Aromatic hydrocarbons carbon atom reactivity

It was shown in Fig. 53 that the hydrocarbon conversion which is reached over the catalyst at fixed reaction conditions depends on the type of the hydrocarbon. Alkenic and aromatic hydrocarbons are more reactive than alkanic hydrocarbons. The reactivity of alkanic hydrocarbons increases with the number of carbon atoms in the molecule. 

The methyl and ethyl esters of cyanoacetic acid are slightly soluble ia water but are completely miscible ia most common organic solvents including aromatic hydrocarbons. The esters, like the parent acid, are highly reactive, particularly ia reactions involving the central carbon atom however, the esters tend not to decarboxylate. They are prepared by esterification of cyanoacetic acid and are used principally as chemical iatermediates. 

Cesium forms simple alkyl and aryl compounds that are similar to those of the other alkah metals (6). They are colorless, sohd, amorphous, nonvolatile, and insoluble, except by decomposition, in most solvents except diethylzinc. As a result of exceptional reactivity, cesium aryls should be effective in alkylations wherever other alkaline alkyls or Grignard reagents have failed (see Grignard reactions). Cesium reacts with hydrocarbons in which the activity of a C—H link is increased by attachment to the carbon atom of doubly linked or aromatic radicals. A brown, sohd addition product is formed when cesium reacts with ethylene, and a very reactive dark red powder, triphenylmethylcesium [76-83-5] (C H )2CCs, is formed by the reaction of cesium amalgam and a solution of triphenylmethyl chloride in anhydrous ether. 

It is of interest that both the methyl affinities and the reactivities of aromatic compounds toward the pheny radical are correlated both by Froax and by atom localization energies. Dewar has shown that the energy required to remove one atom from conjugation (in a hydrocarbon containing an even number of carbon atoms) is greater... 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide form a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.126 For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in Ihe specificity for abstraction of tertiary or secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent.1"6 The exact nature of the complex responsible for this effect, whether a ji-coinplex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved.126- 22... 

Unlike the alkanes, however, the reaction of benzene with the halogens is catalyzed by iron. The relative lack of reactivity in aromatic hydrocarbons is attributed to delocalized double bonds. That is, the second pair of electrons in each of the three possible carbon-to-carbon double bonds is shared by all six carbon atoms rather than by any two specific carbon atoms. Two ways of writing structural formulas which indicate this type of bonding in the benzene molecules are as follows ... 

Naphthalene intermediates [61] are always built up by substitution reactions starting from the cheap and plentiful hydrocarbon using, in the main, only seven basic reactions. Most of these reactions are generally familiar from benzene chemistry but with some modification, since naphthalene has two different possible positions of substitution. These positions are often designated a and [3, the four a-positions being ortho and the four P-positions meta to the nearest carbon atom of the central bond. A further modifying influence is the lower level of aromaticity of naphthalene compared with benzene, leading to increased reactivity. 

The program is reported to carry out simple Hiickel molecular orbital calculations to determine the relative sensitivity of aromatic carbon atoms to oxidation and the relative stability of keto and enol tautomers. Klopman et al. (1999) have reported that for polycyclic aromatic hydrocarbons, adequate reactivity is an essential but not sufficient condition for enzyme catalyzed reaction. The accessibility of the reactive site (i.e., the absence of steric hindrance) was also found to be important. Genetic algorithms have been used to optimize the performance of the biotransformation dictionary by treating the initial priority scores set by expert assessment as adjustable parameters (Klopman et al., 1997). 

In practice, the valence bond picture has probably exerted more influence on how chemists actually think than the HMO picture. However most early applications were primarily qualitative in nature. This qualitative VB picture can be summarized under die name of resonance theory [10]. The basic concept is that in general the more ways one has of arranging the spin pairing in the VB wave function, the more stable the molecule is likely to be. Thus, VB theory predicts that phenanthrene with 14 carbon atoms and 5 Kekule structures should be more stable than anthracene with 14 carbon atoms but just 4 Kekule structures, in complete accord with the experimental evidence. It also predicts that benzenoid hydrocarbons with no Kekule structures should be unstable and highly reactive, and in fact no such compounds are knowa Extensions of this qualitative picture appear, for example, in Clar s ideas of resonant sextets [11], which seem to be very powerful in rationalizing much of the chemistry of benzenoid aromatic hydrocarbons. The early ascendancy of HMO theory was thus largely based on the ease with which it could be used for quantitative computations rather than on any inherent superiority of its fundamental assumptions. 

Many authors elucidated functionalization of polymers containing reactive oxirane moieties. Epoxidized NR, BR, IR and/or the respective model hydrocarbons, poly (butadiene-co-isoprene, various epoxy resins, poly (2,3-epoxypro-pyl methacrylate) and its copolymers or grafted systems were mostly exploited. Stabilizers based on epoxidized unsaturated rubbers are of the top interest. The mechanism of the functionalization process was studied in details by means of 3,4-epoxy-4-methylheptane and 1,2-epoxy-3-ethyl-2-methylpentane as model compounds [289]. The ring opening of the asymmetric oxirane is regiospecific. Aliphatic primary amines attack the least substituted carbon atom and can be involved in crosslink formation. Aromatic primary and secondary amines are less reactive than aliphatic ones because of their lower basicity the attack on the least substituted carbon atom is however preferred too. 

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