The nitro group is a very strong deactivating group. Nitrobenzene undergoes nitration at a rate only `10^(-4)` times that of benzene. The nitro group is a meta director. When nitrobenzene is nitrated with nitric acid and sulfuric acids, `93%` of the substitution occurs at the
meta position.
The carboxyl group `(-CO_2H)`, the sulfo group `(-SO_3H)`, and the trifluoromethyl group `(- CF_3)` are also deactivating groups; they are also meta directors .

The chloro and bromo groups are weak deactivating groups. Chlorobenzene and bromobenzene undergo nitration at rates that are, respectively, `33` and `30` times slower than for benzene. The chloro and bromo groups are ortho - para directors, however. The relative percentages of monosubstituted products that are obtained when chlorobenzene is chlorinated, brominated, nitrated, and sulfonated are shown in Table 1. Similar results are obtained from electrophilic substitutions of bromobenzene.

We have now seen that certain groups activate the benzene ring towards electrophilic substitution, while other groups deactivate the ring. When we say that a group activates the ring, what we mean, of course, is that the group increases the relative rate of the reaction. We mean that an aromatic compound with an activating group reacts faster in electrophilic substitutions than benzene. When we say that a group deactivates the ring, we mean that an aromatic compound with a deactivating group reacts slower than benzene. We have also seen that we can account for relative reaction rates by examining the intermediate state for the rate-determining steps. We know that any factor that increases the energy of the intermediate state relative to that of the reactants decreases the relative
rate of the reaction. It does this because it increases the free energy of activation of the reaction. In thesame way, any factor that decreases the energy of the intermediate state relative to that of the reactants lowers the free energy of activation and increases the
relative rate of the reaction. The rate-determining step in electrophilic substitutions of substituted benzenes is the step that results in the
formation of arenium ion. We can write the formula for a substituted benzene in a generalized way if we use the letter `S` to represent any ring substituent including hydrogen. (If `S` is hydrogen the compound is benzene itself). We can also write the structure for the renium ion in the way shown here. By this formula we mean that `S` can be at any position - ortho, meta, or para - relative to the electrophile, `E^(+)` . Using these conventions, then, we are able to write the ratedeterming step for electrophilic aromatic substitution in the following general way. When we examine this step for a large number of reactions, we find that the relative rates of the reactions depend on whether S withdraws or release electrons. If S is an an electron - releasing group (relative to hydrogen), the reaction occurs faster than the
corresponding reaction of benzene. If `S` is an electron withdrawing group, the reaction occurs slower than that of benzene.

We can account for the electron-withdrawing and electron - releasing properties of a group on the basis of two factors: inductive effect and resonance effects. We shall also see that these two factors determine orientation in aromatic substitution reactions. The inductive effect of a substituent `S` arises from the electrostatic interaction of the polarized `S` to ring bond with the developing positive charge in the ring as it is attacked by an electrophile. If, for example, `S` is a more electronegative atom (or group) than carbon, then the ring will be at the end of the dipole:
Attack by an electrophile will be retarded because this will lead to an additional full positive charge on the ring. The halogens are all more electronegative than carbon and exert an electron - withdrawing inductive effect. Other groups have an electron - withdrawing inductive effect because the atom directly attached to the ring bears a full or partial positive charge. Examples are the following:
Then resonance effect of a substituent `S` refers to the possibility that the presence of `S` may increase or decrease the resonance stabilization of the intermediate areniumion. The `S` substituent may, for example cause one of the three contributors to the resonance hybrid for the areniumion to be stabilized or destabilized than the case when `S` is hydrogen. Moreover, when `S` is an atom bearing one or more nonbonding electron pairs, it may lend extra stability to the areniumion by providing a fourth resonance contributor in which the positive charge resides on `S`.This electron-donating resonance effect applies with decreasing strength in the following order:
This is also the order of the activating ability of these groups. Amino groups are highly activating, hydroxyl and alkoxyl groups are somewhat less activating, and halogen substituents are weakly deactivating. When `X= F`, this order can be related to the lectronegativity of the atoms with the nonbonding pair. The more electronegative the atom is the less able it is to accept the positive charge (f luorine is the most electronegative, nitrogen the least). When `X = Cl, Br`, or `I`, the relatively poor electron-donating ability of the halogens by resonance is understandable on a different basis. These atoms (`CI, Br`, and `I`) are all larger than carbon and, therefore, the orbitals that contain the nonbonding pairs are further from the nucleus and do not overlap well with the `2p` orbital of carbon. (This is a general phenomenon: resonance effects are not transmitted well between atoms of different rows in the periodic table.)