Octet expansion

Before we leave the enhancement of the + R effect of S02Me by octet expansion, it is appropriate to mention its stereochemical aspect. This was examined by Kloosterziel and Backer in the later of their two papers to which reference has already been made78. They measured the pKa value of 4-methylsulfonyl-3,5-xylenol 25 and compared its acidity with that of 4-methylsulfonylphenol. Parallel measurements were made with 4-CN and 4-N02... 

Octet expansion (expansion of the valence shell to more than eight electrons) can occur in elements of Period 3 and later periods. These elements can exhibit variable covalence and be hypervalent. Formal charge helps to identify the dominant resonance structure. 

Write the Lewis structure, including typical contributions to the resonance structure (where appropriate, allow for the possibility of octet expansion, including double bonds in different positions), for (a) sulfite ion (b) hydrogen sulfite ion (c) perchlorate ion (d) nitrite ion. 

The (Tj value for SOMe is slightly different from that given by Taft and Lewis . The continuous increase in a, produced by attached electronegative atoms at S or C is, of course, to be expected. This also encourages octet expansion as measured by The zero value for SOMe presumably means that any tendency to octet expansion [conventionally n (pd) conjugation] is essentially cancelled by the n (pp) conjugation of the sulfur lone pair. For these -f R substituents no exaltation of (Tj,-type values in the direction - (Tj would be expected. The table shows two (T -type constants phenol dissociation. We have already... 

A hybridization scheme is adopted to match the electron arrangement of the molecule. Octet expansion corresponds to the involvement of d-orbitals. 

Structure (a) violates the octet rule of Lewis, but octet expansion became recognised as common and acceptable for a number of heavier... 

Octet expansion or hypervalent state is due to the involvement of d-orbitals of the same principal quantum shell (e.g., 3d in third period, 4d in fourth period, etc.). The two other factors that play a role in octet expansion are ... 

FHF-) formed by hydrogen bonding in F- and HF. It leads to 3c4e model which are an alternative to use of d-orbitals in hypervalent compounds with octet expansion, e.g. XeF2... 

A hypervalent compound contains an atom with more atoms attached to it than is permitted by the octet rule. Empty c/-orbitals are utilized to permit octet expansion. 

Period 3 or higher (Z > 14) The valence of an element may be increased through octet expansion allowed by access to empty rf-orbitals. Two examples arePCIs and SFe. 

However, in this case, although the neon inert gas configurations of the terminal oxygen atoms are maintained, that of the central S atom needs to be expanded to twelve electrons, i.e. six shared electron pairs. This type of octet expansion has been seen previously in SF. ... 

A Lewis acid is any species capable of accepting a pair of electrons, and a Lewis base is a species with a pair of electrons available for donation. The terms acceptor and donor are also commonly used. Lewis acids include H+ and metal cations, molecules such as BF3 with incomplete octets, and ones such as SiF4 where octet expansion is possible (see Topic Cl). Any species with nonbonding electrons is potentially a Lewis base, including molecules such as NH3 and anions such as F. The Lewis acid-base definition should not be confused with the Bronsted one (see Topic E2) Bronsted bases are also Lewis bases, and H+ is a Lewis acid, but Bronsted acids such as HC1 are not Lewis acids. 

Hydrogen and boron stand out in their chemistry. In the other elements, valence states depend on the electron configuration and on the possibility of octet expansion which occurs in period 3 onwards. Multiple bonds are common in period 2, but are often replaced by polymerized structures with heavier elements. 

Hess law, 30, 85, J46 heteropolar bond, 23 heteropolymetallate, 223 high-spin complex, 213 HOMO, 75, 90 homopolar bond, 23 H ckel theory, 82 Hund s first rule, KL, 11, 70 hybridization, 73, 77 hydrazine, 162 hydride, 26,129,150 hydroformylation, 268 hydrogen bond, 79, 91, 151 hydrogen electrode, 140, 143 hydrogen peroxide, 171, 266 hydrogenation, 268 hydrothermal reaction, 41, 114, 257 hydroxide, 130,132,138,119,110 hypervalence, see octet expansion... 

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