Electron pair bonding characteristics

Nucleophilic addition (Section 17.6) The characteristic reaction of an aldehyde or a ketone. An atom possessing an unshared electron pair bonds to the carbon of the C=0 group, and some other species (normally hydrogen) bonds to the oxygen. 

A reasonable interpretation of the 0.72 metallic orbital per atom was not formulated until ten years later.8 It was then suggested that the metallic orbital permits the unsynchronized resonance of electron-pair bonds from one interatomic position to another by the jump of one electron from one atom to an adjacent atom, leading to great stabilization of the metal b3r resonance energy, and to the characteristic properties of metals,... 

The product of a elimination is a neutral species that resembles a carbocation in having only six carbon valence electrons. The simplest carbene is CH2, methylene. Carbenes are highly reactive, so much so that they cannot be isolated. Their involvement in reactions usually has to be inferred from the nature of the products or the reaction kinetics. The characteristic carbene reactions involve forming an electron-pair bond to the carbene carbon by reacting with cr bonds, it bonds, or unshared pairs ( ), Some of these reactions are illustrated here for methylene ( CH2). ... 

This chapter will concentrate primarily on features of the chemistry of diboron compounds of particular interest from the organometallic point of view. While our limited scope does not permit a comprehensive review of all aspects of diboron chemistry, we will initially survey some general features of the subject, with emphasis on unique synthetic aspects and on properties particularly characteristic of compounds containing a simple electron-pair bond between boron atoms. Previously, we published several review articles provided some aspects on boron chemistry,8 16 and including natural boron-containing compounds.17,18... 

Characteristically a covalent bond between two atoms is an electron-pair bond two electrons of opposite spin are in states described by wave functions that have the same shape. But a bond formed by only one electron has most of the same physical characteristics. Look first, therefore, at a one-electron bond, in particular at the simplest instance of it, the hydrogen-molecule ion Hj ... 

The first and third examples illnstrate a characteristic property of electron movanent If an electron pair moves toward an atom, that atom must have a place to put that electron pair, so to speak. In nucleophilic substitution, the carbon atom in a haloalkane has a filled outer shell another electron pair cannot be added without displacement of the electron pair bonding carbon to halogen. The two electron pairs can be viewed as flowing in a synchronous manner As one pair arrives at the closed-shell atom, the other departs, thereby preventing violation of the octet rule at carbon. When you depict electron movement with curved arrows, it is absolutely essential to keep in mind the rules for drawing Lewis structures. Correct use of electron-pushing arrows helps in drawing such structures, because aU electrons are moved to their proper destinations. 

For a classical covalent bond, the basin is disynaptic, its population is close to 2.0, and the variance (and covariance) is significantly smaller than the population, while a classical ionic bond like NaCl has rally core and monosynaptic basins [9, 16, 53]. Scheme 2 summarizes these features, which defines only two electron-pair bond families, either covalent or ionic, in the original ELF formulations. Any bond with very different values, for the basin population and the corresponding fluctuation index, will not qualify as either covalent or ionic. However, as will be shown immediately, CSB possesses unique ELF characteristics, which foretell the repulsive (or slightly attractive) covalent density. 

One of the most useful reactions in forming a P—C bond is the Michaehs-Arbusov reaction, which is a characteristic reaction of tricoordinate phosphoms compounds containing an alkoxy group (22). Alkylation of the electron pair is followed by rearrangement of the initial phosphonium salt. 

Chemical Properties The formation of salts with acids is the most characteristic reaction of amines. Since the amines are soluble in organic solvents and the salts are usually not soluble, acidic products can be conveniendy separated by the reaction with an amine, the unshared electron pair on the amine nitrogen acting as proton acceptor. Amines are good nucleophiles reactions of amines at the nitrogen atom have as a first step the formation of a bond with the unshared electron pair of nitrogen, eg, reactions with acid anhydrides, haUdes, and esters, with carbon dioxide or carbon disulfide, and with isocyanic or isothiocyanic acid derivatives. 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

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). 

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. 

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