Problem with the Friedel Crafts Rxn( Alkylation)
Alkyl groups are electron-releasing and so create a positive inductive effect(+I) when attached to a benzene ring. This means that, with an alkyl group attached, a benzene ring will have a greater electron density than when there is no much group attached. Hence benzene ring will be more attractive to electrophiles, and thus more reactive.
If the product of the reaction is more reactive than the starting material, then the product will react in preference to the un-alkylated starting material. A mixture of mono- , di- and tri-alkylated (and even, possibly tetra- derivatives) will tend to be formed, This reduces the yield of the desired product.
SOURCE: CHEMISTRY FACTSHEET