Stereochemistry constitutional isomers

Two constitutional isomers of molecular formula CgHi20 are formed in the following reac tion Ignoring stereochemistry suggest reasonable structures for these Diels-Alder adducts... 

From a structural point of view a number of different isomers could be present. In principal the three different hydroxyl groups in the ligand are able to interact with the central metal ion thus giving rise to three different constitutional isomers. The stereochemistry of the different configurational isomers of gadobutrol is best described in view of the stereochemistry of the parent compound Gd-DOTA. In this complex chirality results from the restriction of free rotation around the bonds of the ligand caused by the inclusion of the central metal ion. Thus four stereoisomers are generated upon complexation of Gd(III) by DOTA. 

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. 

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. 

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. 

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 ). 

Stereochemistry is the field of chemistry that deals with the stmctures of molecules in three dimensions. Compounds that have the same molecular formula but are not identical are called isomers they fall into two classes constitutional isomers and stereoisomers. Constitutional isomers differ in the way their atoms are coimected. Stereoisomers differ in the way their atoms are arranged in space. There are two kinds of stereoisomers cis-trans isomers and isomers that contain chirality centers. 

Since CISGEN (22) as well as the other programs (23,24) only generate constitutional isomers, stereochemistry was analyzed manually. The lowest energy stereoisomers of each constitution were considered in searching for the stabilomer. 

The enantioselective hydrocyanation of alkenes has the potential to serve as an efficient method to generate optically active nitriles, as well as amides, esters, and amines after functional group interconversions of the nitrile group. As in asymmetric hydroformylation, asymmetric hydrocyanation requires control of both regiochemistry and stereochemistry because simple olefins tend to generate achiral terminal nitrile products. The hydrocyanation of norbomene will give a single constitutional isomer and was studied initially. However, modest enantioselectivities were obtained, and the synthetic value is limited. ... 

Molecules are not easy to handle in silico because of constitutional isomerism, meso-merism, tautomerism, stereoisomerism, chirality and other phenomena. This means, roughly speaking, that molecules are not easily described unambiguously. Precise models are required, since computers can obey orders very well, but cannot read our minds. It is not sufficient to describe a molecule with the molecular formula or a Ust of covalent bonds between the atoms alone. Even the constitutional isomers are not always sufficient, as stereochemistry is sometimes needed when it comes to the consideration of pharmaceutical properties. 

Unlike the constitutional isomers butane and isobutane, which have their atoms connected in a different order (Section 3.2), the two 1,2-dimethyl-cyclopropanes have the same order of connections but differ in the spatial orientation of the atoms. Such compounds, which have their atoms connected in the same order but differ in three-dimensional orientation, are called stereochemical isomers, or stereoisomers. More generally, the term stereochemistry... 

One consequence of three-dimensionality is the existence of stereoisomers, namely cis and trans isomers of substituted cycloalkanes (Section 4-1). We shall see in Chapter 5 that the phenomenon of stereoisomerism is more general and occurs in acyclic molecules as well. These concepts influence such diverse areas as relative reactivities and biological effectiveness. Because of its fundamental importance and its powerful utility in biological applications, stereochemistry constitutes a recurring theme through the remainder of this book. 

Chemotaxonomic studies indicate that the sterol phenotype differs qualitatively and quantitatively in Nature (7,25). From these studies it is evident that enantiomeric purity has been maintained in the sterol constitution, but structural variants, constitutional isomers and diastereoisomers, are produced. The changes in sterol stereochemistry apparently evolved to produce superior compounds that could play multiple roles. 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

Two pathways were found for the chiral hydrogenation, and they give products with different stereochemistries (19). One pathway involves the preferred mode of initial binding of the reactant to the catalyst. The other pathway involves an isomer of the reactant—catalyst complex that is formed in only small amounts, but its conversion is energetically favorable and constitutes the kinetically predominant pathway to products (9) (Fig. 4). Thus the chirahty of the product is determined not by the preferred mode of the initial binding, but instead by the more favorable energetics of the pathway involving the minor isomer of the reactant—catalyst complex. 

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. 

A substantial step towards the understanding of the physical, chemical, or biological properties of a molecule is to study and analyze its spatial shape. Besides the constitution, a major shape-determining feature is the configuration of a molecule, i. e. the stereochemistry. Furthermore, molecular chirality plays a major role in many areas of chemistry. Enantiomers often exhibit quite different physical, chemical, and biological properties. The exploration of the configurational space of a molecule and the analysis of the various isomers a molecule can adopt is therefore of great importance. 

Characteristically, metal ions form complexes that can exist as several isomers. This is a consequence of the stereochemistry, resulting from high numbers of ligands, adopted by most metal complexes. The best known examples of isomerism in complexes are geometrical isomers (such as cis and trans isomers), but these are not the sole type. We can, following a traditional approach, divide the area into two classes constitutional (or structural) isomerism and stereoisomerism. 

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