Para directors

Because substitution in toluene occurs primarily at positions ortho and para to methyl we say that a methyl substituent is an ortho, para director... 

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

The second point is somewhat less obvious but is readily illustrated by the synthesis of 1,3,5-tribromobenzene. This particular- substitution pattern cannot be obtained by direct brornination of benzene because bromine is an ortho, para director. Instead, advantage is taken of the powerful activating and ortho, para-directing effects of the fflnino group in aniline. Brornination of aniline yields 2,4,6-tribromoaniline in quantitative yield. Diazotization of the resulting 2,4,6-tribromoaniline and reduction of the diazonium salt gives the desired 1,3,5-tribromobenzene. 

Ortho, para director (Section 12.9) A group that when present on a benzene ring directs an incoming electrophile to the positions ortho and para to itself. 

Anticipating the products of electrophilic aromatic substitution can be more difficult when two or more substituents compete for control. For example, both methyl and methoxy groups are ortho para directors, and compete for control in electrophilic substitution of 2-methylanisole. The reaction product depends on which substituent has the stronger directing influence. 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

Next, ask yourself what an immediate precursor of the target might be. Since an acetyl group is a meta director while a hydroxyl group is an ortho and para director, acetophenone might be a precursor of m-hydroxyacetophenone. Benzene, in turn, is a precursor of acetophenone. 

B — Cl is an ortho, para director. The possible products are 1,3-dichloro-2-nitrobenzene and 2,4-dichloro-1 -nitrobenzene. 

Table 8.1 Directing effects of substituents in electrophilic aromatic substitution Electron-releasing groups ortho and para directors...

Halogens (a withdrawers, n donors) ortho and para directors ----F -------Cl --------Br -------1... 

Should the activating substituent be at position 2, further substitution will be almost exclusively at position 1 this follows from consideration of resonance structures, where the 2-substituent has minimal effect if attack occurs at position 4. Of course, this would equate to meta attack, which we know is unfavourable for an ortho and para director (see Section 8.4.3). 

An alternative product might be the ort/zo-substituted analogue. Alkyl groups are ortho and para directors for further electrophilic substitution. This follows from stabilization of one of the resonance forms by the electron-donating effect of an alkyl group. This is seen in the para substitution case, and extrapolation to ortho substitution shows a similar stabilization. 

Other substituents are ortho-para-directors, which facilitate attack at these positions. 

In the next section, you see why ortho- and para-directors are a single group. 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

The regiochemistry is important. For example, does the reaction follow Markovnikov s rule or is the reaction anti-Markovnikov Or is the substituent a meta-director or ortho-para-director You learned rules such as this for a reason make sure you continually apply them. 

Halogens are the only deactivating substituents that are ortho-para directors. However, they are the weakest of the deactivators. Halogens withdraw... 

All alkyl groups, not just methyl, are activating substituents and ortho, para directors. This is because any alkyl group, be it methyl, ethyl, isopropyl, tert-butyl, or any other, stabilizes a carbocation site to which it is directly attached. When R = alkyl,... 

Using the developed model, the k values for 2-CP, 3-CP, and 4-CP are 1.12 x 107, 1.004 x 109, and 1.005 x 108 (1/s), respectively therefore, the dechlorination constants for monochlorophenols follow a decreasing order 3-CP > 4-CP > 2-CP. Because chloride ion can be released only after the rupture of the aromatic ring, the faster the hydroxylation of the parent compounds, the faster the dechlorination process should be. Therefore, the above order can be understood in terms of the effect of the substituents on the reactivity of their parent compounds. It is known that both OH and Cl are ortho and para directors. Under the influence of these directors, the following preference of hydroxyl radical attack is expected ... 

The ortho and para director nature of Cl seems to play a less important role when the number of chlorine atoms increases from one to three dechlorination has the following order in terms of decreasing rate constants 2,4,6-TCP... 

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