to form addition product, cyanohydrin.
    Description: 51611.png
Description: 51620.png
  • Addition of sodium bisulphite
    Description: 51631.png
  • Addition of ammonia derivatives
    Description: 51645.png
The various ammonia derivatives and their products are indicated in the adjacent table.
Product structure
Name of the product
H2N-OH, hydroxylamine
Description: 51654.png
Oxime (crystalline solid)
H2N-NH2, hydrazine
Description: 51662.png
(crystalline solid)
H2N-NHPh, phenyl
Description: 51741.png
Phenyl hydrazine
(crystalline solid)
Description: 51727.png
(crystalline solid)
Description: 51718.png
2,4-dinitro phenyl
hydrazine (Brady’s reagent)
Description: 51756.png
2,4-Dinitro phenyl hydrazone (yellow crystalline solid used for identification of aldehydes and ketones)
Brady’s regent with carbonyl compounds gives 2,4-dinitro phenyl hydrazone, which is obtained as a yellow crystalline solid. It is thus used for the identification of aldehydes and ketones.

Reaction with SeO2

Description: 49232.png
Description: 49247.png

Reaction with chloroform

Description: 49255.png

Cannizaro reaction

Description: 51767.png
The reaction requires the presence of strong bases. The rate law with PhCHO is given by
Rate = k[PhCHO]2[OH-]
The slowest (rate-determining) step of the reaction is transfer of hydride ion.
Step I:
Description: 51778.png
Step II:
Description: 51787.png
Step III:
Description: 51797.png
It is evident from the mechanism that the species acting as hydride donor finally forms acid salt, while the one which accepts hydride will form primary alcohol.
Description: 49271.png
Intramolecular Cannizaro
Description: 51810.png

Addition of alcohols

Description: 51822.png
Description: 51830.png

Halogenation of ketones

Description: 49288.png
The rate of this reaction (in the presence of both acid and base) depends on the concentration of acetone and of base/acid but is independent of bromine concentration.
Rate = k[Acetone] [Base] or Rate = k′[Acetone] [Acid]

Aldol condensation

Description: 49298.png
Description: 49309.png
Step I:
Description: 51844.png
Step II:
Description: 51852.png
Step III:
Description: 51860.png
Crossed aldol condensation
Description: 51868.png
Intramolecular aldol condensation
An intramolecular reaction is readily favored if the reaction leads to the formation of a five or six-membered ring. When one of carbonyl group is an aldehyde and other is a ketone, it is the ketone which forms carbanion. This carbanion attacks the carbonyl group of an aldehyde in such a manner that five- or six-membered ring is formed.
Description: 51908.png
Description: 51969.png

Mixed Claisen condensation

Description: 51979.png
Description: 49318.png
Description: 49326.png

Dieckmann condensation

Description: 51997.png

Perkin reaction

Step I:
Description: 52010.png
Step II:
Description: 52019.png
Step III:
Description: 52027.png

Benzoin condensation

Description: 52048.png
Description: 52059.png

Baeyer–Villiger oxidation

Description: 49334.png
Description: 49343.png
Description: 49351.png

Beckmann rearrangement

Description: 52068.png
Description: 52077.png
Description: 52085.png

Pinacol–pinacolone rearrangement

Description: 49362.png
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Description: 52317.png

Periodic acid oxidation

Description: 49382.png
Description: 49393.png
Description: 49400.png
Description: 49407.png

Decarboxylation of β-keto carboxylic acids

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