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  Reactions of Disubstituted Rings Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Disubstituted Rings

When a benzene ring has two substituent groups, each benzene ring exerts an effect on subsequent substitution reactions. The deactivation or activation of the ring can be assumed more or less by the sum of the individual effects of these substituents. The site at which a new substituent is come into existence depends on the orientation of the existing groups and their individual directing effects. We can make out two usual behaviour categories, as displayed in the table below. So, the groups may be oriented in such type of a manner that their directing affects act in concert, reinforcing the result; or are opposed (antagonistic) to each other. Note: the orientations within each category change depending on whether the groups have identical or opposite individual directing effects.

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The products from substitution reactions of compounds having a reinforcing orientation of substituents are easier to assume than those having antagonistic substituents. For an instance, the six equations displayed below are all instances of reinforcing or cooperative directing effects operating in the supposed manner. Symmetry, as in the first two examples, makes it easy to assume the site at which substitution is possible to occur. Note: if two distinct sites are favored, substitution will generally take place at the one that is least hindered by ortho groups.

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The first three instances have two identical directing groups in a meta-relationship to each other. In instances 4 through 6, oppositely directing groups have an ortho or para-relationship. The main results of electrophilic substitution, as displayed, are the sum of the individual group effects. The strongly activating amino (-NH2) and hydroxyl (-OH) substituents favor dihalogenation in instance 5 and 6.

Compounds' Substitution reactions having an antagonistic orientation of substituents need a more careful analysis. If the substituents are similar, as in instance 1 below, the symmetry of the molecule will again make simpler the decision. It will generally exert the product determining affect, instances 2, 4 & 5, although it may be overall deactivating (case 2), When one substituent has a pair of non-bonding electrons presented for adjacent charge stabilization. Example 3 reflects a combination of steric hindrance and the superior innate stabilizing ability of methyl groups relative to other alkyl substituents. Case 6 is interesting in that it illustrates the conversion of an activating ortho/para-directing group into a deactivating meta-directing "onium" cation [-NH (CH3)2(+)] in a strong acid environment.

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