Achiral molecules optical inactivity

The figure given below shows a stereoisomer of tartaric acid. Notice that this compound has two chiral (stereogenic) centers. But, there is a plane of symmetry and thus the molecule itself is achiral and optically inactive. Such compounds that contain one or more stereogenic centers, but are achiral, are called meso compounds. Hence, having a stereogenic center or chiral carbon does not always lead to chirality of the entire molecule. 

When a molecule has two chiral centers that are identically substituted, the number of stereoisomers is reduced from four to three, as is well known for the case of tartaric acid. The three stereoisomers are the d and l forms (enantiomers) and the diastereomeric meso form. The meso form is superimposable on its mirror image, since it has a plane of symmetry and is achiral and optically inactive. The three possible stereoisomers of tartaric acid are shown below ... 

It is again instructive to compare the stereochemical situation in 2,3-dibromobutane with that in an analogous cyclic molecule 1,2-dibromocyclobutane. We can see that trans-, 2-dibromocyclobutane exists as two enantiomers Rfi and 5,5) and may therefore be optically active. The cis isomer, however, has an internal mirror plane and is meso, achiral, and optically inactive (Figure 5-12). 

Achiral molecules that contain chirality centers are called meso forms Meso forms typically contain (but are not limited to) two equivalently substituted chirality centers They are optically inactive... 

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. 

There are two possible approaches for the preparation of optically active products by chemical transformation of optically inactive starting materials kinetic resolution and asymmetric synthesis [44,87], For both types of reactions there is one principle in order to make an optically active compound we need another optically active compound. A kinetic resolution depends on the fact that two enantiomers of a racemate react at different rates with a chiral reagent or catalyst. Accordingly, an asymmetric synthesis involves the creation of an asymmetric center that occurs by chiral discrimination of equivalent groups in an achiral starting material. This can be done either by enan-tioselective (which involves the reaction of a prochiral molecule with a chiral substance) or diastereoselective (which involves the preferential formation of a single diastereomer by the creation of a new asymmetric center in a chiral molecule) synthesis. 

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. 

Structures 1 and 2 are enantiomers, and both are optically active. In structures 3 and 4, there is a plane of symmetry, i.e., there is a mirror image within a single molecule. Such a structure is called a meso structure. Structures 3 and 4 are superimposable, and essentially are the same compound. Hence, we have a meso-tartaric acid and it is achiral (since it has a plane of symmetry, and it is superimposable on its mirror image). Meso-tartaric acid is optically inactive. Therefore, for tartaric acid, we have (-f), (—) and meio-tartaric acid. 

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. 

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. 

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. 

Because aldaric acids have identical functional groups on both terminal carbons, some aldaric acids contain a plane of symmetry, making them achiral molecules. For example, oxidation of D-allose forms an achiral, optically inactive aldaric acid. This contrasts with D-glucaric acid formed from glucose, which has no plane of symmetry, and is thus still optically active. 

It is useful to compare a racemic modification with a compound whose molecules are superimposable on their mirror images, that is, with an achiral compound. They are both optically inactive, and for exactly the same reason. Because of the random distribution of the large number of molecules, for every... 

A meso compound is one whose molecules are superimposable on their mirror images even though they contain chiral centers. A meso compound is optically inactive for the same reason as any other compound whose molecules are achiral the rotation caused by any one molecule is cancelled by an equal and opposite rotation caused by another molecule that is the mirror image of the first (Sec. 4.8). 

As a result, structure (a) is optically inactive. Even though there are two chiral carbons, the rotation of plane-polarized light by chiral carbon-2 is canceled by the opposite rotation of plane-polarized light caused by chiral carbon-3. This molecule is achiral and is termed meso-tartaric acid. Any compound with an internal plane of... 

When polarized light passes through a solution of achiral molecules, the light emerges from the solution with its plane of polarization unchanged. An achiral compound does not rotate the plane of polarization. It is optically inactive. 

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. 

A rotative substance (or mixture of substances) may be optically active or inactive. That is to say, the expected optical rotation may be finite, small or even accidentally zero (because of solvent, temperature, pH, or limited sensitivity of the measurement). A rotative substance consists of chiral molecules, and/or unequal numbers of enantiomorphic molecules. A nonrotative substance (or mixture of substances) is always optically inactive (at any wavelength). A nonrotative substance consists of achiral molecules, and/or equal numbers of enantiomorphic sets of molecules. 

The same kind of spontaneous racemization occurs for any c/s-1,2 disubstituted cyclohexane in which both substituents are the same. Because such compounds are chiral, it is incorrect to speak of them as meso compounds, which are achiral molecules that have chirality centers. Rapid chair-chair interconversion, however, converts them to a 1 1 mixture of enantiomers, and this mixture is optically inactive. 

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