Isomer number

LeoJ77 Leonard, J. E. Isomer numbers of nonrigid molecules. 

The two-digit symbols are used according to the usual isomer numbering convention . 

FIGURE 2.18 Partial deuteriumation of ds-3-phenyl propenoic acids showing the fraction of deuterium in olefinic positions of recovered trans isomers. Numbers in parentheses are over Lindlar catalyst.121... 

Geminal 1H,15N couplings are a sensitive indicator of the stereochemistry of imino compounds, e.g., the cis orientation of the lone electron pair at the nitrogen of (Z)-2-furancarbox-aldehyde oxime enhances the coupling considerably compared with the E-isomer (numbers are Vh.hn)293 295 An analogous sequence has been observed for A-alkylaldimincs, e.g., /V-(methyl)benzenemethanimine296. 

Compound nomenclature is abbreviated as follows (number of C in substituent)-(parent aromatic ring)-(isomer number). 

Isomer Number of conjugated double bonds Absorption maximum (nm) when conjugated to proteins... 

The phosphoranes are derivatives of the pentahydride of phosphorus, PH6, in which the five ligands are covalently bonded to phosphorus. The stereochemistry of pentavalent phosphorus relates to the trigonal bipyramid, just as that of tetravalent carbon relates to the tetrahedron. There are significant differences in the stereochemistry of the compounds of these two elements, other than isomer numbers. Some tetracoordi-nated phosphorus compounds become pentacoordinated rapidly and reversibly. Most pentacoordinated phosphorus compounds change their ligand distribution on the trigonal bipyramidal skeleton very easily by mechanisms that involve the simple deformation of bonds, rather than the rupture and re-formation of bonds. 

For example, hh a disubstituted compound CH Rj (Fig. 8) (1) if the molecule is planar, then two isomers are possible. This planar configuration can be either square or rectangular in each case, there are two isomers only. (2) If the molecule is pyramidal, then two isomers are also possible. There are only two isomers, whether the base is square or rectangular. (3) If the molecule is tetrahedral, then only one form is possible. The carbon atom is at the center of the tetrahedron. In actuality, only one disubstituted isomer is known. Therefore, only the tetrahedral model for a disubstituted methane agrees with the evidence of the isomer number. 

The evidence of electron diifraction, X-ray diffraction, and spectroscopy shows that when carbon is bonded to four other atoms its bonds are directed toward the comers of a tetrahedron. But as early as 1874, years before the direct determination of molecular structure was possible, the tetrahedral carbon atom was proposed by J. H. van t Hoff, while he was still a student at the University of Utrecht. His proposal was based upon the evidence of isomer number. 

As far as compounds of the formula CH3Y are concerned, the evidence of isomer number limits the structure of methane to one of these three possibilities. 

Thus, only the tetrahedral structure for methane agrees with the evidence of isomer number. It is true that this is negative evidence one might argue that isomers exist which have never been isolated or detected simply because the experimental techniques are not good enough. But, as we said before, any compound that contains carbon bonded to four other atoms can be considered to be a derivative of methane in the preparation of hundreds of thousands of. compounds of this sort, the number of isomers obtained has always been consistent with the concept of the tetrahedral carbon atom. 

In testing the superimposability of two of these flat, two-dimensional representations of three-dimensional objects, we must follow a certain procedure and obey certain rules. First, we use these representations only for molecules that contain a chiral center. Second, we draw one of them, and then draw the other as its mirror image. (Drawing these formulas at random can lead to some interesting but quite wrong conclusions about isomer numbers.) Third, in our mind s eye we may slide these formulas or rotate them end for end, but we may not remove them from the plane of the paper. Used with caution, this method of representation is convenient it is not foolproof, however, and in doubtful cases models or pictures of models should be used. 

Molecular formula. Isomer number. Kekul structure... 

Now, Just how did we arrive at the conclusions of the last few paragraphs Most of us—perhaps without realizing it—judge the equivalence of protons by following the approach of isomer number (Sec. 4.2). This is certainlv the easiest way to do it. We imagine each proton in turn to be replaced by some other atom Z. If replacement of either of two protons by Z would yield the same product— or enantiomeric products then the two protons are chemically equivalent. We ignore the existence of conformational isomers and. as wc shall see in Sec. 13.13, this is just what wc should do. 

In the process of evaluating what stereoisomers are likely possibilities for ultimate phosphoranes, the isomer number in each of the sectors related to On is reduced from nine to four by application of an extended principle of microscopic reversibility which states in effect that the stereochemistry (a vs. e) of entry and departure must be the same. The principle of microscopic reversibility (PMR) has been extensively applied to displacement reactions at tetracoordinate phosphorus and it may be instructive to digress at this point in order to clarify the implications of this concept. 

Returning to the evaluation of stereoisomers for candidates as ultimate phosphoranes, the isomer number is further reduced from four to two in each sector since ring strain effectively prevents access to the star-points (eering). Accordingly, the ultimate phosphoranes derived from cis-18 via 15 and 25 are identified as 24 and 14, respectively, for retention and 14 and 24, respectively, for inversion. The ultimate phosphoranes are the same, but the stereochemistry of displacement is reversed, when one starts from trans 18 via 25 and 15. Since enantiomers are indistinguishable under achiral conditions, further discussion need only consider one of the two enantiomeric pathways, e.g., the pathway on the top of the hexagon. 

Leapfrog fullerenes are necessarily rather rare. By construction, they have isolated pentagons. Ceo is both the smallest isolated-pentagon fullerene and the smallest leapfrog. The gap in the fullerene series at C22 leads to a gap at Cgg in the leapfrogs henceforward the counts for leapfrogs follow the isomer numbers for clusters of one-third their size, i.e, C60 (1), C72 (1), C78 (1), Cg4 (2), C90 (3), C96 (6),. Thus, only about 1 in 1(L of the first one and a half million fullerenes are leapfrogs. 

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