For the meta substituted carbocation resonance structures, there are three possible resonance froms that are secondary carbocations. These forms are not as stable as the tertiary carbocation form in the ortho and para substituted carbocations. Therefore, the two major products of the reaction of a monosubstituted benzene ring with an electron donating group and additional electrophile are the ortho and para positions.
It's important to note that the para product is slightly more common than the ortho product due to steric hindrance. H-NMR spectroscopy can be used to determine whether or not a compound has a second substituent at the ortho or para position.
At the ortho position there are four distinct signals, but for the para position there are only two signals because the molecule is symmetrical. Electron donating groups on a benzene ring are said to be activating , because they increase the rate of the second substitution so that it is higher than that of standard benzene. The other circumstance is when you have add an additional electrophile to a monosubstituted benzene ring with an electron withdrawing group on it.
Electron withdrawing groups have an atom with a slight positive or full positive charge directly attached to a benzene ring. Electron withdrawing groups only have one major product, the second substituent adds in the meta position.
Again, this can be explained by the resonance forms of the carbocation intermediates. When the second electrophile is added on to the benzene ring in the ortho position, the same three resonance forms of the carbocation are produced. Again, one form is a tertiary carbocation with the positive charge on the carbon directly attached to the electron withdrawing group.
Unlike in the case with an electron donating group, this resonance form is much less stable. This is due to the electron withdrawing group pulling away electrons from the carbon, creating an even stronger positive charge. This situation holds true for the para substituted tertiary carbocation resonace form as well.
For the meta position, all the carbocations formed are secondary. Although these are not entirely stable, they are more favored than the resonance forms of the ortho and para positions. Substituents with pi bonds to electronegative atoms e. The resonance decreases the electron density at the ortho- and para- positions.
Hence these sites are less nucleophilic, and so the system tends to react with electrophiles at the meta sites. Substituents with several bonds to electronegative atoms e. The net overall effect is similar to that described above for other electron withdrawing groups.
The reason is that they are both inductive electron withdrawing due to their electronegativity but they are also resonance donating lone pair donation. The inductive effect lowers the reactivity of the starting material but the resonance effect controls the regiochemistry due the stability of the intermediate carbocations. Ian Hunt , Department of Chemistry. Intro Lesson: e. Intro Lesson: f. Intro Learn Practice. Do better in math today Get Started Now. How do organic reactions occur?
Nucleophiles and electrophiles 3. Arrow pushing curly arrows in organic chemistry 4. Electron-withdrawing and donating effects 5. Conjugation and resonance structures Back to Course Index. Don't just watch, practice makes perfect. To understand the underlying electronic effects that produce these properties. To apply mesomeric and inductive effects to predict nucleophile and electrophile strength. Notes: When running an organic reaction to make a desired product, the reactivity of both the electrophile and nucleophile need to be thought about.
The more electron rich a nucleophile, or electron poor the electrophile, the better. These conditions will lead to greater reactivity and a higher product yield; the smaller the HOMO-LUMO gap, the greater tendency for reactant bonds to break and product bonds to form.
By increasing electron density on adjacent carbon atoms, EDGs change the reactivity of a molecule: EDGs make nucleophiles stronger. With EDGs attached, a nucleophilic center is even more electron rich and ready to attack electrophilic sites.
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