Disubstituted benzene rings directing effects

What happens in electrophilic aromatic substitution when a disubstituted benzene ring is used as starting material To predict the products, look at the directing effects of both substituents and then determine the net result, using the following three guidelines. 

Proposing a synthesis for a disubstituted benzene ring requires a careful analysis of directing effects to determine which group should be installed first. As an example, consider the following compound. 

But suppose you have a disubstituted benzene ring where the two groups are para to each other. For example, eonsider the directing effects for the following compound ... 

Studies with chloroarenes and Cp/Re(CO)3 under UV irradiation show oxidative addition products derived from C - Cl bond activation. For example, pentachlorobenzene at 350 nm yields fra s-( p,Rc((X))2(Ar)Cl (Ar is 2,3,4,5-C6CI4H) as the primary type of product [121]. Studies of di- and tri-chloro benzenes [122] and of disubstituted dichloroarenes [123] also show C-Cl bond activation. In the latter case, unusual directing effects were observed, for example, a C - Cl bond across the ring from either CF3 and CH3 is selectively activated depending upon the system [123]. 

To synthesize benzene derivatives with more than one substituent, we must always take into account the directing effects of each substituent. In a disubstituted benzene, for example, the directing effects indicate which substituent must be added to the ring first. 

In synthesizing disubstituted benzenes, the order in which the substituents are placed on the ring and the point in a reaction sequence at which a substituent is chemically modified are important considerations. When a disubstituted benzene undergoes an electrophilic aromatic substitution reaction, the directing effect of both substituents has to be considered. 

IDENTIFYING DIRECTING EFFECTS FOR DISUBSTITUTED AND POLYSUBSTITUTED BENZENE RINGS... 

Identifying Directing Effects for Disubstituted and Polysubstituted Benzene Rings... 

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

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