Hybrid orbitals lone-pair problem

Problem 24.20 Draw an orbital picture of thiazole. Assume that both the nitrogen and sulfur atoms are sp2-hybridized, and show the orbitals that the lone pairs occupy. 

To explain the bonding there would be two orbitals needed in carbon which should be concentrated strongly in opposite directions so that maximum overlap for the two bonds to hydrogen and to nitrogen would occur at an angle of 180°. On mathematical analysis of the problem it is found that one s and one p orbital can be hybridized to form two new hybrid orbitals, sp orbitals, to fulfill this requirement. Each new orbital is a mixture of one-half of an s orbital and one-half of a p orbital. The situation on nitrogen is quite similar, only here the second orbital would not be used in bond formation but would hold the lone pair. 

The existence of two different ionization potentials in ethers, R-O-R, shows that their lone pairs are not equivalent. This experimental phenomenon is most easily rationalized by supposing that the oxygen is sp2 hybridized, as shown in 21.12 One of the lone pairs has then sp2 character and the other is a p orbital. When their electron densities are combined, a rabbit ear structure (22) is obtained. This often-used representation is incorrect (because it implies an equivalence of the lone pairs) but convenient, especially when the molecule is deformed slightly to make the oxygen perfectly tetrahedral. Below, we will see how the stability of an ether is increased when one of the lone pairs lies antiparallel to a low-lying neighboring o cx orbital. This is easily seen using representation 22, but less obvious with 21. We will use whichever of these representations is better adapted to the problem at hand. 

A second problem occurs as a result of the possible stereochemical non-activity of the lone pair of electrons associated with the Group 15 element, and this is a problem common to the heavier elements of the neighbouring main groups in the Periodic Table. The VSEPR approach to molecular structure of the compounds of these heavier elements is tantamount to saying that d orbitals become part of any hybridization scheme and all the valence electrons are stereochemically active. 

Problem 1.4. What is the hybridization of the O atom in furan In what kind of orbitals do the lone pairs reside How many lone pairs are used in resonance ... 

Problem 13.3 Use Fig. 8.4 to make a sketch of the electron density contours of the hybrid orbitals pointing towards the ligands or containing the electron lone pair in SiCl2. Is the direction of the hybrid containing the lone pair consistent with the VSEPR model Note that the hybrid orbitals in the Fig. 8.4 are labeled with the p characters. 

Alternatively, consider the valence electrons on the oxygen atoms to be sp hybridized. Each sigma bond would then be formed from the overlap between an sp-hybridized orbital on C and one sp-hybridized orbital on an oxygen. The lone pair on each O atom would occupy the remaining sp orbitals. For the purposes of this problem, either description is acceptable because the molecular orbital description of the pi bonding... 

The fonnation of an alkynide ion (stabilized by having the lone pair on the carbon atom in an hybridized orbital) serves as a driving force for the reaction (See solution to problem 10.8). After the transformation above is complete, water is introduced as a proton source, giving 2. The more strongly basic position is likely protonated first, followed by protonation of the alkoxide ... 

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