Neighboring hydrogens

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Q Base (B ) attacks a neighboring hydrogen and begins lo remove the H at the same time as the alkene double bond starts to form and the X group starts to leave. 

Modest diastereoselectivity was observed for the Michael addition reaction of rac-14 to 13 and these diasteromers 28-a/28-b could be separated and individually identified. The minor isomer 28-b was found to readily undergo conversion to benzoxathiin 30 when treated with BF3 etherate, presumably through the transient intermediate 29-b. The major isomer 28-a was converted by BF3 etherate to intermediate 29-a. Conversion to 30 required the use of the stronger Lewis acid TMSOTf, presumably due to the cis-stereochemistry between the methoxy and the neighboring hydrogen, making it more difficult to eliminate/aromatize. 

Consequently, abstraction of both hydrogens must be taking place in a concerted, or an almost concerted, process, and it has been suggested that the initially formed aziridine intermediate may collapse to the sulpho-namide and a benzyne, the latter going to tars 51>. No evidence has been presented or obtained for the intervention of benzynes in these reactions. The simultaneous abstraction of two neighboring hydrogen atoms has... 

There is also the possibility of Davydov coupling, which is likely to appear when there are double or multiple H-bond systems [7,21-23]. It is responsible for cooperative effects between neighboring hydrogen bonds in cyclic hydrogen bonded dimers, or more generally in hydrogen-bonded chains in solids [ 10,24—34]. 

The 0-0 and H-H RDFs (not shown) indicate that no 0-0 or H-H covalent bonds are formed during the simulations at all densities. The g(Roti) shows a lattice-like structure at 115 GPa, which is consistent with proton diffusion via a hopping mechanism between lattice sites.65 At 34 GPa, the coordination number for the first peak in g(RQH) is 2, which indicates molecular H20. Between 95 GPa and 115 GPa, however, the coordination number for the first peak in g(RQH) becomes four, which indicates that water has formed symmetric hydrogen bonds where each oxygen has four nearest-neighbor hydrogens. 

The elimination of the elements of water from 2-butanol can best be explained by trans elimination reaction involving the anchimeric assistance of hydrogen. The assistance of neighboring hydrogen in reactions in solution has been repeatedly observed 96, 97). The elimination reaction can be presented as follows ... 

Coupling follows the n + 1 rule. According to this rule, a peak splits into n + 1 peaks due to neighboring hydrogen atoms, where n is the number of equivalent hydrogen atoms. The amount of splitting is expressed by the coupling constant (J). 

The internal rotations around the skeletal bonds of PE are hindered due to interaction between the neighboring hydrogens. Since second and higher orders of interactions are not negligible, the internal rotations are interdependent. The statistics of such interdependent rotations is developed and applied to obtain the configurational partition function and the mean-square end-to-end... 

Identify and count neighboring hydrogens that are not chemically equivalent. Use + 1 to figure the number of peaks created by splitting for the chemically equivalent hydrogens. 

The beta phase consists of a combination of arrangements in which each corner and each face-centered palladium atom is surrounded by four and only four nearest neighbor hydrogen atoms. 

The model described is far too elaborate to use in detail to calculate a partition function. However, the average resonance energy can be assigned to pairs of neighboring hydrogen atoms. The calculation of the equilibrium pressure now becomes that of the quasichemical approximation. Lacher (4) has used the quasichemical approximation to calculate the isotherms without justifying the attractive potential between the hydrogen atoms. The partition function assumed lacks the detail to explain either the heat capacities or the hysteresis. 

Solution (a) The signal will be centered at a g value of 2.0022 (Table 11.1) and will be split into a 1 1 doublet by the neighboring hydrogen nucleus, (b) The signal will be centered at a g value of 2.00255 (Table 11.1) and will be split into a quartet (intensity ratio 1 3 3 1) by the three equivalent neighboring hydrogen nuclei. ... 

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