Common Bonding Situations

In Chapter 1 we learned that molecules that satisfy the octet rule are likely to be stable. Furthermore, we learned that the presence of formal changes in a molecule is often a destabilizing factor. The common ways in which atoms are bonded can be understood by using these two criteria. 

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Carbon is sometimes encountered with only three bonds and a negative charge. Such carbanions are less stable than the compounds above, but they are still important. 

Carbon radicals, with only seven electrons in the valence shell for carbon, and carbocations, with only six electrons and a positive charge on the carbon, do not satisfy the octet rule and are quite unstable. These species are only encountered as highly reactive, transient intermediates in certain chemical reactions. 

Discuss the stability of each of these species based on the octet rule and formal charges  

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Carbon in its most stable form has four bonds These may be single, double, or triple bonds. These are by far the most common bonding situations for carbon. 

On the other hand, molecular mechanics (MM) methods, based on classical concepts, are extremely fast, and are able to handle very large systems, such as entire enzymes, with ease. Some MM methods are also as accurate as the best ab initio methods, particularly for hydrocarbons. Most, but not all, MM methods are parameterized only for ground state systems, and only for common bonding situations. By their nature, they are unable to anticipate unusual bonding situations, the making and breaking of most bonds, the chemistry of electronically excited states of molecules—properties that are fundamentally quantum mechanical in nature. 

A summary of commonly encountered formal charges and the bonding situations in which they occur is given in Table 2.2. Although only a bookkeeping device, formal charges often give clues about chemical reactivity, so it s helpful to be able to identify and calculate them correctly. 

Thus, carbon usually forms four bonds. Any other arrangement either does not satisfy the octet rule or has a charge on the carbon. However, carbon is occasionally encountered in one of these less stable bonding arrangements. Both the common and more unusual bonding situations for various atoms are shown in Figures 2.1, 2.2, and 2.3. 

Quickly recognize the common ways in which atoms are bonded in organic compounds. You should also recognize unusual bonding situations and be able to estimate the stability of molecules with such bonds. 

Reactive intermediates are species that are so kinetically unstable that they cannot be isolated or observed under normal conditions. In organic chemistry, the radicals (R3C ), carbanions (R3C ), carbocations (R3C ), and car-benes (R2C ) are the more-common intermediates. The area, and the resulting chemistry, is far richer in organosilicon chemistry, because many of the bonding situations that produce quite stable organic compounds are highly reactive when Si replaces C. 

The ability to draw and manipulate resonance structures is an important skill that will be needed throughout your study of organic chemistry. With practice, you will begin to recognize certain common bonding patterns for which more than one Lewis structure can be drawn. For now, notice that two different re.sonance structures can be drawn in the following situations ... 

Selecting the catalyst and reaction conditions for partial reduction of a triple bond situated in a conjugated system is a challenge. Where the hydrogenation of an isolated alkyne can proceed with nearly complete selectivity, the partial saturation of an enyne takes place selectively with much more difficulty. Selectivities of 85-90% in these latter reactions are common and are considered to be reasonable for synthetic applications. 

A summary of commonly encountered formal charges and the bonding situations in which they occur is given in Table 2.2. 

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