Isomers meso compounds

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

Propylene glycol, dipropylene glycol, and tripropylene glycol all have several isomeric forms. Propylene glycol has one asymmetric carbon and thus there are two enantiomers (R)-I,2-propanediol and (3)-1,2-propanediol. 1,3-Propanediol is a stmctural isomer. Dipropylene glycol exists in three stmctural forms and since each stmctural isomer has two asymmetric carbons there are four possible stereochemical isomers per stmcture or a total of twelve isomers. These twelve consist of four enantiomer pairs and two meso- compounds. Tripropylene glycol has four stmctural isomers and each stmctural isomer has... 

The substituted carbons are stereogenic carbons. This means that there are not only two isomers. In the most general case, where W, X, Y, and Z are all different, there are four isomers since neither the cis nor the trans isomer is superimposable on its mirror image. This is true regardless of ring size or which carbons are involved, except that in rings of even-numbered size when W, X, Y, and Z are at opposite comers, no chirality is present, (e.g., 68). In this case, the substituted carbons are not chiral carbons. Note also that a plane of symmetry exists in such compounds. When W = Y and X=Z, the cis isomer is always superimposable on its mirror image, and hence is a meso compound, while the trans isomer consists of a dl pair, except in... 

Rings with more than two differently substituted carbons can be dealt with on similar principles. In some cases, it is not easy to tell the number of isomers by inspection. The best method for the student is to count the number n of differently substituted carbons (these will usually be asymmetric, but not always, e.g., in 68) and then to draw 2" structures, crossing out those that can be superimposed on others (usually the easiest method is to look for a plane of symmetry). By this means, it can be determined that for 1,2,3-cyclohexanetriol there are two meso compounds and a dl pair and for 1,2,3,4,5,6-hexachlorocyclohexane there are seven meso compounds and a dl pair. The drawing of these structures is left as an exercise for the student. 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

In open-chain compounds, the molecule can usually adopt that conformation in which H and X are anti periplanar. However, in cyclic systems this is not always the case. There are nine stereoisomers of 1,2,3,4,5,6-hexachlorocy-clohexane seven meso forms and a dl pair (see p. 161). Four of the meso compounds and the dl pair (all that were then known) were subjected to elimination of HCl. Only one of these (1) has no Cl trans to an H. Of the other isomers, the fastest elimination rate was about three times as fast as the... 

Up to this point, we have not had to address the unexpected photolysis behavior of the meso-compound, namely, that the stereochemical retention in the products resulting from the decomposition of this stereoisomer actually decreased at concentrations above the CMC. Intuitively, one would expect that the restricted environment created by micellization of the diazenes would enhance stereochemical retention. The (+)-diazene behaves as we would have predicted, but the meso-isomer does not. 

As mentioned in Sect. IV-B, enantiotopic groups even in achiral substances may show nonequivalence. For example, this property sometimes can be used to distinguish meso compounds from their chiral isomers. Thus in the presence of TFPE, the meso isomer of dimethyl 2,3-diaminosuccinate (46) shows two equally intense methoxy singlets and an AB quartet for the now diastereotopic methine hydrogens (88). This coupling clearly shows 46 to be the... 

We can see why a compound with chiral centres should end up optically inactive by looking again at the eclipsed conformer. The molecule itself has a plane of symmetry, and because of this symmetry the optical activity conferred by one chiral centre is equal and opposite to that conferred by the other and, therefore, is cancelled out. It has the characteristics of a racemic mixture, but as an intramolecular phenomenon. A meso compound is defined as one that has chiral centres but is itself achiral. Note that numbering is a problem in tartaric acid because of the symmetry, and that positions 2 and 3 depend on which carboxyl is numbered as C-1. It can be seen that (2R,3S) could easily have been (3R,2S) if we had numbered from the other end, a warning sign that there is something unusual about this isomer. 

We can draw these three stereoisomers as Fischer projections, reversing the configurations at both centres to get the enantiomeric stereoisomers, whilst the Fischer projection for the third isomer, the meso compound, is characterized immediately by a plane of symmetry. For (-l-)-tartaric acid, the configuration is 2R, >R), and for (—)-tartaric acid it is (2S,3S). For both chiral centres, the group of lowest priority is hydrogen, which is on a horizontal line. In fact, this is the case in almost all Fischer projections, since, by convention, the vertical... 

When we considered trans- and c -1,2-dimethyl-cyclohexane, we found that only three configurational isomers exist, enantiomeric forms of the trans isomer, together with the cis isomer, which is an optically inactive meso compound (see Section 3.4.5). The meso relationship could be deduced from the plane of symmetry in the hexagon representation. 

Since the ring C s are sp -hybridized, they may be chiral centers. Therefore, substituted cycloalkanes may be geometric isomers as well as being enantiomers or meso compounds. 

Cyclic compounds Depending on the type of substitution on a ring, the molecule can be chiral (optically active) or achiral (optically inactive). For example, 1,2-dichlorocyclohexane can exists as meso compounds (optically inactive) and enantiomers (optically active). If the two groups attached to the ring are different, i.e. no plane of symmetry, there will be four isomers. 

There is much evidence that when the attack is by Br+ (or a carrier of it), the bromonium ion 2 is often an intermediate and the addition is anti. As long ago as 1911, McKenzie and Fischer independently showed that treatment of maleic acid with bromine gave the dl pair of 2,3-dibromosuccinic acid, while fumaric acid (the trans isomer) gave the meso compound.5 Many similar experiments have been performed since with similar results. For triple bonds, stereoselective anti addition was shown even earlier. Bromination of dicarboxyacetylene gave 70% of the trans isomer.6... 

As an illustration of the stereospecificity of eliminations, the meso compound 4 gives the c/s-alkene 5, whereas the d,l isomers 6 give the trans-alkene 7 with ethoxide. Both reactions clearly proceed by antarafacial elimination ... 

B is correct. The isomer on the left is a meso compound and has no enantiomer. Both isomers have the same bond-to-bond connectivity and are therefore diastereomers. 

A is correct The trans isomer will produce only a meso compound. If you re confused by this question, see the explanation to question 12 below. 

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