Detailed Effects of Substituents-OCH 3 and -OR are also still strongly activating, but less so, because the electron density
is shared on both sides of the oxygen.
-CH 3 and -R in general provide some electron density sharing, but not nearly as much as a
pair of electrons. Thus their effect is only weakly felt.
For deactivating groups we have:
-NO 2 , or nitro and -NR 3 +. The nitro group is very strongly deactivating because of its
resonance structure. The nitro group has two resonance forms: O=N+-O-and O--N+=O.
Both of these forms leave a full positive charge on the nitrogen making it completely unable
to help stabilize the positive carbocation intermediate. The same applies to -NR 3 +.
-CF 3 and -CCl 3 both have an inductive electronegative effect of 3 halides, but with no
electrons to share with the ring, leaving them also very strongly deactivating.
-CN has a triple bond between the carbon and nitrogen with a resonance form of a double
bond between the carbon and nitrogen and a positive charge on the carbon, meaning that
between the electronegativity of the nitrogen and positively charged carbon in the resonance
form, it destabilizes the carbocation and offer no electrons to the ring.
-SO 3 , -COR, -CO 2 R - all of these have electronegative oxygens giving the carbon a positive
partial charge and providing no electrons for stability on the ring.
-F, -Cl, -Br, all have a similar effect. They are electronegative and deactivate the ring,
but have electrons to share that, to some degree, makes up for it, allowing the ortho/para
direction. But to understand their effects better, you need to look at them in terms of
their placement on the periodic chart. Florine is the most electronegative element and it’s
very small and thus very close to the carbon it’s bonded to. This gives its electromagnetic
influence a stronger deactivating character. Chlorine is less electronegative, but it’s also
larger and thus further away from the carbon, making it harder for it to share its electrons.
And so on.