Carbon aromaticity

Aromatic carbon Aromatic compounds Aromatic esters Aromatic ethers... 

The aromaticity of the ring is retained when chlorine bonds to the benzylic carbon aromaticity is lost when chlorine bonds to one of the ring carbons. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Synthetically important substitutions of aromatic compounds can also be done by nucleophilic reagents. There are several general mechanism for substitution by nucleophiles. Unlike nucleophilic substitution at saturated carbon, aromatic nucleophilic substitution does not occur by a single-step mechanism. The broad mechanistic classes that can be recognized include addition-elimination, elimination-addition, and metal-catalyzed processes. (See Section 9.5 of Part A to review these mechanisms.) We first discuss diazonium ions, which can react by several mechanisms. Depending on the substitution pattern, aryl halides can react by either addition-elimination or elimination-addition. Aryl halides and sulfonates also react with nucleophiles by metal-catalyzed mechanisms and these are discussed in Section 11.3. 

Other inhibitors include sulfur, carbon, aromatic azo compounds, and chlorophosphines [Nigenda et al., 1977 Uemura et al., 1977]. 

Some of the more critical properties related to marine fuels include ash content, carbon residue, calculated carbon aromaticity index (CCAI), density, sulfur, total sediment, and viscosity. A description of these properties and the primary reason for their implementation are provided below ... 

It is impractical to determine the cetane number of residual fuels in the ASTM D-613 cetane engine. Because of this, the Calculated Carbon Aromaticity Index and the Calculated Ignition Index were respectively developed by Shell and BR These values can be determined from the following equations where d = Density in kg/m3 59°F (15°C) and v = Viscosity in cSt 122°F (50°C). 

To illustrate this, one quart of xylene, an eight-carbon aromatic compound, weighs approximately 823 g (1.81 lb.) while 1 quart of n-octane, an eight-carbon paraffin, weighs approximately 665 g (1.47 lbs.). 

Afd, change of carbon aromaticity before and after the reaction HDN, nitrogen removal ICW, increment of catalyst weight (THFI on the catalyst). 

These values are summarized in Figure 2 and compared with our results on the soluble fractions from depolymerization. Van Krevelen (9) reported aromatic hydrogen values ranging from 23 to 54% for coals containing 76 to 89% carbon. Aromatic hydrogen content varies directly with rank. Brown (i) obtained values of 19-42% for a series of vitrains and showed a similar relationship with rank displaced toward lower aromatic values. Ladner and Stacey (5) analyzed two additional coals using the procedure developed by Brown, and the results fit Brown s correlation. 

Aromatic carbon content cannot be directly determined from 13C NMR spectrometry because it overlaps with olefinic carbon. Aromatic and olefinic hydrogens can be resolved in H NMR spectrometry, but the chemical shifts of methine hydrogens on esters of secondary alcohols overlap with chemical shifts of olefinic hydrogen in the NMR spectra of fulvic acids. The ring content (0) is a difference determination between the index of hydrogen... 

Acids Bases Neutral nitrogen compounds Saturate hydro- carbons Aromatic hydro- carbons ... 

Figure 5.12 shows QSAR analysis for six-carbon aromatic compounds (see also Table 5.7). 

Compounds Used in the Correlation of LUMO vs. Number of Chlorines for Six-Carbon Aromatics... 

Log P correlated well for six-carbon aromatics and small alcohols and carboxylic acids (in regard to number of carbons). Correlations for two-carbon alkanes were also better than the other descriptors (see Figure 5.15). 

For six-carbon aromatics, log P increases as the number of chlorines increases. The dataset is shown in Table 5.9. 

On the subject of carbon aromaticity of coal, 13C magnetic resonance spectroscopy has also found use in determining the fraction of carbon atoms that are in aromatic locations (fa) as well as attempting to define the structure of the aromatic ring system. However, there is a possibility of serious underestimation of aromaticity by this method (Miknis, 1988 Snape et al., 1989 Sfihi and Legrand, 1990). 

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