Chirality centers meso compounds

A meso compound is an achiral molecule that contains chirality centers. Meso compounds are not optically active. 

To see whether a chirality center is present, look for a carbon atom bonded to four different groups. To see whether the molecule is chiral, look for the presence or absence of a symmetry plane. Not all molecules with chirality centers are chiral overall—meso compounds are an exception. 

We have seen that the presence of chtrahty centers does not necessarily render a compound chiral. Specifically, a compound that exhibits reflectional symmetry will be achiral even though it has chirality centers. Such compounds are called meso compounds. A family of stereoisomers... 

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.)... 

Meso stereoisomer (Section 7.11) An achiral molecule that has chirality centers. The most common kind of meso compound is a molecule with two chirality centers and a plane of symmetry. 

A look at the structure of cis-1,2-dimethylcyclobutane shows that both methyl-bearing ring carbons (Cl and C2) are chirality centers. Overall, though, the compound is achiral because there is a symmetry plane bisecting the ring between Cl and C2. Thus, the molecule is a meso compound. 

Meso compounds contain chirality centers but are achiral overall because they have a plane of symmetry. Racemic mixtures, or racemates, are 50 50 mixtures of (+) and (-) enantiomers. Racemic mixtures and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point. 

Draw the structure of a meso compound that has five carbons and three chirality centers. 

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

For dihydrodiols derived from substituted benzenes, the key to their significance lies in the availability of two adjacent chiral centers with an established absolute stereochemistry. The dihydrodiol from benzene is, of conrse, the meso compound, although enantiomers produced by subsequent reaction with a chiral reagent are readily separated. There are useful reviews containing nnmerous applications (Carless 1992 Ribbons et al. 1989), many of which involve, in addition, the nse of di-flnoro-, di-chloro-, or di-bromobenzene-2,3-dihydrodiols. 

These differences reflect the conformations of (+)- and meso-isomers as they sit at the air-water interface. What is much harder to elucidate is the effect of stereochemistry on intermolecular interactions. How does changing the stereochemistry at one chiral center affect interactions between diastereomers Ab initio molecular orbital calculations have been used to address the problem of separating stereochemically dependent inter- and intra-molecular interactions in diastereomeric compounds (Craig et al., 1971). For example, diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. So far, however, little experimental attention has been focused on this problem. 

A stereoisomer having more than one chiral center and having an internal plane of symmetry. Hence, meso compounds do not exhibit optical activity. 

A meso compound is a compound with chiral centers and a plane of symmetry. The plane of symmetry leads to the optical rotation of one chiral carbon cancelling the optical rotation of another. 

In the products C retains the R configuration since none of its bonds were broken and there was no change in priority. TTie configuration at C, the newly created chiral center, can be either R or S. As a result, two diastereomers are formed, the optically active RR enantiomer and the optically inactive RS meso compound. 

Problem 5.23 Answer True or False to each of the following statements and explain your choice. ( ) There are two broad classes of stereoisomers, (b) Achiral molecules cannot possess chiral centers, (c) A reaction catalyzed by an enzyme always gives an optically active product, (d) Racemization of an enantiomer must result in the breaking of at least one bond to the chiral center, (e) An attempted resolution can distinguish a racemate from a meso compound. <... 

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. 

There are two pairs of mirror images 23a and 24a, as well as 25a and 26a. However, what will not be so immediately clear, but what you must verify for yourself is that 25a and 26a are, in fact, identical. This means that 25a and 26a are representations of a single achiral substance, identical with its mirror image. Substances that have chiral centers but are themselves achiral are called meso compounds. 

The idea that for every n chiral centers there can be 2M different configurations will be true only if none of the configurations has sufficient symmetry to be identical with its mirror image. For every meso form there will be one less pair of enantiomers and one less total number of possible configurations than is theoretically possible according to the number of chiral centers. At most, one meso compound is possible for structures with two chiral centers, whereas two are possible for structures with four chiral centers. An example is offered by the meso forms of tetrahydroxyhexanedioic acid which, with four chiral atoms, have configurations 28 and 29 ... 

The situation is different if the substrate is a prochiial or meso compound. Since these molecules have a center or plane of symmetry the binding of pro-S or pro-R forms is equivalent. The chirality appears only as a result of the transformation. Hence, at least theoretically, the compound can be converted to one enantiomer quantitatively. 

Two chiral centers in a single molecule may offset each other creating an optically inactive molecule. Such compounds are called meso compounds. Meso compounds have a plane of symmetry through their centers which divides them into two halves that are mirror images to each other. Meso compounds are achiral and therefore optically inactive. 

There are no stereogenic centers. Both molecules have planes of symmetry. The cis isomer has two such planes, through opposite corners of the ring. The trans isomer has one such plane, through the opposite methyl-bearing corners. Both compounds are optically inactive and achiral. They are not meso compounds because there are no chiral centers. To summarize, the two isomers are configurational, achiral and diastereomers. 

The tnms-iliastereomer exists as a pair of enantiomers. These stereochemical differences do not depend on the presence of the ring. In fact, suppose the ring is cleaved at the C—C bond opposite the one connecting the chirality centers. The resulting compound, 3,4-dimethylhexane, also has three stereoisomers a weso-diastereomer and two enantiomers of a c/,/-diastereomer. It is just easier to see that the cis- and trans-diastereomers of 1,2-dimethylcyclohexane are different than it is to see that the meso-and c/,/-diastereomers of 3,4-dimethylhexane are different. 

Because both chirality centers have the same groups attached, this compound might be meso. To determine this, the conformation must be changed to a more symmetrical one... 

Fischer projections are especially useful in the case of compounds with more than one chirality center. For example, it is easy to see the plane of symmetry in meso-tartaric acid. As was the case with regular structures, interchanging any two groups in a Fischer projection results in inversion of configuration at the chirality center. Thus, interchanging the H and OH on the lower chirality center of weso-tartaric acid inverts the configuration at that chirality center, resulting in the (27 ,3R)-stereoisomer, (-i-)-tartaric acid. It is also easy to see that this stereoisomer does not have a plane of symmetry. 

MESO COMPOUND An achiral compound that has chirality centers (usually asymmetric carbons). 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

A meso compound with two chirality centers will be (R,S) or (S,R) because the chirality centers must be mirror images of each other, reflected across the internal mirror plane. 

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