Why nitration of toluene is easier than benzene?

As you know from the structures of both the compounds that toluene has a methyl group on the benzene ring which is electron releasing group and hence activate the benzene ring by pushing the elctrons on the benzene ring. On the other hand nitro group on the benzene ring is electron withdrawing group which deactivates the benzene ring by withdrawing the electrons from the benzene ring . Now in the nitration attack of the nucleophile ( NO2 +) takes place. Hence reaction will takes place on that benzene faster which have more electron density on its ring which is the case of toluene.

Why is that the temperature of mono nitration of benzene be maintained at 50 degree Celsius?

So that only one NO2+ is substituted for the hydrogen on one of the carbons in benzene , if it was above 50 degree Celsius then more than one hydrogen would be substituted and mono nitration is only one substitution of hydrogen for NO2+

Why phenol have more intensity than benzene?

Because phenols have a lone pair of electron on the oxygen atom and this is drawn into the benzene ring thereby increasing the electron density of the ring which attracts more electrophiles.

IS chlorobenzene more reactive than benzene or not?

Chlorobenzene is less reactive than benzene as this group's -I effect( electron withdrawing effect) is more than its +R effect ( electron donating effect ) which in turn, actually decreases the electron density of the benzene ring and make the aromatic compound less reactive towards electrophiles.