Isomerism Constitutional Isomers and Stereoisomers

Isomers are different compounds that have the same molecular formula. In our study thus far, much of our attention has been directed toward isomers we have called constitutional isomers. 

We have already seen examples of some types of stereoisomers. The cis and trans forms of alkenes are stereoisomers (Section 1.13B), as are the cis and trans forms of substimted cyclic molecules (Section 4.13). 

Stereoisomers can be subdivided into two general categories those that are enantiomers of each other, and those that are diasteromers of each other. 

The alkene isomers cis- and fran5-l,2-dichloroethene shown here are stereoisomers that are diastereomers. 

Cis and trans isomers of cycloalkanes furnish us with another example of stereoisomers that are diastereomers. Consider the following two compounds  

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The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

The complex [Rh(en)2(N02)(SCN)]+ can exist in 12 isomeric forms, including constitutional isomers and stereoisomers. Sketch the structures of all 12 isomers. 

Isomerism types of isomerism in organic compounds, and we will cover them in detail in Chapter 5 (Stereochemistry). For now, we need to recognize the two large classes of isomers constitutional isomers and stereoisomers. 

Isomers can be broadly divided into two major classes constitutional isomers and stereoisomers. In Chapter 25 we discussed isomerism in coordination compounds, and in Chapter 27 we learned about some isomeric organic compounds. In this chapter we will take a more systematic look at some three-dimensional aspects of organic structures—a subject known as stereochemistry ( spatial chemistry ). 

Isomers are compounds with the same chemical formula but different properties (Section 3.2). Recall the discussion of isomerism in organic compounds (Section 15.2) coordination compounds exhibit the same two broad categories—constitutional isomers and stereoisomers. 

As we have previously learned, compounds that have the same molecular formula but different structural formulas are called isomers. Forms of isomerism abound in organic chemistry. We will encounter two types of isomers in this chapter—constitutional isomers and stereoisomers—but our focus in this section is on constitutional isomers. Constitutional isomers have different bond connectivities and thus different skeletal structures. For example, C4H10 has two constitutional isomers, as shown below ... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

The words isomer and stereoisomer are often tossed around quite loosely, and the subject of isomerism is worth a bit of review. Structural isomers (often called constitutional isomers) are molecules of the same formula, but differing atom-to-atom connectivity. Their constituent parts may well be different (but need not be). Butane (two methyl groups and two methylene groups) and isobutane (three methyl groups and one methine group) are typical examples of structural isomers (Fig. 4.54). 

Excluding enantiomers there are three isomeric cyclopropanedicarboxyhc acids Two of them A and B are constitutional isomers of each other and each forms a cyclic anhydnde on being heated The third diacid C does not form a cyclic anhydride C is a constitutional isomer of A and a stereoisomer of B Identify A B and C Construct molecular models of the cyclic anhy dndes formed on heating A and B Why doesn t C form a cyclic anhydride" ... 

Figure 21. The equilibrium between the helical interlaced system precursor of the trefoil knot and its face-to-face analogous complex leading to the face-to-face complexes. Interconversion between the two isomeric cyclic products is, of course, not possible. For the cyclic compounds, the total number of atoms x connecting two phenolic oxygen atoms is 16 if n=4 (pentakis(ethyleneoxy) fragment) or 19 if n = 5 (hexakis(ethyleneoxy) linker). Each knot is represented by the letter k accompanied by the overall number of atoms included in the cycle. The face-to-face complexes contain two monocycles (letter m), the number of atoms in each ring also being indicated. It can be noted that each knot has a face-to-face counterpart. For instance [Cu2(k-90)]2+ and [Cu2(m-45)2]2+ are constitutional isomers. They are by no means topological stereoisomers [34, 35].

Preparation of the two stereoisomers of perhydro-9b-boraphenalene was originally reported by Koster and Rotermund,6 and the present procedure (part A) is largely based on the procedure described by these authors. However, the original stereochemical assignment was incorrect and has been reversed.3 7 Furthermore, these authors did not use the thermal treatment described above, which appears essential to achieve isomerization of other constitutional isomers into perhydro-9b-boraphe-nalene.3 The original procedure for isomerization of the cis,trans isomer to the all cis isomer has been satisfactory. Contrary to the claim made by these authors,6 however, this isomerization does not lead quantitatively to the all cis isomer, but reaches an equilibrium, which consists of the all cis and cis,trans isomers in the ratio of 88 12 this ratio was also confirmed by reverse isomerization of the pure all cis isomer.5... 

Isomers that differ by their chemical constitution are constitutionally isomeric. As traditionally defined, stereoisomers are molecules with the same chemical constitution that differ with respect to the relative spatial arrangement of their constituent atoms. Since many types of flexible molecules exist whose shape rapidly change with time and that are not adequately representable by any geometric model, stereoisomers must be defined as follows, without reference to molecular geometry ... 

Chemical compounds that have the same molecular formula but different structural formulas are said to be isomers of each other. These constitutional isomers (or structural isomers) differ in their bonding sequence, i.e. their atoms are connected to each other in different ways. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. Stereoisomerism can be further divided into optical isomerism (enantiomerism) and geometrical isomerism (cis—trans isomerism). The relationships between the different types of isomerism are shown in Figure 4.1. 

Constitutional isomers were discussed in Sections 11.6 and 12.3 and geometric isomers, a class of diastereomers, in Section 12.5. The discussion of stereoisomers is completed in this chapter with a discussion of enantiomers and two other classes of diastereomers - optically active and meso compounds. The relationship between different types of isomerism is shown in Fig. 17-1. 

Consider the structures of the constitutional isomers, Compound A and Compound B (below). When treated with aqueous acid. Compound A undergoes isomerization to give a cis stereoisomer. In contrast. Compound B does not undergo isomerization when treated with the same conditions. That is. Compound B remains in the trans configuration. Explain the difference in reactivity between Compound A and Compound B. 

Let me follow this bird s eye view of stereochemical history with an equally brief overview of stereochemical principles. As is shown in Figure 1, molecules of the same molecular formula may be either identical or isomeric. K isomeric, they may or may not differ in constitution or connectivity (a concept well known to information scientists) if they so do, they are constitutional isomers . If they have the same constitution and yet are not identical, they can only differ in their three-dimensional architecture they are stereoisomers . Stereoisomers, in turn, may be classified as enantiomers (molecules whose structure is represented by non-superposable mirror images) or diastereomers (molecules differing in spatial... 

Constitutional isomers are compounds with different carbon skeletons, different functional groups, and different functional group locations. These isomers have different sequential arrangements of atoms. Now let us consider a different type of isomerism. Compounds that have the same sequential arrangement of atoms, but different spatial arrangements, are stereoisomers. This type of isomerism is stereoisomerism. Stereoisomers can exit in several ways in various classes of compounds. For example, cycloalkanes can exist as stereoisomers called geometric isomers. 

The X-ray crystal structure of rapamycin reveals the constitution of the major isomer, compound 1, although in solution at least four isomers are detectable. The most abundant isomers are 1 (major) and the isomeric seven-membered hemiketal (mixture of stereoisomers). For the purposes of this chapter, we will restrict ourselves to structure 1. 

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