Sulfur nonbonding electron pairs

The same equatorial preference is also manifested in the 3,3-disubstituted thietane oxides66,194. Thus, the NMR spectra of 5e,f contain two Me singlets at 1.23 and 1.30 ppm and two methylene multiplets at 3.03 and 3.53 ppm (in CDC13). The large difference in the chemical shifts of the axial and equatorial a-methylene hydrogens is characteristic of an axial nonbonded electron pair on sulfur (conformation 5e equation 73). This conformational preference is corroborated by the small differences in the chemical shifts of the two methyl groups, and fits the contention that 1,3-diaxial interactions are responsible for this ultimate result. Certainly, these interactions are greater in the conformer 5f. 

As Figure 1.5 suggests, the lowest energy transition for molecules with essentially nonbonding electron pairs is of n->n character. These transitions are common for compounds containing oxygen, nitrogen, and sulfur. 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

The size difference between carbon and sulfur atoms leads to relatively inefficient overlap of -tr-orbi-tals in the C=S bond. Consequently, thiocarbonyl compounds are in general highly reactive and have a tendency to di-, oligo- or poly-merize. This is particularly true for thioaldehydes, thioketones, and thio-ketenes. In contrast, thioamides (1) are usually perfectly stable and can be handled without problems. This stability can be understood in terms of a pronounced resonance interaction between the C =S TT-bond and the nonbonding electron pair on nitrogen. The analogous electron delocalization prevails in thiolactams. ... 

When both atoms that constitute the single bond possess nonbonding electron pairs, the barrier often is in the observable range. The high barrier may be due to electrostatic interactions or repulsions between lone pairs. For example, the barrier to rotation about the sulfur-sulfur bond in dibenzyl disulfide (C6H5CH2S-SCH2C6H5) is 7 kcal moF Similar high barriers have been observed in hydrazines (N-N), sulfenamides (S-N), and aminophosphines (N-P). 

Following Woodward, such conversions are called cheletropic reactions. The LUMO of the 1,3-diene surrounds the nonbonding electron pair of sulfur like the claws of a crab (Greek chele). 

How many nonbonding electron pairs are there per atom in chlorine, sulfur, phosphorus, and silicon ... 

Figure 12.20 By drawing Lewis structures you can show that there are three (chlorine), two (sulfur), one (phosphorus), and zero (silicon) nonbonding electron pairs per atom.

Note that the nonbonding electron pairs have been deleted from oxygen, nitrogen, sulfur, and chlorine for simpUcity. 

The simplest Lewis formula for SO2 would place two electron pairs between the S and each of the two O atonos, one electron lone pair at the S and two electron lone pairs at each O atom S( 0 )2. This Lewis formula implies that the sulfur atom in SO2 accomodates five electron pairs in the valence shell (as in SF4). Similarly the simplest Lewis formula of the trioxide would place two electron pairs between the S and each O and two nonbonding electron pairs at each of the latter, S( 0 )3, indicating that the S atom in SO3 accomodates six electron pairs in its valence shell (as in SFe) [3]. Note that both the angular structure of SO2 and the trigonal planar structure of SO3 are in agreement with the VSEPR model. 

Numerous organic and inorganic sulfur compounds are adsorbed at the oxide-free surfaces of Au and Pt electrodes and can be detected by Mode II [62]. These compounds include thioalcohols, thioethers, thiophenes, thiocarbamates, organic thiophosphates, and numerous inorganic compounds. Adsorption is a prerequisite to detection and therefore at least one nonbonded electron pair must reside on the S-atom. The kinetics for detection of adsorbed S-compounds are quite favorable at pHs from 0 to 14. Since alcohol and amine groups are detected only under highly acidic and/or alkaline conditions, the detection of sulfur compounds under mildly acidic conditions is highly selective. 

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