Aromatic compounds, addition substitution reactions

As you learned in Chapter 1, aromatic compounds do not react in the same way that compounds with double or triple bonds do. Benzene s stable ring does not usually accept the addition of other atoms. Instead, aromatic compounds undergo substitution reactions. A hydrogen atom or a functional group that is attached to the benzene ring may be replaced by a different functional group. Figure 2.6 shows two possible reactions for benzene. Notice that iron(III) bromide, FeBrs, is used as a catalyst in the substitution reaction. An addition reaction does not occur because the product of this reaction would be less stable than benzene. 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

We use the term substitution with scheme (138) in the sense that it is used for aromatic compounds. Addition is reserved for processes in which a saturated intermediate is formed. To observe retention, we require only that k 2) > k(3) in (138). By analogy with the SB2 reactions at a saturated carbon (Rreevoy et al., 1967), it is probable that some demetalations with acid in a polar solvent proceed in this way. Certainly, the intermediates are wholly analogous to those proposed for the isomerization, hydration, or hydrogen halide addition to alkenes. 

Aromatic compounds undergo many reactions, but relatively few reactions that affect the bonds to the aromatic ring itself. Most of these reactions are unique to aromatic compounds. A large part of this chapter is devoted to electrophilic aromatic substitution, the most important mechanism involved in the reactions of aromatic compounds. Many reactions of benzene and its derivatives are explained by minor variations of electrophilic aromatic substitution. We will study several of these reactions and then consider how substituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aromatic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. 

A review of the photochemistry of alkaloids includes reactions in which addition to an aromatic compound or substitution in an aromatic ring occurs. The second part of a review of the thermal and photochemical addition of dienophiles to arenes and their vinyl- and hetero-analogues covers additions to styrenes, stilbenes, and related systems. 

Allhough aromatic hydrocarbons are unsaturated, they do not readily undergo addition reactions. The delocalized tt bonding causes aromatic compounds to behave quite differently from alkenes and alkynes. Benzene, for example, does not add CI2 or Br2 to its double bonds under ordinary conditions. In contrast, aromatic hydrocarbons undergo substitution reactions relatively easily. In a substitution reaction one atom of a molecule is removed and replaced (substituted) by another atom or group of atoms. When benzene is warmed in a mixture of nitric and sulfuric acids, for example, hydrogen is replaced by tire nitro group, NO2 ... 

It is a typically aromatic compound and gives addition and substitution reactions more readily than benzene. Can be reduced to a series of compounds containing 2-10 additional hydrogen atoms (e.g. tetralin, decalin), which are liquids of value as solvents. Exhaustive chlorination gives rise to wax-like compounds. It gives rise to two series of monosubstitution products depending upon... 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Styrene undergoes many reactions of an unsaturated compound, such as addition, and of an aromatic compound, such as substitution (2,8). It reacts with various oxidising agents to form styrene oxide, ben2aldehyde, benzoic acid, and other oxygenated compounds. It reacts with benzene on an acidic catalyst to form diphenylethane. Further dehydrogenation of styrene to phenylacetylene is unfavorable even at the high temperature of 600°C, but a concentration of about 50 ppm of phenylacetylene is usually seen in the commercial styrene product. 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

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... 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

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