Benzene and pyridine

McMorrow D and Lotshaw WT 1993 Evidence for low-frequency (=15 cm collective modes in benzene and pyridine liquids Cham. Phys. Lett. 201 369-76... 

Reactions. Quinoline exhibits the reactivity of benzene and pyridine rings, as weU as its own unique reactions. 

In general, the solubility of heterocyclic compounds in water (Table 33) is enhanced by the possibility of hydrogen bonding. Pyridine-like nitrogen atoms facilitate this (compare benzene and pyridine). In the same way, oxazole is miscible with water, and isoxazole is very soluble, more so than furan. 

Draw and compare Lewis structures for benzene and pyridine. How many 7C electrons does each molecule have Where are the most accessible electrons in each Display the electrostatic potential map for pyridine and compare it to the corresponding map for benzene. Would you expect electrophilic attack on pyridine to occur analogously to that in benzene If so, should pyridine be more or less susceptible to aromatic substitution than benzene If not, where would you expect electrophilic attack to occur Explain. 

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

A weighed portion of dried product (referred to as melt-treated coal, MTC) was extracted to exhaustion sequentially with benzene and pyridine in an atmospheric Soxhlet apparatus. The extracts and residue were dried and weighed to determine the solubility of the MTC. 

Extractive solvents reduce the solubilization donor solvents increase it, but involve incorporation. A relation between benzene and pyridine solubility is dependent on wash conditions. Finally, oxygen recovery and corrected solubility are related, the relationship varying with the solvent used. 

In outline of what follows we will begin by brief reference to previous work on coal liquefaction. The present approach will then be motivated from considerations of coal structure and hydro-gen-donor activity. A theoretical section follows in the form of a pericyclic hypothesis for the coal liquefaction mechanism, with focus on the hydrogen transfer step. Experiments suggested by the theory are then discussed, with presentation of preliminary results for hydrogen transfer among model substrates as well as for the liquefaction of an Illinois No. 6 coal to hexane-, benzene-, and pyridine-solubles by selected hydrogen donors. 

Results showing the effectiveness of the A1- and A2-dialins in coal liquefaction relative to control solvents, naphthalene, Decalin, and fetralin, are presented in Tables 3.1 and 3.2. In both these tables, each row provides the conversion of the coal sample to each of hexane-, benzene-, and pyridine-solubles (plus gases) by the indicated solvent. Table 3.1 contains data derived at a temperature of 400 C and a reaction time of 0.5 hr. Among the control solvents, it can be seen that the naphthalene... 

When 2,3,4,5-tetraphenylzirconacyclopentadiene was treated with NiCl2(dppe) under reflux, the 2,3,4,5-tetraphenylnickelacyclopentadiene-dppe complex 36 was obtained as a red solid in 78% isolated yield, along with Cp2ZrCl2 (98% yield by NMR) (Eq. 2.27) [8a]. This complex was the same as that prepared from NiCl2(dppe) and l,4-dilithio-l,2,3,4-tetraphe-nyl-1,3-diene [30], In the case of NiCl2(PPh3)2, a similar transmetalation can proceed. Using this method, benzene and pyridine formation and CO insertion have been developed [8a,8b,46],... 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

An additional contributing factor to the mechanism of the present grafting reaction is the role of radiolytically produced hydrogen atoms. In the radiolysis of binary mixtures of aromatic and aliphatic compounds such as styrene-methanol, the concentration of aromatic strongly influences the G(H2) obtained from the methanol. In the most extensively studied binary mixtures of benzene-methanol (11) and pyridine-methanol (10), it is found that the yield of H atoms is important in determining product yields and types. Small additions (5%) of benzene and pyridine significantly reduce G(H2) from the methanol by scavenging H atoms. Above 5% additive, G(H2) is reduced further, but at a slower rate. These data for benzene-methanol and pyridine-methanol can be extrapolated... 

The SIMS experiments were performed by sputtering both adsorbed benzene and pyridine at various coverages from Ag(lll) surfaces. For... 

