why electrophilic substitution in C6H5CHO occurs mainly at meta position to the CHO group?

Answer 1

Take a look at resonance within the benzene ring. You see that the carbonyl attached is an electron withdrawing group to the aromatic ring. This would cause a partially (+) charge to occur on that carbon on the ring (carbon 1).

So, by adding a group to meta position on either side of the ring (carbon 3) you cause a positive charge to form (and resonate) between carbon 2, 4, and 6. If you add the group to either ortho or para position, you will get resonance between carbon 1, 3 and 5.

If 2 positive charges are near each other, will they be stable? Not really. This happens when you add a group at the meta position.

This all depends on how positive or negative the attached group is. Because the carbonyl is electron withdrawing, it becomes partially (+). If you add a -NH2 group, this group has electrons to donate, so the opposite happens: Your added group will attach to either ortho- or para-.

Answer 2

-CHO deactivates the ring by withdrawing electrons from the Benzene ring. Meta position is deactivated the least.