Why is it difficult to limit Friedel-Crafts alkylation reaction of benzene to monoalklyation step ?

Explanation

If in the borax bead test in oxidising flame a metal salt forms blue coloured bead in hot condition, green coloured bead in cold condition and red bead in reducing flame, then it is certain that the salt under investigation is a copper salt.
The reactions taking place during the borax bead test of copper sulphater are are as follows:
`CuSO_4.5H_2O overset (Delta)rarr CuSO_(4)+5H_2O uarr`
`2CuSO_4 rarr 2CuO+2SO_2+O_2 uarr`
In oxidising flame: `CuO+B_2O_3 underset("cupric metaborate (blue)")(rarr Cu(BO_2)_2)`
In reducing flame:
`2Cu(BO_2)_2 rarr +C overset(Delta)rarr underset("curprous metaborate ")(Cu_(2)(BO_2)_2)+CO+B_2O_3`
`Cu_(2)(BO_2)_(2)+C overset(Delta)rarr underset(underset("opaque red")"metallic copper")(2Cu+B_2O_3+CO)`