Why do some reactions stop cleanly at monosubstitution

The product is a mixture of ortho- and para-nitrophenol from which the ortho compound can be separated by steam distillation. A strong intramolecular hydrogen bond reduces the availability of the OH group for intermolecular hydrogen bonds so the ortho compound has a lower boiling point. The remaining pnra-nitrophenol is used in the manufacture of the painkiller paracetamol. 

We have drawn the substitution at the benzylic centre as an Sn2 reaction as it would normally be with a primary alkyl halide, though it could be S jl in this case as the benzylic cation is stable. Friedel-Crafts alkylation works well with relatively stable cations especially tertiary cations. The cation can be generated in a number of ways such as the protonation of an alkene, the acid-catalysed decomposition of a tertiary alcohol, or the Lewis-acid-catalysed decomposition of a f-alkyl chloride. 

We can, in a qualitative way, combine the directing effects of two or more substituents. In some cases the substituents both direct to the same positions, as in the syntheses of bromoxynil and ioxynil, contact herbicides especially used in spring cereals to control weeds resistant to other weedkillers. They are both synthesized from p-hydroxybenzaldehyde by halogenation. The aldehyde directs meta and the OH group directs ortho so they both direct to the same position. The aldehyde is deactivating but the OH is activating. 

The reaction with NH2OH is the formation of an oxime from the aldehyde and hydroxylamine and was dealt with in Chapter 14. The reaction with P2O5 is a dehydration—phosphorus is used to remove water from the oxime. 

In other cases substituents compete by directing to different positions. For example, in the synthesis of the food preservative BHT (p. 30) from 4-methylphenol (p-cresol) by a Friedel-Crafts alkylation, the methyl and OH groups each direct ortho to themselves. The -OH group is much more powerfully directing than the methyl group because it provides an extra pair of electrons, so it wins and directs the electrophile (a t-butyl cation) ortho to itself. The t-butyl cation can be made from the alkene or t-butanol with protic acid or from t-butyl chloride with AICI3. 

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