Orientation in disubstituted benzenes

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. Depending on the groups and their positions, the effects can either reinforce one another, or work against one another. 

For example, would expect 4-nitrotoluene to be less reactive than toluene (methylbenzene) because of the deactivating effect of a nitro group. Also, the most likely position of substitution should be, and is, ortho to the methyl group and meta to the nitro group:

When the two substituents have opposed orientation effects, it is not always easy to predict what products will be obtained. For example, N-(2-methoxyphenyl)ethanamide has two powerful o,p-directing substituents, −OCH3 and −NHCOCH3. Nitration of this compound gives mainly the 4-nitro derivative, which indicates that the −NHCOCH3 exerts a stronger influence than −OCH3:

When judging, the following rules are useful.  Note: In the schemes below, the green arrow shows the directing effect of the activator, and the red arrow shows the directing effect of the deactivator.  In some cases the green arrow is much smaller, indicating that the directing effect is weaker or less effective than at the other positions.

(a) When two substituents reinforce one another, the new group is introduced at the expected position:

(b) When two groups the same time work against one another, the stronger one wins.

In this example, the methyl group directs o/p, but the methoxy group is a stronger activator and o/p director, so the OCH₃ determines the position of substitution (see stronger green arrows):

(c) When two groups of different types work against one another, usually the activator wins out over the deactivator:

In this case, the OH group directs to three places; however substitution between two substituents is sterically hindered, so it tends to be a minor process (smaller green arrow).  For this reason, only the two major products are shown.