what does alpha hydrogen means..????

explain cannizaro reaction...how do we come to know HCHO makes HCOOK on oxidation

The carbon atom attached very next to a functional group is known as alpha carbon atom. The hydrogen atoms attached to this carbon atom are termed as alpha hydrogen atoms. As an example consider glycine, an amino acid which has the following structure

The functional group is COOH. The carbon atom just next to this COOH group is the alpha carbon atom. There are are two hydrogen atoms attached to this alpha carbon atom, and hence both are alpha hydrogen atoms. Cannizarro reaction is given by only those aldehydes which do not contain an alpha hydrogen, eg : benzaldehyde, formic acid etc. Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction reaction on treatment with a concentrated alkali.

The presence of HCOOH can be easily detected. When the salt of formic acid is treated with dil. HCl, then formic acid is regenerated. On treatment with sodium bicarbonate sodium formate would be formed and brisk effervescence would be observed.

 HCOOK + HCl --------> HCOOH + KCl

 HCOOH + NaHCO₃ ---------> HCOONa + H₂O + CO₂ (↑)  

This effervescence released proves that HCOOK is formed in the Cannizaro reaction.