Ortho-para directing groups 495 table

Ortho-para directing groups, 457 61, 463-466, 464 table, 469 70 Osmium tetraoxide, 589-590, 608 Oxalic acid, 164, 748 Oxane, 593 Oxaphosphetane, 679 Oxazole, 431... 

Except for the alkyl and phenyl substituents, all of the ortho—para-directing groups in Table 15.2 are of the following general type ... 

A comparison of the entries in Table 9.1 shows that almost all ortho-para directing groups have an unshared pair of electrons on the atom bonded to the aromatic ring. Thus, the directing effect of most of these groups is due primarily to the ability of the atom bonded to the ring to delocalize further the positive charge on the cation intermediate. 

Consider now the other ortho,para-directing groups listed in Table 4A.In each of them, the atom attached to the aromatic ring has an unshared electron pair. 

Note that in this last example, we showed nitration of toluene producing only the para isomer. Because methyl is an ortho-para directing group, both the ortho and para isomers are formed (Table 22.1). In problems of this type in which you are asked to prepare the para isomer, assume that both ortho and para isomers are formed but that there are physical methods by which they can be separated and the desired isomer obtained. 

But if the polar effect is not pronounced, then substitution can be powerfully assisted by the substituent. This is ortho-para direction with activation and is provided by groups such as —OH, —OR, —SR, and —NH2. A more comprehensive list of substituents and their orientation effects is provided in Table 22-6. 

Table 11 also shows that, as expected, the -CH2SiMe3 group is ortho/para directing. This activating effect is also evident in the substituent constants calculated for this group as shown in Table 5. The large values for oR illustrate electron donation into the aromatic 7i system.

The Pd(OAc)2-promoted reaction of styrene with benzene was applied to mono-substituted benzenes such as toluene, ethylbenzene, anisole, chlorobenzene and nitrobenzene. The results are given in Table 2.2. Methyl, ethyl, and metho)gr groups are ortho-para-directing, while the nitro group is metadirecting. 

You don t need to resort to memorization to determine whether a substituent is an ortho-para director or a meta director. It is easy to tell them apart All ortho-para directors, except for alkyl, aryl, and CH=CHR groups, have at least one lone pair on the atom directly attached to the ring all meta directors have a positive charge or a partial positive charge on the atom attached to the ring. Take a few minutes to examine the substituents listed in Table 16.1 to convince yourself that this is true. 

---