Stereoisomers achiral molecule

Problem 5.21 Answer True or False to each of the following statements and explain your choice, (a) There are two broad classes of stereoisomers. (/ ) 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. M... 

Merrifield method See solid phase peptide synthesis Meso stereoisomer (Section 7 11) An achiral molecule that has chirality centers The most common kind of meso com pound IS a molecule with two chirality centers and a plane of symmetry... 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

Only three, not four, stereoisomeric 2,3-butanediols aie possible. These three aie shown in Eigure 7.10. The (2R,3R) and (2.S,3.S) fonns aie enantiomers of each other and have equal and opposite optical rotations. A third combination of chirality centers, (2R,3S), however, gives an achiral structure that is superimposable on its (2S,3R) minor image. Because it is achiral, this third stereoisomer is optically inactive. We call achiral molecules that have chirality centers meso forms. The meso form in Eigure 7.10 is known as iwe50-2,3-butanediol. 

C is correct. Diastereomers - epimers, anomers, and geometric isomers - are stereoisomers that are not mirror images of each other. A meso compound is an achiral molecule, which is identical to its mirror image. 

Each stereoisomer in a pair of enantiomers has the property of being able to rotate monochromatic plane-polarized light. The instrument chemists use to demonstrate this property is called a polarimeter (see your text for a further description of the instrument). A pure solution of a single one of the enantiomers (referred to as an optical isomer) can rotate the light in either a clockwise (dextrorotatory, +) or a counterclockwise (levorotatory, -) direction. Thus those molecules that are optically active possess a handedness or chirality. Achiral molecules are optically inactive and do not rotate the light. 

Nonsuperimposable mirror-image molecules are called enantiomers (from the Greek enantion, which means opposite ). The two stereoisomers of 2-bromobutane are enantiomers. A molecule that has a nonsuperimposable mirror image, like an object that has a nonsuperimposable mirror image, is chiral. Each of the enantiomers is chiral. A molecule that has a superimposable mirror image, like an object that has a superimposable mirror image, is achiral. To see that the achiral moleule is superimposable on its mirror image (i.e., they are identical molecules), mentally rotate the achiral molecule clockwise. Notice that chirality is a property of the entire molecule. 

A chiral molecule has a nonsuperimposable mirror image. An achiral molecule has a superimposable mirror image. The feature that is most often die cause of chirality is an asymmetric carbon. An asymmetric carbon is a carbon bonded to four different atoms or groups. An asymmetric carbon is also known as a chirality center. Nitrogen and phosphorus atoms can also be chirality centers. Nonsuperimposable mirror-image molecules are called enantiomers. Diastereomers are stereoisomers that are not enantiomers. Enantiomers have identical physical and chemical properties diastereomers have different physical and chemical properties. An achiral reagent reacts identically with both enantiomers a chiral reagent reacts differently with each enantiomer. A mixture of equal amounts of two enantiomers is called a racemic mixture. 

Chiral compounds are optically active—they rotate the plane of polarized light achiral compounds are optically inactive. If one enantiomer rotates the plane of polarization clockwise (+), its mirror image will rotate the plane of polarization the same amount counterclockwise (—). Each optically active compound has a characteristic specific rotation. A racemic mixture is optically inactive. A meso compound has two or more asymmetric carbons and a plane of symmetry it is an achiral molecule. A compound with the same four groups bonded to two different asymmetric carbons will have three stereoisomers, a meso compound and a pair of enantiomers. If a reaction does not break any bonds to the asymmetric carbon, the reactant and product will have the same relative configuration—their substituents will have the same relative positions. The absolute configuration is the actual configuration. If a reaction does break a bond to the asymmetric carbon, the configuration of the product will depend on the mechaitism of the reaction. 

As is evident from the examples in Scheme 2.2, chirality is not umqueiy associated with asymmetrically substituted atoms. There are examples of chiral molecules lacking asymmetric atoms and chiral molecules containing two or more asymmetrically substituted carbons. Nevertheless, the most important cases of stereoisomerism in organic chemistry are those which involve asymmetrically substituted atoms. In such cases, the maximum number of stereoisomers is given by 2 , where n is the number of asymmetric centers in the molecule. The actual number of stereoisomers is reduced in those cases where elements of symmetry render particular stereoisomers achiral. 

Even isomeric pairs of achiral molecules may exist as separable stereoisomers. Examples include cis- and trans-, or (Z)- and (E)-, alkenes such as 9 and 10, and cyclic compounds such as 11 and 12. The molecules are nonsuperimposable, yet they are not mirror images that is, they are not enantiomers. Rather, they are diastereomers and have different chemical and physical properties. 

We illustrate an alternative strategy, namely asymmetric induction, by E. J. Corey s preparation of a key intermediate in his synthesis of prostaglandins. In asymmetric induction, the reactive functional group of an achiral molecule is placed in a chiral environment by reacting it with a chiral auxiliary. The strategy is that the chiral auxiliary then exerts control over the stereoselectivity of the desired reaction. The chiral auxiliary chosen by Corey was 8-phenylmenthol. This molecule has three chiral centers and can exist as a mixture of 2 = 8 possible stereoisomers. It was prepared in enantiomerically pure form from naturally occurring, enantiomeri-cally pure menthol. 

Illustration of the concept of the stereogenic center in the context of carbon. Whether in a chiral molecule like 2-butanol or an achiral molecule like meso-tartaiic acid, interconversion of two ligands at a stereocenter produces a new stereoisomer. 

Enantiomers have identical physical properties except that one stereoisomer rotates the plane of plane-polarized light by some amount to the right, whereas the other rotates the plane by the same amount in the opposite direction. Enantiomers have identical chemistries with achiral molecules, but interact differently with other chiral molecules. 

In Summary Meso compounds are diastereomers containing a molecular plane of symmetry. They are therefore superimposable on their mirror images and achiral. Molecules with two or more identically substituted stereocenters may exist as meso stereoisomers. 

Up to this point, we have considered whole molecules differing as stereoisomers. We now turn to an atom or group (A/G) within a molecule and examine the three-dimensional shape of the enviromnent of that A/G within the molecule [61], An A/G that resides in a chiral enviromnent in a molecule is called chirotopic. All atoms in a chiral molecule are chiro-topic, but some A/G in some achiral molecules are chirotopic also. For example, bromochloromethane is not chiral and has no stereoisomers, but the enviromnent of each of the hydrogen atoms is chiral (Fig. 3.8) therefore, those hydrogen atoms are chirotopic. 

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. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

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