Iodine, polarizability

Polarizability (Section 4 6) A measure of the ease of distortion of the electric field associated with an atom or a group A fluonne atom in a molecule for example holds its electrons tightly and is very nonpolanzable Iodine is very polanz able... 

Whereas this study focused on reaction protocol, the effect of the methylene source on selectivity was another important factor which demanded attention. Earlier studies have demonstrated that substitution of chloroiodomethane for diiodomethane leads to an increased reaction rate (Scheme 3.10) [22]. It is, thus, surprising that the use of chloroiodomethane in sub-protocol la leads a slower, less selective reaction. In contrast to the use of diiodomethane ( 100% conversion at 300 min), the reaction of chloroiodomethane only reaches 58% conversion after 300 min. Selectivity is severely reduced, dropping to 75 25 er. The failure of this reagent in the chiral process may be attributed to the obvious differences between the highly polarizable iodine and the more electronegative chlorine atom, although an exact analysis of the difference is not clear. 

This actually doesn t work well. Recall (Chapter 10) that nucleophilicity is dependent on polarizability (the size of the atom). Large atoms (like sulfur or iodine) are very polarizable, and therefore, they are excellent nucleophiles. Small atoms are not polarizable, and they make poor nucleophiles. H is as small as they come, and therefore H is not a great nucleophile. H is a good base (in fact, it is an excellent base), but it is not a good nucleophile. So, we use LAH or NaBH4 as a source of nucleophilic H . We can think of LAH and NaBH4 as delivery agents of nucleophilic H. ... 

Because of their low polarizability, BF3 and BCI3 are gases. The larger, more polarizable electron clouds on bromine and iodine make BBt3 a volatile liquid and BI3 a solid at room temperature. 

Gooding SR, Winn PJ, Maurer RI, Ferenczy GG, Miller JR, Harris JE, Griffiths DV, Reynolds CA (2000) Fully polarizable QM/MM calculations an application to die nonbonded iodine-oxygen interaction in dimethyl-2-iodobenzoylphosphonate. J Comput Chem 21(6) 478 t82... 

D. W. Hafemeister In addition there is a recent paper by Rama Reddy, Barros, and deBenedetti in which they cancelled out the iodine chemistry and obtained the ratio of (aR)i2t divided by (aR) i29. They quoted a value of about —0.78 0.04, so they were claiming linearity for the two different isotopes to ca. 5%. Wilets has calculated the polarizability of the nucleus, but I think that this effect is really quite small. 

Triphenyl-X -phosphorin interacts with iodine or other polarizable electron donors, as well as with such electron acceptors as tetrachloro-p-benzoquinone and tetracyanoethylene, to produce deeply colored solutions. Such coloration points to the formation of charge-transfer complexes (see p. 43). In some cases electron transfer occurs with the formation of 2.4.6-triphenyl-X -phosphorin cation radical and tetracyanoethylene anion radical. Weber is currently investigating the details of these reactions (see p. 43). 

It is instructive to look at the refractive indices for a variety of chemical structures (Table 3.1.) What one quickly sees is that polar compounds are not the same as polarizable compounds. Indeed, polarizability is more related to chemical structure features like overall size (higher homologs within a compound family have greater polarizabilities), and presence of conjugated electron systems (benzene is more polarizable than hexane polarizability increases in the order benzene < naphthalene < pyrene). Finally, molecules with large atoms containing nonbonded electrons far from the nucleus (e.g., bromine, iodine) are generally more polarizable. After this brief diversion, now we continue to use refractive indices to estimate polarizabilities. 

The use of wavepacket spectroscopy to follow the solvent-induced dissociation of iodine in solution has been described in detail by Scherer, Jonas, and co-workers [18, 28, 30]. Recently the role of the solvent in inducing the curve crossing has been examined by simulation [29], Remarkably, the experiments show that the wavepacket survives the solvent-induced curve crossing and appears intact (i.e., the atoms are separating ballistically) up to at least 4 A separation [28], The simulations imply that destruction of the wavepacket by the solvent cage (polarizable Ar atoms in this case) occurs between I-I separation of 5-6 A [29]. 

As a result of the influence of the very great polarizability of the iodine ion it can happen that the order of the boiling points of the alkali iodides is just such that lithium iodide shows the lowest boiling point. Thus it can also be made plausible that... 

---