Constitutional isomerizations

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistry. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethanol and dimethyl ether), or different locations of a functional group along the chain (as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

Note An example is a polymer composed of linear and branched molecules another example is a statistical copolymer comprising two isomeric constitutional units. 

In principle, bifunctional aldehydes should be able to engage in twofold enzymatic aldol additions to both of their acceptor carbonyls in a fashion to be classified as a tandem reaction, that is, without the need for isolation of intermediates. Depending on the specificity of the enzyme used and on the functionalization in the starting material, the isomeric constitution as well as the absolute and relative stereochemistry should be deliberately addressable. Therefore, we engaged in a program to evaluate the scope and the Hmitations of such two-directional chain elongation processes for the construction of extended poly functional molecules [36]. 

Any two distinguishable molecules are said to be isomeric, if they have the same empirical formula and are based on the same collection of atoms. The hierarchic classification of molecules, according to isomerism, constitutional isomerism and stereoisomerism, is a centerpiece of the logical structure of chemistry [28]. 

Application of this catalyst system to propargyl alcohols provides a, 3-unsaturated aldehydes (Equation 1.17) and ketones (Equation 1.18) [18]. The ease of accessibility of the substrates by simple addition of terminal alkynes to aldehydes followed by this redox isomerization constitutes a highly chemoselective and atom economic strategy to these unsaturated carbonyl compounds. The chemoselectivity problems of the direct aldol condensation and the poor atom economy of olefination methods make this new strategy the most efficient and reliable approach to these units. 

Furthermore, constitutional heterogeneity occurs. Molecules uniform with respect to elemental composition can vary from molecule to molecule in constitution, e.g. linear and branched molecules, molecules with different number of branches (e.g. distribution of number SCB), statistical copolymers with two isomeric constitutional units, in stereoregular polymers the linking of monomer units with different steric arrangement. Usually, polymers are characterized by multiple distributions. 

Also known as structural isomerism, constitutional isomerism occurs when the same atoms can be connected in two or more different ways. For example, there are three different ways to arrange the atoms in a compound with the chemical formula C5H12. Constitutional isomers have distinct names and generally have different physical and chemical properties. Table 10.3 lists the three constitutional isomers of C5H12 along with their boiling points for comparison. 

Because they have the same molecular formula, these molecules are isomers, but of a different kind from those encountered so far. The preceding chapters dealt with two kinds of isomerism constitutional (also called structural) and stereo (Figure 5-1). Constitutional isomerism describes compounds that have identical molecular formulas but differ in the order in which the individual atoms are connected (Sections 1-9 and 2-5). 

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