Step 1: Scope of the reaction. Cannizzaro's reaction is a disproportionation (self-redox) reaction undergone by aldehydes that have no alpha-hydrogen atoms when treated with concentrated alkali. Step 2: Why only non-alpha-H aldehydes. Aldehydes with alpha-H atoms undergo aldol condensation preferentially with alkali. Only those lacking alpha-H (like formaldehyde or benzaldehyde) undergo Cannizzaro's reaction. Step 3: What happens. One molecule of aldehyde is oxidised to the sodium salt of the corresponding carboxylic acid, while simultaneously another molecule is reduced to the primary alcohol. Both changes occur in the same reaction vessel. Step 4: Reaction equations. \[ 2HCHO + NaOH~(conc) \xrightarrow{\Delta} CH_3OH + HCOONa \] \[ 2C_6H_5CHO + NaOH~(conc) \rightarrow C_6H_5CH_2OH + C_6H_5COONa \] One molecule is oxidised (to carboxylate salt) and one is reduced (to alcohol).