Question

3. In order to make Ethylbenzene, it is always a good way to use a Friedel Craft acylation rather than a Friedel Craft alkylation. Explain why? (5)

Answer 1

Ans: The use of Friedel Craft Acylation over Friedel Craft alkylation in the preparation of Ethylbenzene can be understood by their mechanism:

Friedel Craft alkylation:

+H + AlCl3 E- + Br + CH2CH3 group activates the benzene ring and the polyalkylation occurs

As we can see that the ethyl group attached to the benzene ring after Friedel Craft alkylation is +I and +H group and it activates the benzene ring to the further attack of the electrophile by increasing the electron density of the ring. Thus the Friedel Craft reaction occurs further and in the end, we obtain polyalkylated products.

Whereas, in Friedel Craft acylation, we have:

AlCl3 - + -M Zn(HgYHCI The C=Ogroup is -Mgroup and deactivates the benzene ring for the further attack of electrophile.

After the first step of Friedel Craft Acylation, the C=O group attached to the product is -M group and decreases the electron density in the benzene ring and ultimately deactivating the ring for the further attack of the electrophile. So, no more acylation takes place and we get a single substituted product. The acylated product can then be reduced by Zn(Hg)/HCl to obtain ethylbenzene.

Hence, to avoid the poly alkylation, the Friedel Craft Acylation is preferred over Friedel Craft Alkylation to synthesize ethylbenzene.