Bonds, intermediate type

Does this mean that benzene is in fact an equilibrium mixture of two 1,3,5-cyclohexatriene molecules Or, does this mean that benzene is a single molecule with some intermediate type of bonding ... 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

Sometimes the atomic arrangement of a crystal is such as not to permit the formulation of a covalent structure. This is the case for the sodium chloride arrangement, as the alkali halides do not contain enough electrons to form bonds between each atom and its six equivalent nearest neighbors. This criterion must be applied with caution, however, for in some cases electron pairs may jump around in the crystal, giving more bonds than there are electron pairs, each bond being of an intermediate type. It must also be mentioned that determinations of the atomic arrangement are sometimes not sufficiently accurate to provide evidence on this point an atom reported equidistant from six others may be somewhat closer to three, say, than to the other three. 

Many of these reactions are of great synthetical importance as they all provide facile routes to functionally substituted tin(IV) compounds. One procedure, which is of great industrial interest, is the intermediate addition of HC1 to SnCl2 forming HSnClj which reacts with C = C bonds. This type of reaction is exemplified by... 

DR. NORTON An excellent attempt to observe such hydrogen bonding was made recently by Fachinetti, et al. [Calderazzo, F. Fachinetti, G. Marchetti, F. Zanazzi, P. F. J. Chem. Soc., Chem. Commun. 1981, 181]. They took hydridocobalttetracarbonyl and triethylamine, and crystallized out a species which one can only describe as the tetracarbonylcobaltate of protonated triethylamine. They proposed some type of interaction between the hydrogen and a face of the cobalt tetrahedral complex, but it was clear that the interaction was almost entirely with nitrogens. The conclusion I would draw is that the complex appears to proceed directly to full protonation of the amine without any observable evidence for a hydrogen bonded intermediate. 

A monotonic decrease of benzene yield from methylpentanes is observed as a function of the hydrogen pressure over both metals (27a, 91a). The intermediates of bond shift type dehydroisomerization are likely to be unsaturated. This points to the McKervey-Rooney-Samman mechanism (55). This pathway obviously has a higher energy barrier over platinum than over palladium as compared with the aromatization of -hexane. This is reflected also by the similar aromatization selectivity (iS r) values of -hexane and methylpentanes over palladium (Table IV). 

In many of these systems, the postulated olefin complex intermediate would be labile. Therefore, its role as a pre-equilibrium intermediate is not terribly relevant to the kinetic problem. I think the relevant feature is whether the favorable paths in these insertion reactions involve the first or second type of transition state. This perhaps de-emphasizes the question of whether or not a 7r-bonded intermediate is involved but certainly does focus attention on the question of whether a coordinated unsaturated species is involved as a reactant. This is because the first type of transition state will require two coordination positions and hence involve the elimination of some other ligand before it can form, whereas the second will not. I don t know the answer to this question but this is how I would formulate the problem. 

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

Many bond structures have been proposed for benzene, and no single one may be accepted as fully satisfactory. Probably the best explanation is that the electrons are delocalized over all the six carbon nuclei. Thus benzene does not contain three carbon-carbon single bonds and three carbon-carbon double bonds. Rather, the benzene molecule contains six identical bonds, each one intermediate between a single and a double bond. This type of bond has been called a hybrid bond, a one and one-half bond, or simply a benzene bond. We normally draw the benzene ring as below. Each of the six comers represents a carbon atom, and each carbon atom is bonded to one hydrogen atom. 

Radiolytic studies have now demonstrated that the Wheland intermediate type 1 does in fact form and that earlier work had neglected the possibility that rapid desilylation by bases present in the mass spectrometric study would compete effectively with, and prevent, deprotonation, i.e. that reaction 2 is faster than reaction 3. This would therefore give the appearance that an Si—C bond was not formed62 - 65. If a nitrogen-centred base... 

Since they are identical and contribute equally, there is only one type of nitrogen-oxygen bond, intermediate between double and single in length. 

The side chains of amino acids present a number of nucleophilic groups for catalysis these include RCOO-, R—NH2, aromatic—OH, histidyl, R—OH, and RS. These groups attack electrophilic (electron-deficient) parts of substrates to form a covalent bond between the substrate and the enzyme, thus forming a reaction intermediate. This type of process is particularly evident in the group-transfer enzymes (EC Class 2 see Table 8.1). In the formation of a covalently bonded intermediate, attack by the enzyme nudeophile (Enz-X in Example 8.10) on the substrate can result in acylation, phosphorylation, or glycosylation of the nucleophile. 

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

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