Aromatic Substitution. How do we know where an additonal substituent will most likely be placed? The answer to this is through inductive and resonance effects. Inductive effects are directly correlated with electronegativity. Substituents can either be meta directing or ortho-para directing. Introduction The three general positions of a disubstituted benzene ring are ortho, meta and para. Figure 1: The Effect of an Electron Donating Groups on a Benzene Ring The first scenario for adding an electrophile to a monosubstituted benzene ring is when the substituent is an electron donating group. Electron donating groups are alkyl groups, phenyl groups or substituents that have a lone pair of electrons on the atom directly bonded to the ring. Electron donating groups are donating by induction (Activating and Deactivating Benzene Rings) and resonance. Examples of electron donating groups: -CH3, -OCH3, -OH, -NH2 Electron donating groups cause the second subtituent to add on to the para or ortho position on the benzene ring. The reason for this can be explained by the different carbocation resonance structures of the ortho, meta and para positions.

deactivating groups direct electrophiles to attack the benzene molecule at the ortho- and para- positions, while strongly and moderately deactivating groups direct In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or functional group that donates some of its electron density into a conjugated In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance (mesomerism) or inductive effects (or induction)—called +M or +I effects, respectively—thus making the π system more nucleophilic. As a result of these electronic effects, an aromatic ring to which such a group is attached is more likely to participate in electrophilic substitution reaction. EDGs are therefore often known as activating groups, though steric effects can interfere with the reaction. An electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. The EWG removes electron density from a π system, making it less reactive in this type of reaction, and therefore called deactivating groups. EDGs and EWGs also determine the positions (relative to themselves) on the aromatic ring where substitution reactions are most likely to take place. Electron donating groups are generally ortho/para directors for electrophilic aromatic substitutions, while electron withdrawing groups (except the halogens) are generally meta directors. The selectivities observed with EDGs and EWGs were first described in 1892 and have been… Electron donating groups are typically divided into three levels of activating ability (The "extreme" category can be seen as "strong".) Electron withdrawing groups are assigned to similar groupings. Activating substituents favour electrophilic substitution about the ortho and para positions. Weakly deactivating groups direct electrophiles to attack the benzene molecule at the ortho- and para- positions, while strongly and moderately deactivating groups direct attacks to the meta- position. This is not a case of favoring the meta- position like para- and ortho- directing functional groups, but rather disfavouring the ortho- and para-positions more than they disfavour the meta- position. Oppos…

The first thing to recognize is that the proportions of ortho, meta and para substitution in a given case reflect the relative rates of substitution at each of these sites. If we use the nitration of benzene as a reference, we can assign the rate of reaction at one of the carbons to be 1.0. Since there are six equivalent carbons in benzene, the total rate would be 6.0. If we examine the nitration of toluene, tert-butylbenzene, chlorobenzene and ethyl benzoate in the same manner, we can assign relative rates to the ortho, meta and para sites in each of these compounds. These relative rates are shown (colored red) in the following illustration, and the total rate given below each structure reflects the 2 to 1 ratio of ortho and meta sites to the para position. The overall relative rates of reaction, referenced to benzene as 1.0, are calculated by dividing by six. Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring. From rate data of this kind, it is a simple matter to calculate the proportions of the three substitution isomers.