Some Important Exceptions to the Rule

PROBLEMS Predict the major product for each of the following reactions  

In the beginning of this chapter, we saw a golden rule that helped us understand most of the chemistry that we explored. That rule was always re-form the earhonyl if you can, but never expel H orC . 

The truth is that there are a few, rare exceptions to this rule. In this section, we wiU look at two of these exceptions. 

We will now explore one other exception to the golden rule. There is a reaction where it seems like we are re-forming a carbonyl group to expel C . This reaction, called the Baeyer-Villiger reaction, is extremely useful. If you know how to use it properly, you will find that you might use it many times to solve synthesis problems in this course. So, we will spend some time covering that reaction now. 

The Baeyer-Villiger reaction uses a peroxy acid as the reagent  

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There are some important exceptions to the rules discussed here. For example, tin forms both Sn2+ and Sn4+ ions, and lead forms both Pb2+ and Pb4+ ions. Also, bismuth forms Bi3+ and Bi5+ ions, and thallium forms Tl+ and Tl3+ ions. There are no simple explanations for the behavior of these ions. For now, just note them as exceptions to the very useful rule that ions generally adopt noble gas electron configurations in ionic compounds. Our discussion here refers to representative metals. The transition metals exhibit more complicated behavior, forming a variety of ions that will be considered in Chapter 20. 

There are some important exceptions to the rules discussed here. For example, tin forms both Sn and Sn ions, and lead forms both Pb and Pb ions. Also, bismuth... 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

The octet rule works best for elements in the second period of the periodic table. These elements have only 2s and 2p valence subshells, which can hold a total of eight electrons. When an atom of one of these elements forms a covalent compound, it can attain the noble gas electron configuration [Ne] by sharing electrons with other atoms in the same compound. In Section 8.8. we will discuss some important exceptions to the octet rule. 

Exothermic reactions with a decrease in entropy reach equilibrium (AG = 0) at some temperature and reverse beyond this point. This is evident from Eq. (4.2) where the negative term AH will cancel with the positive term TAS when T gets sufficiently large. Since we already noted that such reactions are common in the chemical industry, should we expect most reactions to be reversible In principle, yes, but in practice we operate many reactors at a temperature far below the equilibrium point and therefore never notice any influence of the reverse reaction. There are, however, industrially important exceptions to this rule. The manufacture of ammonia from nitrogen and hydrogen and the formation of sulfur trioxide from sulfur dioxide and oxygen are two prominent cases. 

In this chapter we will define what we mean by a ceramic and will also describe some of the general properties of ceramics. The difficulty when drawing generalizations, particularly in this case, is that it is always possible to find an exception to the rule. It is because of the wide range of properties exhibited by ceramics that they find application in such a variety of areas. A general theme throughout this book is the interrelationship between the way in which a ceramic is processed, its microstructure, and its properties. We give some examples of these interrelationships in this chapter to illustrate their importance. 

The most important exception to this rule is the specific adsorption, the UPD, of H+ ions or hydrogen adsorption on some noble metal electrodes. These systems were studied from several molar acid concentrations to very high pH values. For detailed discussion of UPD see Sects. 3.2 and 3.3. 

An important exception to this rule is nitric acid (HNO3), which through a different reduction half-reaction dissolves some of the metals below H2 in Ihe activity series. 

It s important that scientists communicate with each other. Communication requires that a standard vocabulary be adopted. In chemistry, part of that vocabulary is chemical nomenclature—the names and formulas of ions and compounds. In this chapter you will learn some of the rules chemists use to name substances and to write their formulas. Please realize that there are over 60 million known chemical compounds. There are always exceptions to the rules given in this chapter however, they are not ones with which you need to be concerned. 

The study of the interaction of electromagnetic radiation with solid or liquid matter requires some understanding of these phases. In the discussion of the interaction of radiation with gases it is generally sufficient to consider the energy levels of an individual molecule of a particular gas. Collision-induced phenomena, where at least two gas molecules are involved in a transition, provide an important exception to this rule (Subsection 3.3.d). However, in most cases the interaction of radiation with a gas can be adequately understood by considering quantum processes involving only one molecule. Such is not the case in interactions of radiation with solids or liquids. 

An important reason for the exceptions to the radius ratio predictions is that ions are not hard spheres but somewhat compressible, hence do not have a truly constant radius. Another reason for the inadequacy of the radius ratio rules, particularly when the anions are much larger than the cations, is that some structures are determined by the close packing of the anions, leaving the cations in holes between the anions. In such a case more anions may be packed around a cation of a given fixed radius than are predicted by the radius ratio, so that although the anions are touching each other, they are not touching the cation. However, if... 

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