Arteleo Chemistry Journal

Geometric Isomers

Geometric isomers (or cis-trans isomers) differ only in the spatial orientation of groups about a plane or direction. Two geometric isomers have the same molecular formula, the same functional groups, the same base chain or ring, and the same order of attachment of atoms; they differ in orientation either (1) around a double bond or (2) across the ring in a cyclic compound. If the larger groups are on opposite sides of the ring or the double bond, the designation trans appears in the name; if they are on the same side, the designation is cis. We learned in Section 27-3 about the geometric isomerism associated with the double bond in alkenes such as the 1,2-dichloroethenes (Figure 27-9). Similarly, two or more substituents can be either on the same side or on opposite sides of the ring, as shown in Figures 28-1 and 28-2. This kind of isomerism is possible when substituents have replaced an H from a —CH2— unit in a ring. Because substituents on an aromatic ring are bonded in the plane of the ring, such substitutions do not lead to geometric isomerism.

Figure 28-1 Models of (a) or-dichlorocyclopropane and (b) frans-dichlorocyclopropane.

Figure 28-2 Models of (a) cis-1-chloro-3-methylcyclopentane, (b) trans-1-chloro-3-methylcyclopentane, and (c) «S-1-chloro-3-methylcyclopentane (stereoview).

Figure 28-2 Models of (a) cis-1-chloro-3-methylcyclopentane, (b) trans-1-chloro-3-methylcyclopentane, and (c) «S-1-chloro-3-methylcyclopentane (stereoview).

Optical Isomers

Many objects are mirror images of each other and cannot be superimposed. Your two hands are a familiar example of this; each hand is a nonsuperimposable mirror image of the other (Figure 28-3). "Superimposable" means that if one object is placed over the other, the positions of all parts will match.

An object that is not superimposable with its mirror image is said to be chiral (from the Greek word cheir, meaning "hand"); an object that is superimposable with its mirror image is said to be achiral. Examples of familiar objects that are chiral are a screw, a propeller, a foot, an ear, and a spiral staircase; examples of common objects that are achiral are a plain cup with no decoration, a pair of eyeglasses, and a sock.

We speak of a screw or a propeller as being "right-handed" or "left-handed.'

Optical isomers that are nonsuperimposable mirror images of each other (chiral) are called enantiomers of one another. Enantiomers can exist in two forms that bear the same relationship to each other as do left and right hands.

As an example of this, consider first the two models of bromochloromethane, CH^BrCl, shown in Figure 28-4. They are mirror images of each other, and they can be superimposed. Thus, this molecule is achiral and is not capable of optical isomerism. Now consider

Figure 28-3 Mirror images. Place your left hand in front of a mirror; you will observe that it looks like your right hand. We say that the two hands are mirror images of each other; each hand is in every way the "reverse" of the other. Now try placing one hand directly over the other; they are not identical. Hence, they are nonsuperimposable mirror images. Each hand is a chiral object.

bromochloroiodomethane, CHBrCll (Figure 28-5). This molecule is not superimposable with its mirror image, so it is chiral, and the two forms are said to be enantiomers of each other. Any compound that contains four different atoms or groups bonded to the same carbon atom is chiral; that is, it exhibits optical isomerism. Such a carbon is said to be asymmetric (meaning "without symmetry"). Most simple chiral molecules contain at least one asymmetric carbon atom, although there are other ways in which molecular chirality can occur.

Stereoisomers have the same type and number of atoms, connected in the same order, but arranged differently in space. Optical isomers (enantiomers) and geometric isomers are subgroups of stereoisomers. They differ, however, in that geometric isomers have different physical and chemical properties, whereas optical isomers have physical properties that are identical (e.g., melting point, boiling point, and density). Optical isomers also undergo the same chemical reactions, except when they interact with other chiral compounds. Consequently, their properties in biological systems may be very different. They also often exhibit different solubilities in solvents that are composed of chiral molecules.

Mirror plane

Mirror plane

Figure 28-4 Models of two mirror-image forms of bromochloromethane, CH^BrCl. The two models are the same (superimposable), so they are achiral. CH^BrCl does not exhibit optical isomerism.

Isomers

Constitutional (structural)

Stereoisomers

Geometric

Figure 28-5 (a, b) Models of the two mirror-image forms of bromochloroiodomethane, CHBrClI. (Colors: CI, green-, Br, reel; I, purple.) (c) The same model as in (a), turned so that H and I point the same as in (b); however, the Br and CI atoms are not in the same positions in (b) and (c). The two models in (a) and (b) cannot be superimposed on each other no matter how we rotate them, so they are chiral. These two forms of CHBrClI represent different compounds that are optical isomers of each other.

Optical isomers also differ from each other in one notable physical property: They interact with polarized light in different ways. The main features of this subject were presented in Chapter 25. Separate equimolar solutions of two optical isomers rotate a plane of polarized light (Figures 25-4 and 25-5) by equal amounts but in opposite directions. One of the optical isomers is designated as the D-isomer and its nonsuperimposable mirror image is designated as the L-isomer. The phenomenon in which a plane of polarized light is rotated by samples of either isomer is called optical activity. It can be measured with a polarimeter (Figure 25-5). A racemic mixture is a single sample containing equal amounts of the two optical isomers of a compound. Such a solution does not rotate a plane of polarized light because the equal and opposite effects of the two isomers exactly cancel. The isomers must be separated from each other to exhibit optical activity.

One very important way in which optical isomers differ chemically from one another is in their biological activities. «-Amino acids have the general structure

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