Disubstituted Benzenes Ortho, Meta, and Para Substitution

PROBLEM 14.24 In addition to the mechanism following the path shown for pyrrole in Rgure 14.61, there is another reasonable mechanism for aromatic substitution of furan. Suggest one. Hint Think simple What are the possible reactions between electrophiles and double bonds (Chapters 9 and 10) Use the chlorination of furan as an example. 

PROBLEM 14.25 There is a difficulty with the answer to Problem 14.24. Rnd what is wrong (or unlikely) and suggest an alternative mechanism. Recall what you know of the stereoelectronic requirements of the E2 reaction (Section 7.9, p. 301). 

9 Disubstituted Benzenes Ortho, Meta, and Para Substitution 

We have already mentioned (Problem 14.9) that the synthesis of toluene from benzene is made difficult because once a significant amount of toluene is formed, it competes successfully with the unsubstituted benzene in further methyiation reactions. Unless one is careful to use a vast excess of benzene, the result is a horrible mixture of benzene, toluene, dimethylbenzenes (xylenes), and even tri- and tetra-substituted molecules. 

WORKED PROBLEM 14.26 If one attempts to methylate benzene completely with CH3CI/AICI3, the product is a greenish solid of the formula C13H21AICI4. Propose a structure for the compound and a mechanism for its formation. 

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The general question now arises of how the substituted benzenes we have learned to make will react with electrophilic reagents. What are the possibilities There are three isomers of disubstituted benzene, ortho, meta, and para, as shown in Figure 14.63. 

Substituents affect the orientation of the reaction. The three possii disubstituted products—ortho, meta, and para—are usually not for in equal amounts. Instead, the nature of the substituent already pr ent on the benzene ring determines the position of the second subsi tution. Table 16.1 lists experimental results for the nitration ofsoij substituted benzenes and shows that some groups direct substitute... 

A hypothesis on the existence of correlations between molecular stmcture and physico-chemical properties was reported in the work of Kbmer [1874], which dealt with the synthesis of disubstituted benzenes and the discovery of ortho, meta, and para derivatives the different colors of disubstituted benzenes were thought to be related to differences in molecular stmcture and the indicator variables for ortho, meta, and para substitution can be considered as the first three molecular descriptors [Kbrner, 1869, 1874]. 

Disubstituted benzenes are named using the prefixes ortho-, meta-, and para- to specify the substitution patterns. These terms are abbreviated o-, m-, andp-. Numbers can also be used to specify the substitution in disubstituted benzenes. 

With substituted benzene rings, an alternative way of identifying the positions of the substituents is to use the terms ortho, meta, and para. Ortho compounds are 1,2-disubstituted, meta compounds are 1,3-disubstituted, and para compounds are 1,4-disubstituted. Some examples should make this clear. 

Regiochemistry and stereochemistry When both components of a cycloaddition reaction are unsymmetrically substituted, two regioisomeric cycloadducts are possible. In the case of Diels-Alder reactions, these are shown in the reactions of both C-1 and C-2 substituted dienes and monosubstituted dienophiles. Isomeric adducts can be referred as ortho, meta and para in reference to similar disubstitution isomers of benzene. 

The effect of para substituents on the OH torsional barrier in phenols and nitrogen inversion barrier in anilines has been examined by Pople and co-workers (8,9). These topics are discussed in Sections V.A.4 and V.A.5. The results show that in a para-substituted benzene, a w donor and a tt acceptor interact favorably with one another whereas the situation of two tt donors leads to resonance saturation and a destabilizing interaction. Wepster et al. (93,94) have reached similar conclusions on the basis of experimental studies. The relative stabilities of ortho-, meta-, and para-disubstituted benzenes for the substituents CN, OH, and F have been studied by von Niessen (66) using a Gaussian lobe minimal basis set. Radom has calculated the effect of substituents on the acidities of phenols and noted good agreement with available gas-phase data (65). 

There are directly measured enthalpy of formation data for the ortho, meta, and para isomers of the disubstituted benzenes R, R = Me, Me (1, g) Me, Et (1, g) Me, n-Pr (1) Me, i-Pr (1) Me, t-Bu (g) Et, Et (1). For all the meta and / ara-disubstituted species and for o xylene, the /7(g, 1) are less than the root-mean-square (rms) of the experimental uncertainty levels. All of these compounds are, therefore, unstrained and so too presumably would be the meta and para isomers of other dialkyl-substituted benzenes. Enthalpies of formation for those compounds where /7( ) is about 0 can be estimated from eqs 16 and 17 and the parameters in Table 3. 

The mass spectra of the xylene isomers (Figs. 4.22 and 4.23 for example) show a medium peak at m/z = 105, which is due to the loss of a hydrogen atom and the formation of the methyltropylium ion. More importantly, xylene loses one methyl group to form the tropylium (m/z = 91). The mass spectra of ortho-, meta-, and para-disubstituted aromatic rings are essentially identical. As a result, the substitution pattern of polyalkylated benzenes cannot be determined by mass spectrometry. 

In Summary Simple monosubstituted benzenes are named by placing the substituent name before the word benzene. For more highly substituted systems, 1,2-, 1,3-, and 1,4- (or ortho-, meta-, and para-) prefixes indicate the positions of disubstitution. Alternatively, the ring is numbered, and substituents labeled with these numbers are named in alphabetical order. Many simple substituted benzenes have common names. 

Substituted benzenes are named by adding prefixes or suffixes to the word benzene. Disubstituted systems are labeled as 1,2-, 1,3-, and 1,4- or ortho, meta, and para, depending on the location of the substituents. Many benzene derivatives have common names, sometimes used as bases for naming their substituted analogs. As a substituent, an aromatic system is called aryl the parent aryl substituent, CsHs, is called phenyl its homolog C6H5CH2 is named phenylmethyl (benzyl). 

Substituted aromatic compounds are named using the suffix -benzene. Thus, C6H5Br is bromobenzene, C6H5CH3 is methylbenzene (also called toluene), C6H5N02 is nitrobenzene, and so on. Disubstituted aromatic compounds are named using one of the prefixes ortho-, meta-, or para-. An ortho- or o-disubstituted benzene has its two substituents in a 1,2 relationship on the ring a meta- or m-disubstituted benzene has its two substituents in a 1,3 relationship and a para- or p-disubstituted benzene has its substituents in a 1,4 relationship. When the benzene ring itself is a substituent, the name phenyl (pronounced fen-nil) is used. 

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