Constitutional isomers, generation

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

Except for the biochemical exanple just cited, the stnactures of all of the alcohols in Section 5.9 (including those in Problem 5.13) were such that each one could give only a single alkene by p elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the sane extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

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. 

They have the same molecular mass, but differ in their manner of linkage, their branching, and in the order (sequence) of their atoms (constitutional isomers). The number of possible isomers increases rapidly in higher generations - by analogy with the classical case of the alkanes. 

Moreover, in the multichromophoric second generation p-C2P4, a dual annihilation process was observed. The fast annihilation process occurs between a short distance pair of chromophores comparable in distance to the one in p-C IP4, while the longer time annihilation process occurs among the more prevalent pair of chromophores at longer distance. The origin of this can be traced back to the distribution of constitutional isomers as a result of the synthesis as mentioned earlier. 

Bohanec, S. and Zupon, J., Structure generation of constitutional isomers from structural fragments, J. Chem. Inf. Comput. Sci., 31, 531, 1991. 

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. 

Stereoselective is a similar term, but it refers to the preferential formation of a stereoisomer rather than a constitutional isomer. If a reaction that generates a carbon-carbon double bond or an asymmetric carbon in a product forms one stereoisomer preferentially over another, it is a stereoselective reaction. In other words, it selects for a particular stereoisomer. Depending on the degree of preference for a particular stereoisomer, a reaction can be described as being moderately stereoselective, highly stereoselective, or completely stereoselective. 

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. ... 

Exercise Use MOLGEN-ONLINE, via http //www.molgen.de, to evaluate the number of constitutional isomers of HCNO without prescribing valences forthe atoms involved. Thereby, standard valences will be used, and consequently fulminic acid will not be generated. 

Exercise As MOLGEN-ONLINE is based on MOLGEN 5.0, you can prescribe valences different from the standard ones. Evaluate the constitutional isomers of HCNO with chemically less usual valences, e.g. with carbon ofvalence3and nitrogen of valence 4. Examples 4 and 5 online show how valences can be prescribed. Submit C[val = 3]N[val = 4]0H, and fulminic acid, among others, will be generated. 

In order to cover such cases with some success, we can use probabilistic methods. There is in fact an easy algorithm that allows the generation of orbit representatives uniformly at random, for example constitutional isomers corresponding to a given chemical formula. Uniformly at random means that the generated representatives are uniformly distributed over the orbits. The underlying mathematical method was in-... 

For the substituents, we first have to generate all alkyl groups of the form Z-CnH2n+i, 1 < n < 6, i.e. constitutional isomers with the molecular formula C H2n+iZ. A molecular formula-based generation of these isomers yields 33 connectivity isomers (Figure 5.7), that are distributed in the following way ... 

Next, we examine whether the match value Is able to rank structure candidates according to their relevance for the experimental spectrum. We generate all constitutional isomers of molecular formula C6H12O2,1313 candidates in total. If these are ordered by decreasing match values, the correct candidate methyl n-pentanoate Is at position 16. Figure 8.18 shows the 24 highest-ranked structure candidates and their match values. The top 13 positions are occupied by cyclic structures, although the ratio of cycUc and acyclic structures for 6 1202 is rather balanced (641 acyclic, 672 cycUc structures). If the acycUc nature could be determined somehow from the spectrum, then the correct candidate would be at position 2. In Section 8.5 we will try to find criteria for these structural properties empirically. 

---