Chlorine compounds, electron density

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

In contrast, methyl-for-chlorine substitution is decidedly nonlinear, a feature also displayed by the lighter Group 14 compounds. This curvature is not an artifact of the BAC-MP4 predictions, since it is observable in the (admittedly limited) experimental data for these compounds (Fig. 10). In fact, the deviations from linearity are even greater in the experimental data. Such behavior is also observed in the analogous Si compounds and is related to the negative hypercongugation (anomeric) effect, in which electron density from... 

The clue to the reaction is the polarity of the double bond. The lowest electron density is at the carbon linked to hydrogen, and is due to the strong inductive effect of the difluoromethylene groups and a slight effect of the vinylic chlorine. The attacking species, ethoxide anion, which is in an equilibrium with hydroxide ion in alcoholic potassium hydroxide, reacts in an SN2 reaction by joining the carbon bonded to hydrogen. The subsequent shift of the double bond facilitates ejection of fluorine as an anion and leads to an ether, compound O [77]. 

In order for dopant atoms to be stabilized within a host lattice, both types of atoms must have similar electronegativities. If this prerequisite were not met, electron density would transfer to the more electronegative atoms, forming a compound between lattice and dopant atoms. For instance, the exposure of an iron lattice to chlorine gas would result in a yellow compound of FeCls instead of reddish crystals of Cl-doped iron. 

We will not spend much time on the other mode of interaction, the one that occurs in compounds such as PCI3, CCI4, or H2O. For the moment, we can think of the covalent interaction between the atoms in these compounds as a sharing of electron density. This mode occurs between atoms that have relatively similar electronegativities. For example, the difference in the electronegativities of phosphorus and chlorine is not sufficient to cause an exchange of electrons. Instead, some of the eleetron density of the phosphorus is shared with the chlorine and vice versa. 

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