O-p-directing substituents

The nature of the substituent in a substituted aromatic compound influences the position of hydroxylation. Thus, o-p-directing substituents, such as amino groups, result in o-and... 

When the two substituents have opposed orientation effects, it is not always easy to predict what products will be obtained. For example, iV-(2-methoxy-phenyl)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 ... 

N-Ben2ylidenaniline gives 2-phenylphthalimidine in 84 % yield. N-Ben-zylidenanilines with o,p-directing substituents also undergo this reaction. 

Azobenzenes containing o,p-directing substituents react in a similar manner. If only one aromatic ring is substituted, cyclization always takes place on this substituted ring. Azobenzenes containing m-directing substituents do not cyclize [123]. An explanation for this is given in the sections on reaction mechanism. 

Many students get into trouble when writing synthetic procedures because they overlook steric effects. For example, if the task is to synthesize the ortho product starting with propylbenzene, you can t rely on the o-p-directing ability of the propyl group. One solution is to place an easily removable substituent in the para position, leaving only the ortho position available for further attack. (If the para substituent is a meta-director, then both the meta-director and the propyl group favor attack at the Scime position.)... 

If the chlorination were performed first, a considerable portion of product would have the chlorine para to the methyl group. Also, note that in the second step, both substituents direct the chlorine to the desired position (—CH3 is o,p-directing, -NO2 is m-directing). 

This relationship holds reasonably well for p-disubstituted derivatives of benzene and is only applicable to substituents of opposite electronic effects, involving an o-p-directing group in the presence of an m-directing group. The combination of two similarly orienting groups results in a... 

Two types of shift are observed in disubstituted compounds. If both substituents are of the same type (o, p-directing or m-directing) there is no additive effect due to the second substituent the band displacements are those observed for the most displaced monosubstituted compound. Thus the L band (203 nm in benzene) is shifted by... 

The nature of the substituent in a substituted aromatic compound influences the position of hydroxylation. Thus, o-/7-directing substituents, such as amino groups, result in o- and p-hydroxylated metabolites such as the o- and /7-aminophenols from aniline (figure 4.8). Meta-directing substituents such as nitro groups lead to m- and /7-hydroxylated products, for example nitrobenzene is hydroxylated... 

Meth. 10, 600 13, 767).—The relative directive influences of o- and p-directing substituents for tritylation are OH>NH2>NHAc>OGH3>GH3. This is the same as in inorganic electrophilic substitution, except that there NH2>OH. F. e., also with triphenylcarbinol, s. G. Ghuchani, Soc. 1959, 1753 1960, 325. 

The nitration of nitro- and dinitro-biphenyls has been examined by several workers. i - As would be expected, nitration of the nitro-biphenyls occurs in the phenyl ring. Like a phenyl group, a nitrophenyl group is 0 -directing, but like certain substituents of the type CH CHA ( 9.1.6) it is, except in the case of w-nitrophenyl, deactivating. Partial rate factors for the nitration at o °C of biphenyl and the nitro-biphenyls with solutions prepared from nitric acid and acetic anhydride are given below. The high o p-v2X o found for nitration of biphenyl... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

FIGURE 1. Correlation of selected AHf° differences (Aj, kJ mol-1) with covalent potential, VjfA-1) O, direct differences, Ai x, scaled differences, A j p (see text for definition of p). Primed substituents (H SiHj.CHj) indicate compounds with scaled differences... 

Methyl is an ortho, para-directing substituent, and toluene yields mainly o-nitrotoluene and p-nitrotoluene on mononitration. Some m-nitrotoluene is also formed. 

Another probe technique that has been used is to compare the effects of trifluoromethyl, at the meta and para positions, in both phenol and benzoic acid [29]. Only in the case where the substituent is in the para position in phenol is it directly conjugated with the ionising centre and therefore allowing a resonance effect to be important. Values of pK for the phenols led to the following substituent parameters o-(p-CF3) = +0.54 and o-(ot-CF3) = +0.43, the ratio o-(p-CF3)/o-(w-CF3) being 1.25, and this is essentially... 

---