Organic molecules constitutional isomers

The stereospecificity of living organisms is imperative to their efficiency. The reason is that it is just not possible for an organism to be so constructed as to be able to deal with all of the theoretically possible isomers of molecules with many asymmetric centers. Thus a protein molecule not uncommonly has 100 or more different asymmetric centers such a molecule would have 2100 or 1030 possible optical isomers. A vessel with a capacity on the order of 107 liters would be required to hold all of the possible stereoisomeric molecules of this structure if no two were identical. An organism so constituted as to be able to deal specifically with each one of these isomers would be very large indeed. 

Structural (or constitutional) isomers are compounds with the same molecular formulas but different structural formulas (that is, different arrangements of the atoms in the molecule). Isomerism is especially important in organic chemistry because of the capacity of carbon atoms to be arranged in so many different ways continuous chains, branched chains, and rings. Structural formulas can be written so that every bond is shown, or in various abbreviated forms. For example, the formula for n-pentane (n stands for normal) can be written as ... 

As an example, consider the molecule formed from one atom each of carbon, hydrogen, and nitrogen. We can assemble these atoms in two different ways and still satisfy the octet rule. (This is another example of compounds that have the same molecular formula but a different arrangement of bonded atoms. Such compounds are called constitutional isomers or structural isomers and are very common in organic chemistry.) The structure on the left was the one used in Figure 1.6. The structure on the right is obtained if the hydrogen is bonded to the N rather than the C. 

In Chapter i you learned the geometry of the bonds around an atom. For example, the four bonds of an -hybridized carbon have a tetrahedral geometry. But what happens when several such carbons are bonded together What is the geometrical relationship between the bonds on different carbons What is the overall shape of the molecule Is more than one shape possible If so, are they different in energy Can they interconvert If so, how fast These and other questions will be answered in this chapter and the next, which discuss the stereochemistry, or three-dimensional structures, of organic molecules. In these chapters you will encounter a new type of isomer stereoisomers. Unlike the constitutional isomers that you have already seen, stereoisomers have the same bonds or connectivity, but the bonds are in a different three-dimensional orientation. 

Stereochemistry is the study of the three-dimensional structure of molecules. No one can understand organic chemistry, biochemistry, or biology without using stereochemistry. Biological systems are exquisitely selective, and they often discriminate between molecules with subtle stereochemical differences. We have seen (Section 2-8) that isomers are grouped into two broad classes constitutional isomers and stereoisomers. Constitutional isomers (structural isomers) differ in their bonding sequence their atoms are connected differently. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. 

A tautomerization reaction interconverts two constitutional isomers of an organic molecule, referred to as tautomers, which differ only by the positions of hydrogen atoms. The scheme below shows how the reaction typically takes place at the ketone functional group of an organic molecule. The ketone form of the molecule is commonly referred to as the keto form of the molecule, while the alcohol form is commonly referred to as the enol form of the molecule. 

In conventional organic nomenclature, a polymer is not considered to be an isomer of the repeating molecular unit, because the molecular formulas formally differ. This is a somewhat arbitrary distinction, however, because it is never really an isolated, single molecule of monomer that is compared with the polymer. In an aggregate of monomer molecules, intermolecular forces exist and the constitutional difference from an aggregate of polymer molecules is simply that some intermolecular forces have been converted into true chemical bonds. In any case, the term polymerization isomerism has had a long-standing use in coordination chemistry. It may refer... 

The first three chapters constitute a review of bonding and an introduction to organic compounds. Functional groups are introduced. Resonance is covered extensively, and numerous examples are provided. Acid-base chemistry is discussed in Chapter 4, and this reaction is used to introduce many of the general features of reactions, including the effect of structure on reactivity. Nomenclature of all of the functional groups is covered in Chapters 5 and 12. In this edition, stereochemistry is covered in two chapters to break up the material Chapter 6 discusses cis trans isomers and conformations, and Chapter 7 addresses chiral molecules. 

The heat of combustion, and hence also the heat of formation of organic compounds depend in a marked degree upon their constitution, that is to say, they depend not only upon the number and nature of the atoms in the molecule, but also upon the manner in which they are bound together. Isomeric compounds may have totally different heats of formation. Thus acetone CHg. CO. CHg has a heat of combustion equal to 423000 cal., while the heat of combustion of its isomer allyl alcohol CHg. CHCHg(OH) is 465000 cal., and of its other isomer propionic aldehyde CHgCHgCHO is 441000 cal. Similarly, the heat of combustion of polymeric substances (per gram) varies with the size of the molecule, as the following table shows ... 

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