Constitutional isomers definition

The contribution of Cyvin, Cyvin and Brunvoll on the enumeration of benzenoid chemical isomers is the continuation of a series that began with two articles in Volume 162 of this journal. These three articles constitute a definitive and exhaustive treatment of the given topic. 

The second major category of isomers and the focus of this chapter are stereoisomers. Being isomers they too have the same number and kind of building block atoms, but, unlike constitutional isomers, they have identical topologies. Stereoisomers, in turn, are divided into two groups enantiomers and diastereomers. Enantiomers are isomers that are not superimposable on their mirror images. And, by definition, diastereomers are all other stereoisomers that are not enantiomers. 

The answer is D. Constitution isomers are compounds with the same molecular formula, but differ in the order with which the atoms are bonded. This definition matches the relationship between n-butane and isobutane. 

Now we introduce the notions of molecular formula, structural formula and constitutional isomer. To be acceptable for a computer, these terms need a precise mathematical definition. They are of central importance in the following discussions and in the applications ... 

Definition (Molecular formula, constitutional Isomer) Assume a set of chemical elements. The molecular formula of a molecule consists of a set of chemical elements, e.g. 4 = (H, C, N, 0) together with their occurrence numbers in the molecule, for example 3, 0,1, 0, resulting in the formula NH3 in the usual notation, where numbers 1 are left out. In mathematical terms this can be defined as follows ... 

Example (Numbers of constitutional isomers) Here are a few example molecular formulas of molecular mass 78 (that of benzene), consisting of the elements in 4, as well as the numbers of constitutional isomers possible. The numbers are those obtained for RC (see Definition 1.14). 

We will now discuss at some length the many ways in which deviations from standard bonding parameters lead to energetic destabilization of a molecule. We will focus on "stable" structures (i.e., not on reactive intermediates), but the notions we develop here also apply to reactive intermediates. We first explore acyclic systems, wherein molecular motions directly lead to strained forms. Note that we are not yet considering conventional chemical reactivity. We will be considering conformers, or conformational isomers. Recall that conformers are stereoisomers that interconvert by rotation around single bonds (see Chapter 6 for definitions of stereochemical concepts). These isomers are not to be confused with constitutional isomers, where the molecular formula is the same, but the atoms are arranged differently. 

All the structures in a given column are constitutional isomers of one another, but the structures in column 1 are not constitutional isomers of structures in other columns. Based on this information, write a definition for the term constitutional isomers that starts ... 

This classification, defined in Table 13 with examples, appears very clear and logical in view of the standard classification of isomers. However, the historical development followed a rather curious course. The term constitutional selectivity, unfortunately a somewhat clumsy word which is rarely used, appeared in the literature as late as 1979 -2. This was after an inspiring, but not completely clear, discourse by Hassner on the almost equivalent term regioselectivity which greatly appealed to chemists and was immediately accepted. It is important to note that the now universally accepted definition of stereoselectivity and its subclasses enantio- and diastercoselectivity did not appear in print until as late as 19714. Before that, the term stereoselectivity apparently had the special meaning of the present term diastereoselectivity5. One consequence of this was discussed in the previous section. Furthermore, in the past, the terms selectivity and specificity were usually coupled. The latter term will be discussed in Section 1.2.3.3, but it is currently regarded with suspicion and best avoided. 

Along with the isomerism of linear copolymers due to various distributions of different monomeric units in their chains, other kinds of isomerisms are known. They can appear even in homopolymer molecules, provided several fashions exist for a monomer to enter in the polymer chain in the course of the synthesis. So, asymmetric monomeric units can be coupled in macromolecules according to "head-to-tail" or "head-to-head"—"tail-to-tail" type of arrangement. Apart from such a constitutional isomerism, stereoisomerism can be also inherent to some of the polymers. Isomers can sometimes substantially vary in performance properties that should be taken into account when choosing the kinetic model. The principal types of such an account are analogous to those considered in the foregoing. The only distinction consists in more extended definition of possible states of a stochastic process of conventional movement along a polymer chain. 

The physical and chemical properties of the first three trinitrotoluenes—the alpha, beta and gamma— are quite well known, since these isomers have been known for some time, and have been prepared in sufficient quantities to enable research, which has embraced many reactions, to be carried out. The last three trinitrotoluenes—the delta, epsilon and zeta— have been discovered in too recent years to enable the scientist to reach definite conclusions concerning their chemical reactions. Practically all that is known concerning these last three isomers is the melting-point. So far as the commercial manufacture of TNT is concerned, the chemical and physical properties of the alpha, beta, and gamma trinitrotoluenes are of vastly more importance than the properties of the others, because the first-mentioned isomers constitute practically 100 per cent of the TNT. Narrowing down the relative importance still more, it is found that interest has centered on but one of the six trinitrotoluenes— the alpha. This is because the alpha or symmetrical trinitrotoluene forms about 98 per cent of the commercial product and the reactions of this product are... 

Structures 1 and 2 are isomers because they have the same empirical formula, but they have the same constitution (same point of attachment). According to the simple definition of an isomer in Chapter 4, this fact poses a problem. They do have the same points of attachment, but they differ in the spatial arrangement of the atoms, so they are stereoisomers. Therefore, nonsuperim-posable mirror images are different stereoisomers and they are recognized as enantiomers. In other words, an enantiomer is a stereoisomer that has a nonsuperimposable mirror image. 

Cis and trans isomers are stereoisomers—compounds that have identical connectivities (i.e., their atoms are attached in the same sequence) but differ in the arrangement of their atoms in space. They are distinct from constitutional or structural isomers (Sections 1-9 and 2-5), which are compounds with differing connectivities among atoms. Conformations (Sections 2-8 and 2-9) also are stereoisomers by this definition. However, unlike cis and trans isomers, which can be interconverted only by breaking bonds (try it on your models), conformers are readily equilibrated by rotation about bonds. Stereoisomerism will be discussed in more detail in Chapter 5. 

All constitutive elements of a compound having been duly considered in compliance with the existing rules, still another set of descriptors has to be envisaged to account properly for all stereochemical features involved. Definitive status has already been attained by the rules for characterizing cisitrans isomers, stereogenic (formerly asymmetric) centers, and exolendo relationships in bicyclic compounds, as will be outlined in subsequent sections. A unified procedure for the treatment of other stereogenic units will be given at the end of this chapter. 

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