Although the full electronic structure of an arene can only be computed using
quantum mechanics, the directing effects of different substituents can often be guessed through analysis of
resonance diagrams. Specifically, any formal negative or positive charges in minor resonance contributors (ones in accord with the natural polarization but not necessarily obeying the
octet rule) reflect locations having a larger or smaller density of charge in the
molecular orbital for a bond most likely to break. A carbon atom with a larger coefficient will be preferentially attacked, due to more favorable orbital overlap with the electrophile. The perturbation of a conjugating electron-withdrawing or electron-donating group causes the π electron distribution on a benzene ring to resemble (
very slightly!) an electron-deficient benzyl cation or electron-excessive benzyl anion, respectively. The latter species admit tractable quantum calculation using
Hückel theory: the cation withdraws electron density at the
ortho and
para positions, favoring
meta attack, whereas the anion releases electron density into the same positions, activating them for attack. This is precisely the result that the drawing of resonance structures would predict. For example,
aniline has resonance structures with negative charges around the ring system: can donate electron density through
resonance.|394x394px|center|frameless Attack occurs at
ortho and
para positions, because the (partial) formal negative charges at these positions indicate a local electron excess. On the other hand, the
nitrobenzene resonance structures have positive charges around the ring system: can withdraw electron density through
resonance.|center|framelessAttack occurs at the
meta position, since the (partial) formal positive charges at the
ortho and
para positions indicate electron deficiency at these positions. Another common argument, which makes identical predictions, considers the stabilization or destabilization by substituents of the Wheland intermediates resulting from electrophilic attack at the
ortho/
para or
meta positions. The Hammond postulate then dictates that the relative transition state energies will reflect the differences in the ground state energies of the Wheland intermediates.
Carbonyls, sulfonic acids and nitro Because of the full or partial positive charge on the element directly attached to the ring for each of these groups, they all have a moderate to strong electron-withdrawing inductive effect (known as the -I effect). They also exhibit electron-withdrawing resonance effects, (known as the -M effect): Thus, these groups make the aromatic ring very electron-poor (δ+) relative to benzene and, therefore, they strongly deactivate the ring (i.e. reactions proceed much slower in rings bearing these groups compared to those reactions in benzene.) ===
Anilines,
phenols, and
ethers (such as
anisole) === Due to the electronegativity difference between carbon and oxygen / nitrogen, there will be a slight electron withdrawing effect through
inductive effect (known as the –I effect). However, the other effect called resonance add electron density back to the ring (known as the +M effect) and dominate over that of inductive effect. Hence the result is that they are EDGs and
ortho/
para directors. Phenol is an ortho/para director, but in a presence of base, the reaction is more rapid. It is due to the higher reactivity of
phenolate anion. The negative oxygen was 'forced' to give electron density to the carbons (because it has a negative charge, it has an extra +I effect). Even when cold and with neutral (and relatively weak) electrophiles, the reaction still occurs rapidly.
Alkyl groups Alkyl groups are electron donating groups. The carbon on that is
sp3 hybridized and less electronegative than those that are
sp2 hybridized. They have overlap on the
carbon–hydrogen bonds (or
carbon–carbon bonds in compounds like
tert-butylbenzene) with the ring p orbital. Hence they are more reactive than benzene and are
ortho/
para directors.
Carboxylate Inductively, the negatively charged
carboxylate ion moderately repels the electrons in the bond attaching it to the ring. Thus, there is a weak electron-donating +I effect. There is an almost zero -M effect since the electron-withdrawing resonance capacity of the carbonyl group is effectively removed by the delocalisation of the negative charge of the anion on the oxygen. Thus overall the
carboxylate group (unlike the
carboxyl group) has an activating influence.
Alkylammonium and trifluoromethyl group These groups have a strong electron-withdrawing inductive effect (-I) either by virtue of their positive charge or because of the powerfully electronegativity of the halogens. There is no resonance effect because there are no orbitals or electron pairs which can overlap with those of the ring. The inductive effect acts like that for the carboxylate anion but in the opposite direction (i.e. it produces small positive charges on the
ortho and
para positions but not on the
meta position and it destabilises the
Wheland intermediate.) Hence these groups are deactivating and
meta directing:
Halides' competing effects Induction versus resonance Fluorine is something of an anomaly in this circumstance. Above, it is described as a weak electron withdrawing group but this is only partly true. It is correct that fluorine has a -I effect, which results in electrons being withdrawn inductively. However, another effect that plays a role is the +M effect which adds electron density back into the benzene ring (thus having the opposite effect of the -I effect but by a different mechanism). This is called the
mesomeric effect (hence +M) and the result for fluorine is that the +M effect approximately cancels out the -I effect. The effect of this for
fluorobenzene at the
para position is reactivity that is comparable to (or even higher than) that of
benzene. Because inductive effects depends strongly on proximity, the
meta and
ortho positions of fluorobenzene are considerably less reactive than benzene. Thus, electrophilic aromatic substitution on fluorobenzene is strongly
para selective. This -I and +M effect is true for all halides - there is some electron withdrawing and donating character of each. To understand why the reactivity changes occur, we need to consider the
orbital overlaps occurring in each. The
valence orbitals of fluorine are the 2p orbitals which is the same for carbon - hence they will be very close in energy and
orbital overlap will be favourable.
Chlorine has 3p valence orbitals, hence the orbital energies will be further apart and the geometry less favourable, leading to less donation the stabilize the carbocationic intermediate, hence
chlorobenzene is less reactive than
fluorobenzene. However,
bromobenzene and
iodobenzene are about the same or a little more reactive than chlorobenzene, because although the resonance donation is even worse, the inductive effect is also weakened due to their lower electronegativities. Thus the overall order of reactivity is U-shaped, with a minimum at chlorobenzene/bromobenzene (relative nitration rates compared to benzene = 1 in parentheses):
PhF (0.18) >
PhCl (0.064) ~
PhBr (0.060) 13 times more acidic than hydrofluoric acid)
Directing effect Due to the lone pair of electrons, halogen groups are available for donating electrons. Hence they are therefore
ortho /
para directors.
Nitroso group Induction Due to the electronegativity difference between carbon and nitrogen, the nitroso group has a relatively strong -I effect, but not as strong as the nitro group. (Positively charged nitrogen atoms on alkylammonium cations and on nitro groups have a much stronger -I effect)
Resonance The nitroso group has both a +M and -M effect, but the -M effect is more favorable. Nitrogen has a lone pair of electrons. However, the lone pair of its monomer form is unfavourable to donate through resonance. Only the dimer form is available for +M effect. However, the dimer form is less stable in a solution. Therefore, the nitroso group is less available to donate electrons. Oppositely, withdrawing electron density is more favourable: (see the picture on the right).As a result, the nitroso group is a deactivator. However, it has available to donate electron density to the benzene ring during the
Wheland intermediate, making it still being an
ortho / para director. ==
Steric effects ==