Figure 17-4 shows the effect of the halogen atom graphically, with an energy para-directing, and deactivators diagram comparing energies of the transition states and intermediates for electrophilic are meta-directing, except for the attack on chlorobenzene and benzene.
The reactions of halogens need higher energies.
The intermediate for meta substitution is less stable than the other two.
The reactivity of an aromatic ring is affected by two or more substituents.
The result is easy to predict if the groups reinforce each other.
The two methyl groups are both activated, so we can predict that all the xylenes are activated.
In the case of a nitrobenzoic acid, both substituents are deactivating, so we think that the acid is going to be attacked.
It is easy to predict the orientation of addition.
There are two different positions for xylene, one ortho to one of the groups and the other para to the other.
The two equivalent positions are wherephilic substitution occurs.
There may be some substitution at the position between the two groups, but it is not as effective as the other two positions.
The methyl group directs an electrolyte towards its ortho positions.
The locations of the nitro group are its meta positions.
When two or more substituents conflict, it's more difficult to predict where an electrophile will react.
In many cases, the result is a mixture.
mixtures of substitution products are given because xylene is activated at all the positions.
When there is a conflict between two groups, the activated group usually directs the substitution.
The deactivating groups are usually stronger than the active groups.
It is helpful to separate substituents into three different classes.
Powerful ortho, para-directors are used to stable the sigma complexes.
The substituent in the stronger class is more dominant if there are two substituents.
A mixture is likely if both are in the same class.
The incoming substituent is directed by the stronger group.
The methoxy group is a stronger group than the nitro group.
substitution at the crowded position ortho to both the methoxy group and the nitro group is prevented by Steric effects.
The aromatic ring has a nitrogen atom with a nonbonding pair of electrons bonding to it.
The amide group is a stronger group than the chlorine atom, and substitution occurs mostly at the positions ortho to the amide.
The amide is a particularly strong activated group, and the reaction gives some dibrominated product.
The site of substitution for a biphenyl is determined by two factors: which phenyl ring is more activated (or less deactivated) and which position on that ring is most reactive.
To show why a phenyl substituent should be para-directing, use resonance forms of a sigma complex.
A new carbon-carbon bond can be formed by substitution onto aromatic rings.
The first studies of reactions with aromatic compounds were done in 1877 by the French alkaloid chemist Charles Friedel and his American partner, James Crafts.
alkylate benzene was found to give alkylbenzenes in the presence of Lewis acid catalysts.
The gas is evolved.
The butyl cation is acting as aphile.
The catalyst is aluminum chloride.
The catalyst is regenerated in the final step.
A wide variety of primary, secondary, and tertiary alkyl halides are used in Friedel-Crafts alkylations.
The carbocation is most likely caused by secondary and tertiary halides.
Friedel-Crafts alkylation is an aromatic substitution in which an alkyl cation acts as the electrolyte.
The alkylated product is regenerated by the loss of a protons.
The free primary carbocation is too unstable with primary alkyl halides.
A complex of aluminum chloride and alkyl halide is what the actual electrophile is.
There is a weakened carbon-halogen bond and a positive charge on the carbon atom in this complex.
Friedel-Crafts alkylations can be made using most of the ways we have seen.
Two common methods are treatment of alkenes and alcohols.