Next page chemical structure

For many molecules it is possible to formulate valence-bond structures that are so reasonable and that account so satisfactorily for the properties of the substances that they are accepted by everyone without hesitation. The structures given on the next page may be shown for illustration. The physical and chemical properties of substances and the configurations of molecules associated with structures of this type are well understood, and this understanding forms the basis for a large part of chemical reasoning. 

Figure 6.1 Chemical structures of compounds discussed in Section V (continued on next page).

Thus the salt water can be converted into four simple substances hydrogen, oxygen, sodium, and chlorine (Figure 2.6 on the next page). Chemists are unable to convert these four substances into simpler ones. They are four of the building blocks of matter that we call elements, substances that cannot be chemically converted into simpler ones. (We will get a more precise definition of elements after we have explored their structure in more detail.)... 

The next few pages are an update on the state of the Celastraceae family in Latin America, detailing the different chemical structures found and the results of the studies of biological activity carried out since the last publication on the subject [1]. 

The molecular orbital picture we developed in Figure 9.7 does a good job of showing the stabilization, but it is not particularly well suited for bookkeeping purposes. In practice, the ease of bookkeeping—of easily drawing the next chemical reaction—makes the resonance picture simpler to use. The price is that you have to either draw two structures (or more in other cases) or adopt some other code that indicates the presence of more than one representation for the molecule. We will see many examples in the next pages and chapters. 

One can also use the chemical shifts of a reference compound as the base value if its structure is closely related to that assumed for the unknown. The increments corresponding to the structural elements missing in the reference compoimd are then added to the base value, while those of structural elements present in the reference but absent in the unknown are subtracted (see example on next page). 

Kekule structures are like Lewis structures except lone pairs are normally omitted. Structures are often further simplified by omitting some (or aU) of the covalent bonds and hsting atoms bonded to a particular carbon (or nitrogen or oxygen) next to it (with a subscript if there is more than one of a particular atom). Lone-pair electrons are usually not shown, unless they are needed to draw attention to some chemical property of the molecule. These structures are called condensed structures. Compare the condensed sttuctures shown here with the Lewis structures shown on page 17. 

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