Chem 316 Notes-2 H. D. Roth Resonance and Inductive Effects in Aromatic Compounds

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1 hem 316 otes-2. D. Roth Resonance and Inductive Effects in Aromatic ompounds In order to understand the patterns of aromatic substitution it is imperative to be familiar with the mechanisms by which substituents direct the incoming substituents. The substitution is governed by resonance effects and by inductive effects. Let s begin with resonance effects. We differentiate between resonance electron-donating and resonance electron-withdrawing groups in conjugation with an aromatic ring. Typical electron-withdrawing groups (e.g., O2, SO3,, OOR, (=O)R), generate partial positive charges in the o- and p-positions (note the curved arrows), whereas electron-donationg groups (e.g., R2, OR, F, l, I) cause partial negative charges in the o- and p-positions. The mechanism of delocalization (resonance) leaves the m-positions of either type unaffected. ( 3 ) 2 ( 3 ) 2 Because the typical aromatic substitution is electrophilic, it follows that electron-donating groups direct the substitution to the o- and p- positions, because the partial negative charges favor this approach. In addition, the rates of the o- and p-substitutions are enhanced relative to the reaction of benzene. These electronic effects are kinetic in nature. Once the electrophile has been added, we consider the stability of the resulting carbocation: it is most stable if it is conjugated with the ED group. This is a thermodynamic effect. Both kinetic and thermodynamic effects work in the same direction. Only the case of the ortho-substitution is shown below; you may want to go over the corresponding m- and p-substitution. ( 3 ) 2 ( 3 ) 2 1

2 hem 316 otes-2. D. Roth In contrast, resonance electron withdrawing groups disfavor the substitution in the o- and p-positions. Therefore, by default, they direct the substitution to the m-positions, however at a reduced rate (because of the nearby partial positive charges). 2 Once the electrophile has been added (in the m-position), the resulting carbocation has three resonance contributors; in none of them is the positive charge conjugated with the EW group. You may want to review the corresponding substitution in the o- and p-positions. They are unfavorable because the positive charge conjugated with the EW group. Inductive effects are strongly distance dependent. Their effect is strongest in the o-position and falls off toward the m- and p-positions. D A Alkyl groups directly attached to the aromatic ring are inductively electron-donating (remember hyperconjugation?). All groups containing electron pairs are inductively electron-withdrawing, including F3, R2, OR, F, l,, I (in addition to those that are also electron-withdrawing by resonance, i.e., O2, SO3,, OOR, (=O)R). In addition to directing the regiochemistry of electrophilic aromatic substitution, the different substituents, particularly the pattern of positive or negative charges induced by them, also have a profound effect on the MR 2

3 hem 316 otes-2. D. Roth spectra of aromatic compounds. Recall the drawing below, showing how the electrons of a nucleus generate different electronic environments for an adjacent 1 nucleus (or group of nuclei). This effect also applies to the 1 nucleus itself. Because the bonding electrons induce a field, that at the nucleus opposes the external field, they shield the 1 nucleus. On the other hand, a 1 nucleus without an electron (a proton, ) does not experience an induced magnetic field: is OT shielded. The chemical shift for a carboxylic acid 1 nucleus is strongly deshielded, to ppm. From this observation we can deriive that a positive charge at or near the nucleus deshields whereas a negative charge shields. We begin by looking at the effects of electron-donating or withdrawing groups in conjugation with a double bond; these effects can be explained or predicted by delocalization or resonance considerations. For example, oxacyclohexene is resonance-stabilized by electron donation, creating a negative charge on the β alkene carbon, thereby shielding the attached 1 nucleus (4.65 ppm). In contrast, cyclohexenone is resonance-stabilized by the electron withdrawing carbonyl function. The positive charge created on the β alkene carbon deshields the attached 1 3

4 hem 316 otes-2. D. Roth nucleus (6.88 ppm). These considerations allow us to rationalize and assign the divergent chemical shifts of these compounds. O O O O 4.65 ppm 6.37 ppm 5.93 ppm 6.88 ppm We can also check the chemical shifts of cycloheptatrienyl cation (9.2 ppm) and cyclopentadienyl anion (5.5 ppm). Applying this concept to the electron-donating or withdrawing groups in conjugation with an aromatic ring, we conclude that typical electronwithdrawing groups (e.g., O2, ), cause incrementally deshielded MR shifts in the o- and p-position, whereas electron-donating groups (e.g., R2, OR) cause incremental shielding effects in the o- and p-position. The mechanism of delocalization (resonance) leaves the m-positions of either type unaffected. ( 3 ) 2 ( 3 ) 2 ow consider an aromatic compound with a resonance electronwithdrawing group, i.e., nitrobenzene. The spectrum has three signals at ~6.5, 6.7, and 7.1 ppm. Please note that all resonances are upfield, less deshielded than benzene (7.23 ppm). ow do we assign these resonances? ( 3 ) 2 ( 3 ) 2 4

5 hem 316 otes-2. D. Roth o m O 2 p o m Please note that there are two o- and two m-nuclei, and one p-nucleus. The o-nuclei have only one nearest neighbor whereas the m- and p-nuclei each have two nearest neighbors. Thus, the signal appearing as a doublet must represent the o-s, the signals appearing as triplets must be the m-, and p-nuclei. One of these is obviously weaker than the other: the signal at 6.7 ppm must represent the p-nucleus. Given this assignment, we note that the o- and p-1 nuclei are significantly shielded and even the m-1 nuclei are slightly shielded relative to the benzene 1 nuclei (7.25 ppm). We ascribe these chemical shifts to the partial negative charges (causing shielding) in the o- and p-positions. The spectrum of,-dimethylaniline, an aromatic compound bearing a resonance electron-donating substutuent, is significantly different. 5

6 hem 316 otes-2. D. Roth o m ( 3 ) 2 p o m Again, the spectrum has three signals, at ~7.5, 7.7, and 8.2 ppm; all resonances are downfield, more deshielded, than that of benzene (7.23 ppm). We assign these resonances on similar considerations as above. The doublet (8.2 ppm) must represent the o-s, the triplets the m-, and p-nuclei. The weaker triplet (7.7 ppm) must represent the p-nucleus. The o-1 nuclei are significantly deshielded whereas the p- nucleus and even the m-1 nuclei are slightly deshielded. We ascribe these chemical shifts to the partial positive charges (causing deshielding) in the o- and p-positions. The significantly greater effect on the o-nuclei must be an inductive effect. 6

Background A nucleus with an odd atomic number or an odd mass number has a nuclear spin that can be observed by NMR spectrometers.

Background A nucleus with an odd atomic number or an odd mass number has a nuclear spin that can be observed by NMR spectrometers. NMR Spectroscopy I Reading: Wade chapter, sections -- -7 Study Problems: -, -7 Key oncepts and Skills: Given an structure, determine which protons are equivalent and which are nonequivalent, predict the