Disubstituted benzenes, orientation

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

The 5 ring H s of monosubstituted benzenes, C H G, are not equally reactive. Introduction of E into Cf,HjG rarely gives the statistical distribution of 40% ortho, 40% meta, and 20% para disubstituted benzenes. The ring substituent(s) determine(s) (a) the orientation of E (meta or a mixture of ortho and para) and (b) the reactivity of the ring toward substitution. 

The orientation and reactivity effects of substituents discussed for the substitution of monosubstituted benzenes also hold for disubstituted benzenes, except that the directing influences now come from two groups. Qualitatively, the effects of the two substituents are additive on the reactivity. We therefore would expect 4-nitromethylbenzene to be less reactive than methylbenzene... 

Start by analyzing whether the bromo. nitro, or pentyl groups can be added to the appropriate disubstituted benzene with the required orientation. 

Optical activity, retention in the Stevens rearrangement 418 Optical-rotatory dispersion, in configuration assignment 136 of amino acids 134, 135 Orientational control, in exchange reactions 260 in halogenation 257, 263 in tritylation 263 of disubstituted benzenes 263 Orientational effect 250, 251 Orton rearrangement 637 Oxamidinium salts 734—737 from tetraaminoethylencs 721, 722 hydrolysis of 731... 

Some data relating to orientation of disubstituted benzenes are shown in Table 26. Thus the halogenation and tritylation ° of aniline derivatives results in substitution chiefly as expected fi om the -f-iZ effect. References on nitration and sulphonation have not been included in Table 26, since these processes often lead to oxidation products, making the orientational comparison in these compounds doubtful. 

---