Electrophilic substitution Of benzene

So far we ve been concerned only with electrophilic substitution of benzene Two impor tant questions arise when we turn to substitution on rings that already bear at least one substituent... 

Aromatic hydrocarbons, like paraffin hydrocarbons, react by substitution, but by a different reaction mechanism and under milder conditions. Aromatic compounds react by addition only under severe conditions. For example, electrophilic substitution of benzene using nitric acid produces nitrobenzene under normal conditions, while the addition of hydrogen to benzene occurs in presence of catalyst only under high pressure to... 

Certain groups attached to an aromatic ring can donate electrons into its delocalized molecular orbitals. Examples of these electron-donating substituents include —NH2 and —OH. Electrophilic substitution of benzene is much faster when an electron-donating substituent is present. For example, the nitration of phenol, C6H5OH, proceeds so quickly that it requires no catalyst. Moreover, when the products are analyzed, the only products are found to be 2-nitrophenol (ortho-nitrophenol, 37) and 4-nitrophenol (pnra-mtrophcnol, 38 . 

We have seen how an —OH group can accelerate a reaction. Are there substituents that can slow down the electrophilic substitution of benzene One way to reduce the electron density in the benzene ring and to make it less attractive to elec-... 

Aromatic rings are much less reactive than their double-bond character would suggest they commonly undergo substitution rather than addition. Electrophilic substitution of benzene with electron-donating substituents is accelerated and takes place at the ortho and para positions preferentially. Electrophilic substitution of benzene with electron-withdrawing substituents takes place at a reduced rate and primarily at the meta positions. 

Scheme 18 Orbital phase properties for the electrophilic substitutions of benzenes with an electron-donating group...

Some substituted guanidines have been obtained [457] by reaction of amines with the disulphide H2N(HN )C S S C( NH)NH2. Papers on the structure and p/fa s [458], and the synthesis [458, 459] of acylguanidines have been published. Reaction of guanidine with alkyl-, alkenyl-, and benzyl-halides, followed by distillation under basic conditions, is reported to give useful yields of amines [460]. A novel electrophilic substitution of benzene to give A -methyl-A -phenyl-guanidine amongst other products has been published [461 ]. 

Furans substituted with electron releasing groups usually undergo polymerization with mineral acids due to facile protonation at the 2-position. Aluminum chloride also causes resinification of furan but benzo[6 ]furan and compounds with electron withdrawing groups are, however, more stable. The reversion to type so characteristic of the electrophilic substitution of benzene is by no means prevalent in the chemistry of furan and benzo[6]furan and many apparent electrophilic substitutions in reality proceed by addition. 

It is thus probably impossible at present to interpret unambiguously the magnitude of p, and this difficulty is compounded by the paucity of data. Some p factors for electrophilic substitutions of benzene and thiophene are gathered in Table 6.14 only the acetylation, detritiation, and protio-... 

Reaction coordinate diagrams for electrophilic substitution of benzene and electrophilic addition to benzene. 

Because electrophilic substitution of benzene involves the reaction of an electrophile with an aromatic compound, it is more precisely called an electrophilic aromatic substitution reaction. In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound. 

Cyclohexadienyl cations are well known as intermediates in the electrophilic substitution of benzene. Indeed benzene is protonated by superacids to give solutions containing the ion, which can be characterized spectroscopically. These ions are, however, far too reactive to be isolated as salts, and as they are generated only in acidic media, their reactions with nucleophiles cannot be studied. In contrast, cyclohexadienyltricarbonyliron hexafluorophosphate is a stable yellow solid which can be stored indefinitely at room temperature and can even be recrystallized from water. This is an especially striking example of the stabilization of a reactive organic species by coordination to a transition element. 

The mechanism of electrophilic substitution of benzene is discussed in Section 7.2... 

In this chapter, the first reaction mechanism encountered is the important and general electrophilic substitution of benzene. A host of aromatic substitution reactions will be studied in... 

Practice Problem Draw mechanisms for (a) nucleophilic addition of H to CH3COCH3 and (b) electrophilic substitution of benzene with CU. 

In two other typical electrophilic substitutions of benzene, the electrophiles are the nitronium ion (N02 ), leading to nitrobenzene, and sulfur trioxide (SO3), giving ben-zenesulfonic acid. 

A problem of another sort arises in the attempt to prepare an o-disubstituted benzene, even when one of the groups is an ortho, para director. Although appreciable amounts of ortho isomers may form in electrophilic substitutions of benzenes containing such groups, the para isomer is the major product in most such cases (Sections 16-2 and 16-3). Suppose you required an efficient synthesis of l-(l,l-dimethylethyl)-2-nitrobenzene [o-(fert-butyl)nitrobenzene]. Direct nitration of (l,l-dimethylethyl)benzene (tert-butylbenzene) is unsatisfactory. 

Using curved arrows to indicate the movement of electrons, draw the mechanism for each of the following reactions (a) nucleophilic addition of CN to CH3CHO and (b) electrophilic substitution of benzene with SO3H. (Draw aU resonance structures for the carbocation intermediate.)... 

---