A student wants to make propylbenzene, C₆H₅CH₂CH₂CH₃, from benzene…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student wants to make propylbenzene, , from benzene. She first tries a Friedel-Crafts reaction with 1-chloropropane and anhydrous aluminium chloride. The main product is not propylbenzene but the isomer with a branched side chain, and the two are hard to separate. Her teacher says that a two-step route will work.
In the first step, benzene reacts with propanoyl chloride, , and anhydrous aluminium chloride to give propiophenone, . In the second step, the ketone is reduced to propylbenzene with zinc amalgam and hydrochloric acid.
She starts with of benzene, which is . She keeps the glassware dry and adds the aluminium chloride in portions, because it reacts with water. The first step has a yield of . The second step has a yield of . The molar mass of propylbenzene is .
- Why does the direct route fail? (b) Find the moles of propiophenone and the mass of propylbenzene.
Need a reminder? The key ideaShow
Electrophilic substitution of benzene: nitration (conc. and conc. ), halogenation ( with ), sulphonation (oleum), Friedel-Crafts alkylation and acylation ( or with anhydrous ). Alkylation can rearrange; acylation cannot. Sulphonation is reversible.
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. (a) The primary propyl cation shifts a hydrogen atom and becomes the secondary isopropyl cation. That cation gives isopropylbenzene. An acylium ion does not rearrange.
Step 2. Moles of ketone. of propiophenone.
Step 3. Moles of propylbenzene. .
Step 4. Mass. .
Answer: the direct route rearranges. The two-step route gives of ketone and of propylbenzene.
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