In chemistry, synthesis indicates a reaction by which small molecules are converted into larger molecules. For example, reaction of N2 with H2 to give NH3 is of the synthesis type. Students may be asked" How glucose is synthesized in the green leaves of plants?" .
If a chemist converts methanol (CH3OH) into bromomethane (CH3Br), the reaction ought to be described as preparation. There are chemists who depict this reaction as synthesis, mistakenly.