Our teacher told us that five membered ring can’t be formed through Friedel-Crafts alkylation but acylation is OK. Why is that? Is it because alkylation is an unfavored 5-endo-trig reaction but acylation is 5-endo-dig, thus favored?
Maybe your teacher means to say that because further reactions would happen due to the further activation of the ring due to the alkyl group as opposed to the deactivation due to the acyl group. Also further catalyst induced polymerization reaction can occur apart from possible polyalkylation
In general alkylation on rings are not preferred because of the above reasons as well