Hybrid orbitals lone electron pairs

Pyridines form stable salts with strong acids. Yellow ionic picrates were used for characterization in the past. Pyridine itself is often used to neutralize acid formed in a reaction and as a basic solvent. The basicity of pyridine (as measured by the dissociation constant of its conjugate acid, pKa 5.2) is less than that of aliphatic amines (cf. NH3, pA"a 9.5 NMe3, pKd 9.8). This reduced basicity is probably due to the changed hybridization of the nitrogen atom in ammonia the lone electron pair is in an sp3-orbital, but in pyridine it is in an s/r-orbital. The higher the s character of an orbital, the more it is concentrated near the nucleus, and the less available for bond formation. Nitriles, where the lone electron pair is in an. vp-orbital, are of even lower basicity. 

How is the nitrogen atom of the pyridine ring hybridized The N atom is surrounded by three groups (two atoms and a lone electron pair), making it sp hybridized, and leaving one unhybridized p orbital with one electron that overlaps with adjacent p orbitals. The lone pair on N resides in an sp hybrid orbital that is perpendicular to the delocalized n electrons. 

In the case of ammonia, the five valence electrons surrounding nitrogen— two electrons occupying the outermost 2s orbital and three electrons in three 2p orbitals— hybridize to form four sp orbitals that, if equal, would separate themselves in three dimensional space by pointing at the comers of a tetrahedron. One of these orbitals, however, contains the lone electron pair, and the three remaining sp orbitals make additional space available by pointing at the comers of a pyramid with a triangular base. 

The geometries found for the complexes of formaldehyde with Brst and second row cations in theoretical studies were analyzed in terms of molecular orbitals. Based on the results of photoelectron spectroscopy, it was argued that the carbonyl group contains two nonequivalent lone pairs an sp-hybridized orbital contains one pair of electrons along the C—O axis and a second, higher energy lone pair in a p-like orbital lies perpendicular to the C=0 axis (Figure 7). ... 

The w-system is, in addition, destabilized by a repulsive interaction between the lone electron pairs on the fluorine atoms and the r-orbitals on the sp hybridized carbon atoms. Nucleophilic attack on the carbon induces re-hybridization to the sp state, relieving some of this repulsive strain. 

Describe the bonding in 1-methoxypropanone (CH3OCH2COCH3) in terms of (a) hybridization of C and O, (b) type of bonds between C and O, and (c) type of orbitals that hold the lone electron pairs on O. 

Just as in the case of the oxofluoramido complexes (see Section 2.1) the tungsten-nitrogen bond in complexes with ketimines also exhibits essentially a double bond character, which is due to the Pir-dir interaction between the lone electron pair of the nitrogen atom and the vacant orbital of the tungsten atom. As a result of this interaction, the nitrogen atom of the coordinated ketimine acquires a hybridization close to the sp type, an allene-like linear fragment W N=C is formed, and the ligand plane is oriented in the equatorial plane of the complex. The hindered character of rotation about the W-N bond in ketimines with unlike substituents at the carbon atom leads to the appearance of two enantiomers at -30 °C ... 

The lone pair of the nitrogen atom is localized and occupies an y> -hybridized orbital, and as a result, pyridine is a stronger base than pyrrole. Nevertheless, pyridine is still a weaker base than alkyl amines, because the lone pair is housed in an r -hybridized orbital, rather than an y> -hybridized orbital (as seen in Figure 18.14). By occupying an -hybridized orbital, the electrons of the lone pair have more s character and are therefore closer to the positively charged nucleus, rendering them less basie. 

As a result of an analysis of numerous data on the direction of hydrolysis reactions of ethers and amides which proceed via formation of the type XXVII intermediates, a simple phenomenological theory was evolved of stereoelectronic control of fragmentation of tetrahedral intermediates [120, 121]. This theory rests on the classical notion of the hybrid orbitals of electron lone pairs (ELP) of heteroatoms. 

Hybrid orbitals may contain lone electron pairs ammonia and water... 

Figure 17-1 Orbital picture of the carbonyl group. The sp hybridization and the orbital arrangement are similar to those of ethene (Figure 11-2). However, both the two lone electron pairs and the electronegativity of the oxygen atom modify the properties of the functional group.

Comparison with the electronic structure of an alkene double bond reveals two important differences. First, the oxygen atom bears two lone electron pairs located in two sp hybrid orbitals. Second, oxygen is more electronegative than carbon. This property causes an appreciable polarization of the carbon-oxygen double bond, with a partial positive... 

In the nitriles, both atoms in the functional group are sp hybridized and a lone electron pair on nitrogen occupies an sp hybrid orbital pointing away from the molecule along the C-N... 

Figure 20-3 (A) Orbital picture of the nitrile group, showing the sp hybridization of both atoms in the C=N function. (B) Molecular structure of ethanenitrile (acetonitrile), which is similar to that of the corresponding alkyne. (C) Electrostatic potential map of ethanenitrile, depicting the positively polarized cyano carbon (blue) and the relatively negatively polarized nitrogen (green) with its lone electron pair (red).

In contrast to amides (Section 20-1), the nitrogen orbitals in amines are very nearly sp hybridized (see Section 1-8), forming an approximately tetrahedral arrangement. Three vertices of the tetrahedron are occupied by the three substituents, the fourth by the lone electron pair. As we shall see, it is this electron pair that is the source of the basic and nucleophilic properties of the amines. The term pyramidal is often used to describe the geometry adopted by the nitrogen and its three substituents. Figure 21-1 depicts the structure of methanamine (methylamine). 

Figure 25 1 Orbital pictures of (A) pyrrole and (B) furan p< = O), and thiophene = S). The het-erocitom in each is sp hybridized and bears one delocalized lone electron pair.

Pyridine contains an 5/ -hybridized nitrogen atom like that in an imine (Section 17-9). In contrast to pyrrole, only one electron in the p orbital completes the aromatic rr-electron arrangement of the aromatic ring as in the phenyl anion, the lone electron pair is located... 

Figure 25-2 (A) Orbital picture of pyridine. The lone electron pair on nitrogen is in an sp -hybridized orbital and is not part of the aromatic 7T system. (B) The electrostatic potential map of pyridine reveais the location of the lone electron pair on nitrogen (red) in the molecular plane and the electron-withdrawing effect of the electronegative nitrogen on the aromatic tt system (green compare to the electrostatic potential map of pyrrole in Section 25-3).

Replacement of one (or more) of the CH units in benzene by an sp -hybridized nitrogen gives rise to pyridine (and other azabenzenes). The p orbital on the heteroatom contributes one electron to the 77 system the lone electron pair is located in an sp hybrid atomic orbital in the molecular plane. Azabenzenes are electron poor, because the electronegative nitrogen withdraws electron... 

Oxygen atom has initially six electrons in the 2s and 2p orbits and then captures two more electrons from each of its B neighbors. The eight electrons fuUy occupy the 2s and 2p levels of an oxygen atom that hybridizes its sp orbits then to form four directional orbits. The eight electrons repopulate in the four directional orbitals with two electron pairs shared between O and B. The remaining two orbits are occupied by the lone electron pairs of oxygen. 

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