Owing to the very reactive nature of RuO relatively few solvents are suitable for its reactions. It is soluble in water to the extent of some 2% and is stable in such solutions, but reacts violently with diethyl ether, benzene and pyridine [236]. It has often been used catalytically in a biphasic system, with the co-oxidant in the aqueous layer. Under these circumstances the RuO formed from reduction of RuO by the substrate is re-oxidised at the organic - aqueous interface, so that oxidations with such systems can be much enhanced by stirring, shaking or sonication. In some cases (e.g. oxidation of aUcenes) it may be necessary to cool the reactants below room temperature, but in most cases ambient temperatures suffice, as indeed they do for the vast majority of organic oxidations catalysed by Ru complexes. 

The increased stability of 4n + 2 cyclic planar polyenes, relative to their imaginary classical counterparts, comes about because all the bonding energy levels within the ji-system are completely filled. For benzene and pyridine there are three such levels, each containing two spin-paired electrons. There is then an analogy between the electronic constitutions of these molecules and atoms that achieve noble gas structure. 

The formation of benzene and pyridine presumably occurs via the bicyclo[4.2.0] valence isomers (175) and (176), although adducts of these structures with dienophiles were not obtained on heating or during irradiation. The difference in products in thermal and photochemical reactions could be explained either by interconversion of (173) and (174) and rate-limiting isomerizations of (173) to (176) and (174) to (175), or alternatively, rate-limiting isomerization of (173) to (174) and symmetry-allowed 4ir photocyclization of (173) to (175) (79JOC1264). 

The IR spectra of the corresponding benzene, pyridine and X -phosphorin derivatives resemble each other, although X -phosphorin is spectrally closer to benzene than to pyridine. Only in the region 1200—1400 cm additional bands are to be detected. However, these alone do not suffice to identify the X -phosphorin system. Fig. 8 shows the IR spectra of 2.4.6-tri-tert-butyl-X -phosphorin compared with those of the anologous benzene and pyridine derivatives. 

Quinoline and isoquinoline, known as benzopyridines, are two isomeric heterocyclic compounds that have two rings, a benzene and a pyridine ring, fused together. In quinoline this fusion is at C2/C3, whereas in isoquinoline this is at C3/C4 of the pyridine ring. Like benzene and pyridine, these benzopyridines are also aromatic in nature. 

Skinner (56) precipitated an asphaltic portion from the dry Santa Maria Valley crude with propane and then extracted the asphaltic portion with a succession of solvents in the following order w-pentane, n-hexane, n-heptane, 2,2,4-trimethylpentane, cyclohexane, benzene, and pyridine. 

The difference in stability noted above is of the same order of magnitude as that found for the formation of adducts 22 and 23 for the reaction of 1,3,5-trinitrobenzene with the OH" and MeO" ions, respectively. Examination of the activation parameters shows some relevant differences between the heterocyclic and the homocyclic systems, however. The higher reverse rate for the methoxy adduct of trinitrobenzene 23 relative to the corresponding hydroxy adduct 22 is controlled by the higher activation entropy, whereas the higher reverse rate for the methoxy adduct of the dinitropyridine 1 relative to 3 is determined by the activation enthalpy. The entropy effect has been related to the different role of intramolecular interactions of the hydroxy adducts in the benzene and pyridine series. The reason for this difference is still unclear, however.45... 

A well understood case is that of quinoline reaction at position 2 is kinetically favored as compared with reaction at position 4, but the adduct from the latter is thermodynamically more stable. This situation, where the site of attack leading to the more stable adduct is the y position, is analogous with those regarding the formation of Meisenheimer adducts from benzene and pyridine derivatives and RCT nucleophiles. Presumably, with quinoline kinetic control favors the position that is more strongly influenced by the inductive effect of the heteroatom. The fact that position 2 of quinoline is the most reactive toward nucleophilic reagents is probably related to the lower 71-electron density at that position.123 However, the predominance of the C-4 adduct at equilibrium can be better justified by the atom localization energies for nucleophilic attachment at the different positions of quinoline. Moreover, both 7t-electron densities and atom localization energies indicate position 1 of isoquinoline to be the most favored one for nucleophilic addition. 

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