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 Iodoform Reaction

The iodoform test is a test for the presence of carbonyl compounds with the structure  RCOCH3  and alcohols with the structure  RCH(OH)CH3.

A solution of  I2  is added to a small amount of your unknown, followed by just enough  NaOH  to remove the colour.  

The formation of a pale yellow precipitate of iodoform (with a characteristic "antiseptic" smell) is a positive result.

Aldol condensation

 Aldol condensation: In the presence of a dilute alkali, aldehydes and ketones (having at least one α-hydrogen atom) produce β-hydroxyl aldehyde (aldol) and β-hydroxyl ketone respectively. The aldol and ketol then readily lose water to give α, β-unsaturated carboxyl compounds. Such reactions are called aldol condensation.  

For example, in the presence of dil. NaOH, ethanal (aldehyde) produces 3-hydroxybutanal (aldol), which then readily loses water to produce but-2-enal.

Cannizzaro reaction

Cannizzaro reaction: Aldehydes having no α-hydrogen atoms undergo self oxidation and reduction (disproportionation) reactions on treatment with a concentrated alkali. In such reactions, one molecule of aldehyde gets oxidised to form an acid and the other molecule of aldehyde gets reduced to form an alcohol.  

For example, two molecules of formaldehyde, in the presence of concentrated NaOH, produce methanol and sodium formate.

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