Structure alkane aromatization

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

While a majority of laboratory-scale dehydrocyclization studies involve carefully chosen feedstocks, often a single alkane, commercial operators use a naphtha fraction consisting of a complex mixture of hydrocarbons. At least some of these will be incapable of easily undergoing direct dehydrocyclization and need to be isomerized into reactive structures if aromatics are to be formed. The work of Davis suggests that the acidity of dual function catalysts is an important added factor in these isomerizations, one which likely complements the different set of isomerizations that may be catalyzed by the platinum function. 

As we recognize additional chemical types which display the H bond interaction, we find that this interaction is important in an increasing segment of our chemical knowledge. With H bonding discernible for such weak acids as mercaptans, thiophenols, and halogen-substituted alkanes, and for such we2ik bases as aromatics, olefins, and thioethers, it becomes profitable to look for even weaker interactions which have properties in common with the H bond. Just such an example has already been mentioned in Section 12.2.1 the crystal structure of aromatics provides evidence of attractive interaction between the aromatic C—H protons and the pi electrons of adjacent molecules. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

A substantial portion of fhe gas and vapors emitted to the atmosphere in appreciable quantity from anthropogenic sources tends to be relatively simple in chemical structure carbon dioxide, carbon monoxide, sulfur dioxide, and nitric oxide from combustion processes hydrogen sulfide, ammonia, hydrogen chloride, and hydrogen fluoride from industrial processes. The solvents and gasoline fractions that evaporate are alkanes, alkenes, and aromatics with relatively simple structures. In addition, more complex... 

Benzene derivatives. Tbe nomenclature is a combination of the lUPAC system and traditional names. Many of the derivatives are named by the substituent group appearing as the prefbt. These may be considered a subclass of the aliphatic-aromatic hydrocarbon family, which contains both aliphatic and aromatic units in its structures. Thus, alkylbenzenes are made up of a benzene ring and alkane units alkenylbenzenes are Composed of a benzene ring and alkene units and alkynylbenzenes comprise a benzene ring and alkyne units. Examples of alkylbenzenes include... 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

In addition to nonheme iron complexes also heme systems are able to catalyze the oxidation of benzene. For example, porphyrin-like phthalocyanine structures were employed to benzene oxidation (see also alkane hydroxylation) [129], Mechanistic investigations of this t3 pe of reactions were carried out amongst others by Nam and coworkers resulting in similar conclusions like in the nonheme case [130], More recently, Sorokin reported a remarkable biological aromatic oxidation, which occurred via formation of benzene oxide and involves an NIH shift. Here, phenol is obtained with a TON of 11 at r.t. with 0.24 mol% of the catalyst. 

Compounds consisting of only carbon and hydrogen have the simplest compositions of all organic compounds. These compounds are called hydrocarbons. It is possible to classify the hydrocarbons into four series, based on the characteristic structures of the molecules in each series. These series are known as (1) the alkane series, (2) the alkene series, (3) the alkyne series, and (4) the aromatic series. There are many subdivisions of each series, and it is also possible to have molecules that could be classified as belonging to more than one series. 

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 ... 

Other Cg hydrocarbons. The dehydrogenation of normal hexane and 2,3-di methyl butane also proceeds but not as voraciously on small platinum clusters. Figure 8 is a plot of the hydrogen content in the first adduct as a function of the size of the platinum metal cluster. The metal atom reacts via dihydrogen elimination to produce PtC6Hi2 products. The platinum trimer is now the smallest cluster that will produce a C H near one. The similarity of size dependent dehydrogenation of the normal hexane and the branched molecule suggest that these systems may not readily aromatize these alkanes. Further structural studies are needed to identify the reaction products. 

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