The directing effects of two or more substituents can work with or against one another. Bromoxynil and ioxynil are contact herbicides especially used in spring cereals to control weeds resistant to other weedkillers, and both are synthesized from p-hydroxybenz aldehyde by double halogenation. The aldehyde directs meta and the OH group directs ortho: both effects work together to promote bromination or iodination at the same two positions.

In other cases substituents compete by directing to different positions. The antioxidant BHT (p. 58) is made from 4-methylphenol (known as p-cresol) by a Friedel–Crafts alkylation. Usually, both the methyl and OH groups are ortho, para directors. The para positions are obvi ously both blocked, but the positions ortho to each of the groups are different. Since the –OH group is much more powerfully directing than the methyl group it ‘wins’ and directs the electrophile (a t-butyl cation) ortho to itself.

In this case the t-butyl cation is made from the alkene and protic acid; alternative reagents would be t-butanol with protic acid or t-butyl chloride with AlCl3. Even a ‘watered-down’ activating group like the amide –NHCOMe, which provides an extra pair of electrons, will ‘win’ over a deactivating group or an activating alkyl group. Bromination of this amide goes ortho to the –NHCOMe group but meta to the methyl group.

When looking at any compound where competition is an issue it is sensible to consider electronic effects first and then steric effects. For electronic effects, in general, any activating effects are more important than deactivating ones. For example, the aldehyde below has three groups—two methoxy groups that direct ortho and para and an aldehyde that directs meta.

Despite the fact that the aldehyde group withdraws electron density from positions 2 and 6, C6 is still the position for nitration. The activating methoxy groups dominate electronically and the choice is really between C2, C5, and C6. Now consider steric factors: reaction at C2 or C5 would lead to three adjacent substituents. Substitution occurs at position 6.

مواضيع ذات صلة

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Phenols react rapidly with bromine

Alkyl and acyl substituents can be added to a benzene ring by the Friedel–Crafts reaction

Sulfonation of benzene

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers