Organic Compounds --- A First Look

Now that you have refreshed your basic understanding of bonding, you are ready to examine organic compounds in more detail. The purpose of this chapter is to provide more experience with simple organic molecules. You will learn about how their atoms are connected (the structure of the molecule) and how their structures affect some of their properties. 

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. 

Look for this logo in the chapter and go to OrganicChemistryNow at http //now.brookscole.com/hornback2 for tutorials, simulations, problems, and molecular models. 

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. 

1 In estimating the stability of a species, the first thing to consider is the octet rule. Species that satisfy the octet rule are likely to be stable. If the octet rule is not satisfied, the species is not very stable. Next look for the presence of formal charges. Their presence causes some destabilization. For example, the structure in part (a) satisfies the octet rule at all atoms. However, one of the carbon atoms has a formal charge so it is destabilized somewhat. 

9 Use the following procedure to calculate the DU of formulas that have atoms other than C and H  

1) Add the number of halogens in the formula to number of hydrogens in the formula  

11 Intermolecular forces between molecules result from the attraction of opposite 

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The choice of a given database as source of auxiliary values may not be straightforward, even for a thermochemist. Consistency is a very important criterion, but factors such as the publication year, the assignment of an uncertainty to each value, and even the scientific reputation of the authors or the origin of the database matter. For instance, it would not be sensible to use the old NBS Circular 500 [22] when the NBS Tables of Chemical Thermodynamic Properties [17], published in 1982, is available. If we need a value for the standard enthalpy of formation of an organic compound, such as ethanol, we will probably prefer Pedley s Thermodynamic Data and Structures of Organic Compounds [15], published in 1994, which reports the error bars. Finally, if we are looking for the standard enthalpy of formation of any particular substance, we should first check whether it is included in CODATA Key Values for Thermodynamics [16] or in the very recent Active Thermochemical Tables [23,24],... 

Before we inspect Kf values of a variety of organic compounds for seawater (the most important natural saline environment), we first take a look at the salting-out efficiencies of various ion combinations. Since it is very difficult to quantify the contribution of individual ions, salting constants are available only for combined salts. Nevertheless, the data in Table 5.6 illustrate that smaller ions that form... 

Let us first look at some qualitative aspects of how CMOSs affect the activity coefficient, and thus the solubility and partitioning behavior, of a given organic compound when present in a water/CMOS mixture. The following general conclusions are illustrated by the examples given in Figs. 5.6 to 5.8 and in Table 5.8. 

For rapid access to information such as mp, bp, density, solubility, optical rotation, A max, and crystal form, one turns first to the Handbook of Chemistry and Physics where information is found on some 15,000 organic compounds, including the Beilstein reference to each compound. These compounds are well known and completely characterized. The majority are commercially available. The Merck Index contains information on nearly 10,000 compounds, especially those of pharmaceutical interest. In addition to the usual physical properties, information and literature references to synthesis, isolation, and medicinal properties, such as toxicity data, are found. The last third of the book is devoted to such items as a long cross index of names (which is very useful for looking up drugs), a table of organic name reactions, an excellent section on first aid for poisons, a list of chemical poisons, and a listing of the locations of many poison control centers. 

When one wishes to synthesize a given compound, it is normal practice to search the literature as described above in order to find out by what methods the compound has been synthesized, or in the case of new compounds, how related compounds have been synthesized. The procedures reported in the literature are of variable reliability and are often rather incomplete. Therefore it is advisable to look in some secondary sources to see if more complete or reliable information may be found. The first such source would be Organic Syntheses. This series of publications gives complete methods for the preparation of a large number of organic compounds. Each procedure has been submitted by one laboratory, checked by another, and is supplied with a complete set of notes which should enable anyone to prepare the compound satisfactorily. Similar volumes on related subjects have appeared and are entitled Inorganic Syntheses and Biochemical Preparations. ... 

The most efficient way to use this index is to look for the pertinent property (e.g., vapor pressure, entropy),/process (e.g., disposal of chemicals, calibration), ov general concept (e.g., units, radiation). Most primary entries are subdivided into several secondary entries, e.g., under heat capacity there are 17 secondary entries such as air, metals, water, etc. Primary entries will be found for certain classes of substances, such as alloys, elements, organic compounds, refrigerants, semiconductors, etc. Primary entries are also given for the individual chemical elements and for a few compounds such as water and carbon dioxide. However, only the most important tables are listed under these substances. Therefore, the user will find in most cases that it is best to look first for the property of interest, then examine the table or tables that are referenced. 

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