Rule-breaking theory

The idea that there is a stability associated with the electron arrangements of the atoms of noble gases is sometimes called the octet rule. This rule applies to many compounds, but it is not always obeyed (unfortunately scientific theories are often not as neat as we would like them to be ). When does the octet rule break down ... 

First approximation theory leads to certain wave mechanical selection rules on the basis of which a radiative electronic transition may be classified as allowed (high probability) or forbidden (vanishingly low probability). Some forbidden transitions are indeed too weak to observe easily but in actual practice with polyatomic molecules the selection rules often break down sufficiently to permit reasonably strong absorption processes to occur. The following kinds of transition are forbidden... 

As becomes evident from the two preceding examples, the VBSCD has the shape it has because of the crossing of the two HL structures that describe reactant and product bonding. In fact, as long as the bonds that break and form in the reaction are covalent (polar-covalent), this interchange of the HL structures will always occur along the reaction coordinate. This is a simple fundamental rule of VB theory ... 

A chemical reaction always involves bond-breaking/making processes or valence electron rearrangements, which can be characterized by the variation of VB structures. According to the resonance theory [1, 50], the evolution of a system in the elementary reaction process can be interpreted through the resonance among the correlated VB structures corresponding to reactant, product and some intermediate states. Because only symmetry-adapted VB structures can effectively resonate, all VB structures involved in the description of a reaction will thus retain the symmetry shared by both reactant and product states in the elementary process. Therefore, we postulate that the VB structures of the reactant and the product states for concerted reactions should preserve symmetry-adaptation, called the VB structure symmetry-adaptation (VBSSA) rule. 

Success in scientific theory is won, not by rigid adherence to the rules of logic, but by bold speculation which dares even to break those rules if by that means new regions of interest maybe opened up. 

Oil-Water Versus Water-Oil Emulsions. If oil and water are vigorously shaken, they form a dispersion of water droplets in oil and oil droplets in water. When shaking is stopped the phases start to separate small water drops fall toward the interface, and oil drops rise. The emulsion quickly breaks. Adding an emulsifier to the system changes the outcome after standing, one phase becomes continuous, while the other remains dispersed. The nature of the emulsion is determined by the emulsifier. As a general rule, the continuous phase is the one in which the emulsifier is soluble. Thus sodium stearate promotes an oil-in-water (o/w) emulsion, while zinc distearate promotes a water-in-oil (w/o) emulsion. Several qualitative theories have been advanced to explain this empirical mle. 

Theories of Demulsification. Within commercial emulsion breaking, a number of general rules help to form the basic philosophy of how emulsions behave ... 

The classic carbonium theory of this reaction (/3-rule) postulates the formation of a carbonium ion at one of the carbon atoms close to the bridgehead carbon. Among the possible Wagner-Meerwein transpositions, the ones leading to four-membered rings are not observed, and may, instead, produce ring breaking (Fig. 16). 

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