Ethanal is a two-carbon aldehyde, and propanone is a three carbon ketone. The first step is to make a Carbon-Carbon bond, and the Grignard reagent would be the best choice. The aldehyde ethanal is less of a methyl carbon so to introduce methyl substituent, methyl magnesium bromide is preferred. The carbon of the Grignard reagent is a nucleophile (CH3-) and will effortlessly attack the electrophilic carbonyl carbon (-CHO) of the aldehyde.
The attack of the nucleophile (-CH3-) will maintain the tetravalency of the carbon by pushing the pie (π) electrons to the electronegative oxygen forming a negatively charged anion . The negative anion will attract the electropositive magnesium halide to balance the charges and form a salt. The magnesium salt formed would be then subjected to hydrolysis to give isopropyl alcohol (2o alcohol) as the product. The formation of the alcohol is the first step for the conversion to the ketone.
The alcohol formed is then oxidized to the ketone by oxidizing agents such as- Na2Cr2O7, CrO3, KMnO4, PCC. Upon oxidation, propanone is obtained as the final product.