Free electron pairs

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

Some systematic studies on the different reaction schemes and how they are realized in organic reactions were performed some time ago [18]. Reactions used in organic synthesis were analyzed thoroughly in order to identify which reaction schemes occur. The analysis was restricted to reactions that shift electrons in pairs, as either a bonding or a free electron pair. Thus, only polar or heteiolytic and concerted reactions were considered. However, it must be emphasized that the reaction schemes list only the overall change in the distribution of bonds and ftee electron pairs, and make no specific statements on a reaction mechanism. Thus, reactions that proceed mechanistically through homolysis might be included in the overall reaction scheme. 

The two reaction schemes of Figures 3-13 and 3-15 encompass a large proportion of all organic reactions. However, these reactions do not involve a change in the number of bonds at the atoms participating in them. Therefore, when oxidation and reduction reactions that also change the valency of an atom ate to be considered, an additional reaction scheme must be introduced in which free electron pairs are involved. Figure 3-16 shows such a scheme and some specific reaction types. 

The other two coordination positions are available for groups of the fiber having free electron pairs, as ia (51) where B is —NH2, —NH—, or —OH. 

In -isosparteine (47) both tertiary protons at C-6 and C-11 are arranged in the CIS position to the free electron pair on nitrogen as indicated by the NMR spectrum (80). Much earlier X-ray analysis showed that all rings in crystalline a-isospa teine (46) are present in stable chair conformations (SJ). A comparative rate (extrapolated) at 65 for a-isosparteine (5.0) and sparteine (1.0) has been calculated (82). Also it has been reported that (8-isosparteine gave the dehydro derivative under mild conditions and the didehydro under more drastic conditions (times, temperatures not given) (60). 

Another convenient method for the preparation of tertiary enamines involves the dehydrogenation of saturated bases with mercuric acetate (111-116). A trans-1,2 elimination occurs, which requires an antiperi-planar position of the nitrogen-free electron pair and the eliminated atom. A preferential elimination of the hydrogen atom from the tertiary carbon atom is supposed. Overoxidation can be avoided by adding disodium ethyl-enediaminotetraacetate to the reaction mixture (117). 

Bohlmann et al. (118-121) observed that an infrared absorption band between 2700-2800 cm is characteristic of a piperidine derivative possessing at least two axial carbon-hydrogen bonds in antiperiplanar position to the free-electron pair on the nitrogen atom. The possibility of forming an enamine by dehydrogenation can be determined by this test. Compounds which do not fulfill this condition cannot usually be dehydrogenated (50, 122,123). Thus, for example, yohimbine can be dehydrogenated by mercuric acetate,whereas reserpine or pseudoyohimbine do not react (124). The quinolizidine (125) enamines (Scheme 4), l-azabicyclo(4,3,0)-nonane, l-azabicyclo(5,3,0)decane, l-azabicyclo(5,4,0)undecane, and l-azabicyclo(5,5,0)dodecane have been prepared in this manner (112,126). 

The ultraviolet spectra were also used for determination of the pyrroline structure (1,158-160). They exhibit a bathochromic shift to 225-235 m, caused by the auxochromic action of the nitrogen-free electron pair which is in conjugation with n electrons of the enamine double bond (161,162). 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

The simplest examples of this type of compound are enamines derived from the quinuclidine skeleton (67). The formulation of enamines of qflmuclidine in a inesomeric form would violate Bredt s rule. Actually, the ultraviolet spectrum of 2,3-benzoquinuclidine shows that there exists no interaction of aromatic ring tt electrons and the nitrogen-free electron pair (160,169). The overlap of the olefinic tt orbital and the lone pair orbital on nitrogen is precluded. 

The typical properties of water arise from the ability of the water molecule to participate in four hydrogen bonds due to its two protons and its two lone electron pairs (2s)2 (2pz)2 which act as proton acceptors. In the condensed state, the angle between the 2px and the 2py orbital of oxygen is enlarged by hydridisation to a mixture of s- and p-state to 109°. Because both of the free electron pairs are situated in a plane... 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

In comparison with the decomposition of taws-azoalkanes 20 a much larger group size effect has been found for the thermolysis rates of a few c/s-azoalkanes 24. Due to the repulsion of the free electron pairs on the two nitrogen atoms and due to steric interaction between the cis oriented alkyl groups cis azoalkanes 24 decom-... 

RG12 Valence electrons are transformed from a free electron pair into bonding electrons and a change in the valence state of atom X occurs. Before this scheme is applied in EROS, a table of valence states for each atom is scanned to determine whether this change in the valence for atom X is allowed. The scheme has importance in representing oxidations at atom X as exemplified with the change Sn - SIV (Fig. 13). 

RG221 and RG222 Both reaction schemes involve the breaking and making of two bonds and the shifting of a free electron pair ... 

A positive charge in an empty rc-orbital can be stabilized by the + M effect exerted by the free electron pair on an adjacent atom X, or by a filled tr-orbital of a double bond C = D. In delocalizing a positive charge into a double bond C = D, the larger orbital coefficient is again on atom D. The higher the electronegativities of the orbitals on X or on D, the less they are available for donation of electron density into the... 

In which the terms Ha refers to the number of free electron pairs, MW is the molecular weight, and clogP is the computed lipophilicity. While this method could be stated to be "partially in silico" because it utilizes some chemical descriptors, the need for in vivo animal data and their dominance in the individual terms really makes this approach more of an animal-human correlation than an in silico method. Finally, in the same report, the authors describe a regression based solely in animal data. Overall, the performance of these... 

The other two electrons are found around the nitrogen atom as a free electron pair. Since the free electron pair does not form a bond, it occupies a larger volume in space then the bonding electron pairs between the nitrogen and hydrogen atoms. 

Substituents with lone electron pairs, such as alkoxy, hydroxy, alkyl, and aryl-amino groups, are known as electron donors. The CH3 group, despite the absence of such free electron pairs, is also considered an electron donor. Functional groups with conjugated rr-electron systems, such as NOz, COOH, COOR, SOz, or SOzAr act as electron acceptors. 

TT-electron interactions between the aromatic ring and adjacent conjugated free electron pairs, either nonbonding electron pairs or double bonds of substituents, improve electron delocalization, which in turn increases absorption intensity, that is, color strength. 

The commercially interesting metal complex pigments usually contain the co-ordinative tetravalent Cu+ + or Ni+ + ions, less commonly Co++ ions. The fourth coordination site is typically occupied by a solvent molecule with a free electron pair. It may also be engaged by the second nitrogen atom of a different pigment molecule, a phenomenon which is observed in azo complexes and similar materials. In the latter case, sandwich structures are obtained [5]. The copper and nickel complexes are mostly planar molecules. 

Not only heteroatom-H bonds but also activated C-H bonds can add to the jr-system of an allene. Since carbon lacks a free electron pair, the transition metal catalyst must first activate the C-H bond the new species formed will then react with the C=C double bond. For efficient activation of that kind, two acceptors (typically esters, nitriles and/or sulfones) are necessary. In accord with this mechanistic picture is the fact that the reaction does not benefit from an additional base (which would deproto-nate the pronucleophile). Hence neutral conditions are even better. 

Amongst the hydro-closo-polyborates B H 2- members with n = 6 to 12 are known. All hydro-closo- but also hydro-nido-polyborates act as bases they are, however, not typical Lewis-bases as they miss free electron pairs. However, the negative charge at the hydrogen atoms allows for an interaction with Lewis acids A by formation of hydride bridge bonds (3c2e bonds) ... 

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