Ring hydrogenation

Nitration Warming benzene with a mixture of nitric acid and sulfuric acid gives nitrobenzene A nitro group (—NO2) replaces one of the ring hydrogens... 

Cycloahphatic diamines such as (13) [115172-12-8] which retain some aromatic character have been made from end-ring hydrogenation (44) of l,3-bis(p-aminocumyl)ben2ene [2687-27-6] the double alkylation adduct of aniline to y -diisopropenylben2ene [3748-13-8] (45) using Ru catalysts (46). 

Substitution of the ring hydrogen atoms by electrophilic attack occurs with all of the same reagents that react with ben2ene. Some of the common groups with which toluene can be substituted direcdy are... 

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

The net cr-charge on ring hydrogen atoms can also be significant for thiazole the order of decreasing acidity of the hydrogens is H-2 > 5 > 4, consistent with experiment. 

Table 3 h NMR Spectral Data for Ring Hydrogens of Nitrogenous Azoles (a) NH Derivatives... 

Table 10 H NMR Spectral Data (5, p.p.m.) for Ring Hydrogens of Non-aromatic Azoles with Two Ring Double...

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Ring hydrogen atoms can be abstracted from the a-carbon atoms of azolium ions b strong bases, as demonstrated in base-catalyzed hydrogen exchange (Section 4.02.1.7.2... 

Isoxazoles are also rather stable to nucleophilic attack by OH at carbon. For reactions with base at a ring hydrogen atom, leading, for example, to ring opening of isoxazoles, see Section 4.02.1.7.1. 

Another interesting fact is that hydrogen scrambling, i.e. randomization of the ring hydrogens of pyrazole to lose positional identity on electron impact, has not been observed to any significant extent (see however 780MS575). 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Exocyclic conjugation causes a small upheld shift of the ring hydrogen resonances, as can be seen in Table 6. The increase in tr-bond hxation also results in an increase in the 4,5 coupling constant to about 6.0 Hz. The use of coupling constants for the investigation of tautomerism is discussed in Section 4.17.5. 

Nucleophilic attack on ring hydrogen (proton abstraction) (Section 5.05.3.5)... 

Nucleophilic Attack on Ring Hydrogen (Proton Abstraction)... 

LUMO +1 of propylene oxide reveals the likely site of nucleophilic attack. The LUMO is primarily localized on the ring hydrogens. 

The NMR spectrum of the cyclophane shows three signals at 6 4.27, 6.97, and 7.24 (1 2 1 ratio) due to the benzene ring hydrogens. Examine cyclophane and identify which hydrogens are responsible for each signal. 

The study of the NMR spectra of thiophenes has attracted considerable interest, 22,24-3sb partly because the spectra of substituted thiophenes containing only a few ring hydrogens are quite suitable for complete analysis and partly because in a series of related compounds the chemical shifts observed are related to differences in the electron distribution about chemically nonequivalent hydrogens (for review, see reference 39), especially for hydrogens far removed from the substituent. 

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