Plane of symmetry, and chirality

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

We describe the situation by saying that the receptor provides a chiral environment for the substrate. In the absence of a chiral environment, the two red substituents are chemically identical, but in the presence of the chiral environment, they are chemically distinctive (Figure 9.18a). The situation is similar to what happens when you pick up a coffee mug. By itself, the mug has a plane of symmetry and is achiral. You could, if you wanted, drink from on either side of the handle. When you pick up the mug, however, your hand provides a chiral environment so one side becomes much more accessible and easier to drink from than the other (Figure 9.18b). 

Chiral (Section 9.2) Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, one right-handed and one left-handed. The most common cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents. 

Although the ultimate criterion is, of course, nonsuperimposability on the mirror image (chirality), other tests may be used that are simpler to apply but not always accurate. One such test is the presence of a plane of symmetry A plane of symmetry (also called a mirror plane) is a plane passing through an object such that the part on one side of the plane is the exact reflection of the part on the other side (the plane acting as a mirror). Compounds possessing such a plane are always optically inactive, but there are a few cases known in which compounds lack a plane of symmetry and are nevertheless inactive. Such compounds possess a center of symmetry, such as in a-truxillic acid, or an alternating axis of symmetry as in 1. A... 

There is no plane of symmetry and the molecule is chiral many such compounds have been resolved. Note that groups in the para position cannot cause lack of symmetry. Isomers that can be separated only because rotation about single bonds is prevented or greatly slowed are called atropisomers. 9,9 -Bianthryls also show hindered rotation and exhibit atropisomers. °... 

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. 

Conformationally dependent chirality is a common feature of molecules that contain carbon atoms with identical substituents. Another example is presented in Fig. 7.10. In dihydroxymethane, the conformation in which the H-O-C-O angles are trans, has a plane of symmetry, and the carbon atom is of the type C(a2b2). In contrast, when one of the O-H bonds is rotated out of the plane, the structural parameters change in an asymmetric way. The system is now of type C(aa bb ) and the mirror images cannot be superimposed. 

Chlorobutane does not possess a plane of symmetry and is chiral. 

Turning to enzymatic hydration, we see from the data in Table 10.1 that phenanthrene 9,10-oxide Fig. 10.10, 10.29) is an excellent substrate for epoxide hydrolase. Comparison of enzymatic hydration of the three isomeric phenanthrene oxides shows that the Vmax with the 9,10-oxide is greater than with the 1,2- or the 3,4-oxide the affinity was higher as well, as assessed by the tenfold lower Km value [90]. Furthermore, phenanthrene 9,10-oxide has a plane of symmetry and is, thus, an achiral molecule, but hydration gives rise to a chiral metabolite with high product enantioselectivity. Indeed, nucleophilic attack by epoxide hydrolase occurs at C(9) with inversion of configuration i.e., from below the oxirane ring as shown in Fig. 10.10) to yield the C-H9.S, 10.S )-9,10-dihydro-9,10-diol (10.30) [91],... 

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. 

Only three of the compounds shown have chiral centres these are indicated. These three compounds could, therefore, exist in optically active form. None of the compounds shown is a meso compound. Note that we have to consider part of the ring system as a group attached to the centre in question. Follow this around until a decision can be made. The last two are not exactly trick questions, but require care. One has a plane of symmetry, and the carbon carrying the hydroxyl has two of its attached groups the same the benzene ring is planar, so none of its carbons has the potential to be chiral. 

The achiral things are (c) and (h). with planes of symmetry (e). with a center of symmetry and two planes of symmetry and (/), with a center of symmetry. This leaves (a), (b), (d). (g). and (/) as chiral. [Chiral derives from the Greek for hand.)... 

Note that the cis isomer lacks an improper axis of rotation and is therefore chiral, but that the trans isomer has a plane of symmetry and will be achiral in the absence of an asymmetric carbon in the phosphine ligand-28 As in the case of the previously encountered cyclopentadienyl complex (page 476), it can be argued whether the coordination number is 5 or 9. In either semantic interpretation these compounds are of considerable interest since isomerism in nine-coordinate complexes Is even less well documented than in those with coordination number 5. 

Chelate complexes with two ethylenediamine rings in a cis configuration lack a plane of symmetry and thus have the potential to be separated into enantiomeric (A, A) (12) forms. Inert cis-bis(en) complexes of Co111,247 Crmn or Rh111248 can be resolved by the method of racemic modification 249 or using chromatographic techniques,35 but labile systems, such as Ni(en)2+, which occasionally crystallize in one chiral form,220 rapidly racemize in solution. 

Phenyl vinyl sulfoxide lacks a plane of symmetry and is chiral. Phenyl vinyl sulfone is achiral a plane of symmetry passes through the phenyl and vinyl groups and the central sulfur atom. 

Structure (c) is also the only one which has a chiral carbon atom (the carbons of the two CH groups). However, the cis isomer has a plane of symmetry and cannot have an optical isomer. Only the trans isomer has an optical isomer. 

It is important to note that high molecular weight trans-isotactic poly(methy-lene-1,3-cyclopentane) contains no mirror or mirror glide planes of symmetry and is thus chiral by virtue of its main chain stereochemistry (it exhibits optical activity) this is in contrast to high molecular weight polypropylene and other poly(a-olefin)s, which contain an effective mirror plane perpendicular to the molecular axis in the middle of the molecule and are thus achiral [30,497],... 

There are six isomers of difluorocyclobutane (see below). The vicinal di-substituted isomers B and C (both with a twofold proper axis of symmetry, symmetry point group C2) are chiral and are enantiomers of each other. The cis-configured compound D (with a plane of symmetry, symmetry point group Cs) is achiral and is a meso compound. The compounds A and F (both with two planes of symmetry and on the line of intersection of both planes a twofold axis of symmetry, symmetry point group C2V) and E (with a plane of symmetry, a twofold axis of symmetry perpendicular to it and a centre of symmetry, symmetry point group C21O are all achiral. These results can be verified from the flow chart given in the appendix. 

This compound does not have a plane of symmetry, and we suspect that it is chiral. Drawing the mirror image shows that it is nonsuperim-posable on the original structure. These are the enantiomers of a chiral compound. 

A structure without a plane of symmetry is chiral and not superimposable on its mirror image and can exist as two enantiomers... 

An aldehyde cyanohydrin is chiral because it does not have a plane of symmetry. In fact, it cannot have a plane of symmetry, because it contains a tetrahedral carbon atom carrying four different groups OH, CN, RCH2, and H. Such a carbon atom is known as a stereogenic or chiral centre. The product of cyanide and acetone is not chiral it has a plane of symmetry, and no chiral centre because two of the groups on the central carbon atom are the same. 

If a molecule contains one carbon atom carrying four different groups it will not have a plane of symmetry and must therefore be chiral. A carbon atom carrying four different groups is a stereogenic or chiral centre. 

The last pair of diastereoisomers on the other hand, is chiral. We know this because they do not have a plane of symmetry and we can check that by drawing the mirror image of each one it is not superimposable on the first structure. 

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