MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsNumerical
A chemist makes benzaldehyde from toluene in three steps…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A chemist makes benzaldehyde from toluene in three steps. Toluene is oxidised to benzoic acid in a yield of . The acid is turned into benzoyl chloride with thionyl chloride in a yield of . The acid chloride is reduced by the Rosenmund method in a yield of . The chemist starts with of toluene () and wants benzaldehyde (). Find the overall yield and the mass of benzaldehyde. Then find the mass of toluene needed for of benzaldehyde.
Need a reminder? The key ideaShow
Work backwards from the target and count the carbons first. Cyanide and RMgX with CO add one carbon, an aldol joins two molecules, and soda lime or the haloform reaction removes one. The overall yield of a route is the product of the step yields. The degree of unsaturation is . The tests are 2,4-DNP (any aldehyde or ketone), Tollens (aldehydes), Fehling (aliphatic aldehydes) and iodoform (CHCO-).
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Overall yield , which is .
Step 2. . Mass: .
Step 3. For of benzaldehyde: . Divide by to get of toluene, which is .
Answer: , and .
Check. Less than 100 % in every step, so the mass of toluene needed must be larger than the that a perfect route would take.
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