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16 -- Part 2: AROMATIC COMPOUNDS
Sulfur trioxide is a powerful oxidizer.
An aromatic ring is regenerated when a protons is lost.
The sulfonate group may become acidic.
alkylbenzene sulfonates are widely used as detergents.
A alkylbenzenesulfonic acid is given by the shponation of an alkylbenzene.
Section 25-4 covers detergents in more detail.
The dipolar sigma complex shown in the sulfonation of benzene has its positive charge and negative charge delocalized over three carbon atoms.
The aromatic ring is given by the first synthetic detergents of sulfur trioxide.
SO3 is removed from had branched alkyl groups.
The equilibrium is made up of hydrating it to sulfuric acid.
The sigma complex can lose either of the two protons if benzene is attacked.
We can show that the product has a deuterium atom in place of hydrogen by using a deuterium ion.
Adding SO3 to some D2O will generate D2SO4.
Benzene gives a deuterated product.
The final products reflect the D/H ratio of the solution.
A large amount of deuterium gives a product with all six of the benzene hydrogens.
Until now, we have only considered benzene as a substitution for aromatics.
If we want to make more complicated aromatic compounds, we need to consider the effects other substituents might have.
Under the same conditions, toluene reacts 25 times faster than benzene.
substitution at the ortho and para positions give a mixture of products.
The product ratios show that the orientation of substitution is not random.
There would be equal amounts of ortho and meta substitution and half as much para substitution.
The prediction is based on the two ortho positions, two meta positions, and one para position available for substitution.
The first step in forming the sigma complexample is matic substitution.
The enhanced reaction rate and the preference for ortho and para substitution by Ultrasuss is considering the structures of the intermediate sigma complexes.
There was no positive charge distributed over the carbon atoms.
The sigma complexes for ortho and para attack are more stable than the sigma complex for nitration of benzene.
The intermediate for substitution of benzene has the same positive charge spread as the sigma complex for meta substitution.
The large rate enhancement seen with ortho and para substitution is not shown in the meta substitution of toluene.
The rate-limiting transition state leads to the formation of the methyl group in toluene.
When the positive charge is delocalized onto the tertiary carbon atom, the stabilizing effect is large.
When substitution occurs at the meta position, the positive charge is not delocalized onto the tertiary carbon, and the methyl group has a smaller effect on the stability of the sigma complexample.
The states leading to them are CH3.
The other substituent that is activated by the benzene ring is the methyl group.
In the next section, we look at groups that have the opposite effect.
The results with toluene are general for any alkylbenzene.
A transition state and intermediate with a positive charge shared by the tertiary carbon atom can be achieved with substitution ortho or para.
The products of alkylbenzenes are mostly ortho and para-substituted.
Another example of an aromatic substitution that is enhanced by stabilization is the reaction of ethylbenzene with bromine.
With respect to the meta isomer, the rates of formation of the ortho- and para-substituted isomers are greatly enhanced.
The sigma complex is lower in energy for substitution at the ortho and para positions than it is for substitution at the meta position.
Styrene undergoes an aromatic substitution much faster than benzene, and the products are mostly ortho- and para-substituted.
The results can be explained using resonance forms of the intermediates.
The nitration of methoxybenzene is about 10,000 times faster than that of benzene and 400 times faster than toluene.
Oxygen is a strongly negative group, yet it donates electron density to stabilizing the transition state and the sigma complexample.
The EPM of anisole shows the aromatic ring to be electron-rich, consistent with the observation that anisole is only six each atom has eight strongly activated toward reactions.
The second resonance form puts a positive charge on the oxygen atom, but it has more covalent bonds, and it provides each atom with an octet in its valence shell.
The ortho and para positions are activated by the methoxy group of anisole.
If the sigma complex is para to the site of substitution, the methoxy group can be effective in stabilizing it, but not if it is meta.
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