21.1.3—Analyse

For every arrow in a synthesis, identify the starting functional group, reaction type, reagent/conditions, product functional group and any by-products. Then check atom and charge balance.

Common by-products include H₂O in condensation, HBr or HCl in substitution, and inorganic salts after neutralisation or hydrolysis. A plausible route must explain where they go.

Ethanol → bromoethane with PBr₃ is substitution; bromoethane → propanenitrile with KCN is substitution and adds one carbon; hydrolysis then gives a carboxylic acid.

A correct product name alone is not enough: a route can be impossible if the reagent cannot cause the stated functional-group change.