Ortho and para director

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. 

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. 

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

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... 

Eqn. (a) shows the interaction between thiophene-2-carboxylic acid chloride and 2, 3-dichloro-anisole in the presence of CS2, anhydrous aluminium chloride and subsequent hydrolysis yields [2, 3-dichloro-4-(2-thiophen carbonyl) phenoxy]-methyl (I). The methoxy function present in the latter reactant, being an ortho- and para-director, helps to hook on the thiophene-2-carbonyl moiety at the para-position of the 2, 3-dichloro anisole to give (I). Obviously, the ori/io-position is pre-occupied by a chloro group. 

Explain the statement - electron releasing groups are ortho and para directors and electron withdrawing groups are metadirectors. 

The influence of meta-directing substituents can he explained using the same kinds of arguments used for ortho and para directors. Look at the nitration of henzaldehyde, for instance (Figure 9.19). Of the three possible carhocation intermediates, the meta intermediate has three favorable resonance forms, while the ortho and para intermediates have only two. In both ortho and para intermediates, the third resonance form is unfavorable because it places the positive charge directly on the carbon that hears the aldehyde group, where it is disfavored by a repulsive interaction with the positively polarized carbon atom of the C=0 group. Hence, the meta intermediate is more favored and is formed faster than the ortho and para intermediates. 

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