Constitutional isomers defined

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). 

An immediate consequence of Pasteur s law is that the relationship between enantiomers is established by symmetry alone and does not require any knowledge of molecular bonding connectedness (constitution). This is in contrast to diastereomers, the other class of stereoisomers Diastereomers are not related by symmetry, and their relationship can be defined only by first specifying that their constitutions are the same—otherwise, there would be nothing to distinguish them from constitutional isomers. Thus enantiomers, which have identical scalar properties and differ only in pseudoscalar properties, have more in common with homomers than with diastereomers, while diastereomers, which differ in all scalar properties, have more in common with constitutional isomers than with enantiomers.51, 52 It therefore makes more sense, in an isomer classification scheme, to give priority to isometry rather than to constitution.52 In such a scheme there is no need for the concept stereoisomer the concept retains its usefulness only because it normally proves convenient, in chemical reaction schemes, to combine enantiomers and stereoisomers in a common class. 

Isomers can be defined as molecules which closely resemble each other, but fail to be identical due to one difference in their chemical structure. Thus, structural isomers are chemical entities which share the same molecular formula (i.e., the same atomic composition), but which differ in one aspect. When they differ in their constitution (i.e., in the connectivity of their atoms), they are called constitutional isomers, for example 1-propanol and 2-propanol. When structural isomers have identical constitution but differ in the spatial arrangement of their atoms, they are designated as stereoisomers. 

For a technical short chain chlorinated paraffin (SCCP) mixture containing 60% chlorine by weight, the theoretical number of congeners (defined as constitutional isomers and homologues) is 4,200 [11, 16]. It should be noted that the complexity would actually be an order of magnitude greater than that indicated in Table 1 because chlorine substitution at a secondary carbon atom usually produces a chiral carbon atom so that enantiomers and diastereoisomers would be generated. 

There are five constitutional isomers with molecular formula CgH. We are now able to name three of them (hexane, isohexane, and neohexane), but we cannot name the other two without defining names for new stmctural units. (For now, ignore the names written in blue.)... 

Constitutional isomerism is defined as that type of isomerism (i.e., different structures corresponding to the same molecular formula) resulting from differences in vicinity relationships between atoms. Examples of pairs of constitutional isomers are -butane and isobutane [CCCC versus CC(C)C in Smiles notation], ethanol and dimethyl ether (CCO versus COC), 1 - and 2-methylbutene (C=CCC versus CC=CC), and 1- and 2-propanol [CCCO versus CC(0)C]. Constitutional isomerism is adequately accounted for in chemical graph theory by the adjacency or distance matrices, which consider only the vicinity relationships. ... 

Different isomers are possible if the alkane molecule contains more than three carbon atoms. In the case of butane, which has the composition defined by the formula C4H10, the carbon atoms can be interconnected in two different ways forming two constitutional isomers. While the compound with the linear carbon chain is usually called w-butane its branched isomer is called wo-butane. 

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 ... 

There are five constitutional isomers with molecular formula C6H14. Again, we are able to name only two of them, unless we define new stmctural units. 

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. 

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 ... 

We define the order of the singular values as a > a2 > 31. The planar and collinear configurations give a3 0 and a2 a3 = 0, respectively. Furthermore, we let the sign of a3 specify the permutational isomers of the cluster [14]. That is, if (det Ws) = psl (ps2 x ps3) > 0, which is the case for isomer (A) in Fig. 12, fl3 >0. Otherwise, a3 < 0. Eigenvectors ea(a = 1,2,3) coincide with the principal axes of instantaneous moment of inertia tensor of the four-body system. We thereby refer to the principal-axis frame as a body frame. On the other hand, the triplet of axes (u1,u2,u3) or an SO(3) matrix U constitutes an internal frame. Rotation of the internal frame in a three-dimensional space, which is the democratic rotation in the four-body system, is parameterized by three... 

The simplest, from the viewpoint of topological structure, are the linear polymers. Depending on the number m of the types of monomeric units they differentiate homopolymers (ra=1) and copolymers (m>2). In the most trivial case molecules in a homopolymer are merely identified by the number Z of monomeric units involved, whereas the composition of a copolymer macromolecule is defined by vector 1 with components lly..., Za,..., Zm equal to the numbers of monomeric units of each type. At identical composition these molecules can vary in microstructure which is characterized by the manner of alternation of different units in a copolymer chain. Because the values of the average degree of polymerization l=lx+... +Zm in synthetic copolymers normally constitute 102-104 it becomes clear that the number of conceivable types of isomers with different microstructure turns out to be practically infinite. Naturally, a quantitative description of any polymer specimen comprising macromolecules with such an impressive number of configurations can be performed exclusively by statistical methods. 

Define each type of isomer (a) constitutional (b) geometric ... 

---