Grossman, CHE 230 5.1 5. Chirality, Acyclic Stereochemistry, Optical Activity. 5.1 Enantiomers and Chirality. We’ve learned about enantiomers, compounds whose internal dimensions — atom–to–atom connections, bond angles, dihedral angles, interatomic distances — are identical in all respects but whose structures are non-superimposable. We’ve seen that certain structures are identical to their mirror images, and do not have enantiomers, while other compounds are non-superimposable with their mirror images, and do have enantiomers. There is a particular property of shape, called chirality, that allows one to determine whether a compound has an enantiomer or not. A compound that is not identical to its mirror image is called a chiral compound (from the Greek word for hand). A compound that is identical to its mirror image is said to be achiral. Note that “chiral” and “achiral” refer to structures, while “enantiomer” refers to the relationship of one structure to another. Every structure with a shape is either chiral or achiral, whether it is microscopic like a molecule or macroscopic like a hand, chair, or building. Helices like DNA, screws, and Slinkies are chiral; that’s why screws have right-handed or left-handed threads. You can tell whether an object is achiral or chiral simply by examining its shape. There is a way of classifying objects according to the symmetry characteristics of their shapes. This is called group theory. We won’t go into the details now. At this point, all we need to know is that any object (including a molecule) that has a plane of symmetry must be achiral. Such an object is identical to its mirror image. Almost any object which lacks a plane of symmetry must be chiral, i.e. not identical to its mirror image. (An object might also be achiral if it has a center of symmetry or an improper axis of symmetry. Most achiral organic compounds, though, have a plane of symmetry, so we don’t need to worry about these other symmetry elements.) Let’s look at what is a plane of symmetry. First, consider a human being. If we think about the plane going through the center of a person that separates the left and right sides, we can see that the plane relates the two sides as a mirror does. That is, if someone stood to my left and looked toward the plane, that person couldn’t tell whether she was seeing my right side in the plane or whether she was seeing a reflection of my left side. This means that the human body is achiral. A broom is achiral. A chair is achiral. The frame of an automobile is achiral. (But the inside is not, because the driver’s side has a wheel and the passenger side does not.) All of these objects have planes of symmetry. Now look at your hand. You can't find a plane of symmetry in your hand. Therefore it is chiral. Try to find a plane of symmetry in an extended Slinky™; you won't be able to find one.Grossman, CHE 230 5.2 How about molecules? The situation here is a little more complicated, because many molecules constantly change their shape by rotation about σ bonds. Let’s start with compounds that don't undergo rotations about σ bonds. We can certainly say that ethylene is achiral. Same for methane, chloromethane, dichloromethane, and chloroform. Cyclopropane is achiral, and so is chlorocyclopropane, 1,1-dichlorocyclopropane, and cis-1,2-dichlorocyclopropane. All these compounds have planes of symmetry. Bromochloromethane is also achiral: it has a plane of symmetry containing the C, Br, and Cl atoms and relating the two H atoms to one another. On the other hand, bromochlorofluoromethane is chiral. It has no plane of symmetry, and it is nonsuperimposable with its mirror image. trans-1,2-Dichlorocyclopropane is also chiral, as is cis-1-bromo-2-chlorocyclopropane. HHHHHHHHHHHClHHClClClClHCl HHHHHHClHHClHHHHHHHClHHClHHCl HFBrClHHBrClFHBrCl ClHHHClHHClHClHHClHHBrHHBrHHClHHGrossman, CHE 230 5.3 How about compounds that change their shape? Let’s look at ethane first. We have to consider all the different conformers, because different conformers have different shapes, and chirality is a property of shape. Staggered ethane has a plane of symmetry (several, in fact), as does eclipsed ethane. But if we look at a conformation of ethane in between staggered and eclipsed, e.g. by starting with eclipsed ethane and twisting the H–C–C–H dihedral angle 30° in one direction, we find that this conformer has no plane of symmetry. This conformer of ethane is chiral. Its nonidentical mirror image is a different conformer of ethane, arrived at by starting with eclipsed ethane and twisting it 30° in the opposite direction. These two enantiomers can interconvert easily by rotation about the C–C σ bond, so they are conformational enantiomers. eclipseddihedral angle 0°achiralstaggereddihedral angle 60°achiralpartly staggereddihedral angle 30°chiralpartly staggereddihedral angle –30°chiral Almost any compound with more than two or three C atoms has an infinite number of chiral conformers. Even local minima can be chiral; for example, gauche butane. In practice, though, we say that a compound is achiral if any of its low-energy conformers are achiral. Ethane's staggered and eclipsed conformers are achiral, so ethane is said to be achiral, even though it has many chiral conformers. In practice you can ignore the H atoms in CH3, NH2, and OH groups (but not CH2 groups!) when you are deciding whether a compound is chiral. You might ask, how low is “low-energy”? This is a very good question. The answer depends on what time scale we’re talking about. If we look at a compound on the time scale of µs, we have a very different sense of “rapid” than if we look at it on the time scale of days. For example, when X = H in the aromatic compound below, the two conformational enantiomers interconvert with a half-life less than seconds. However, when X = OH, the interconversion is slow on a time scale of days, even at very high temperatures. Is the X = H compound chiral or achiral? It depends on whether you’re talking about a microsecond or a laboratory time scale! We will use the laboratory time scale, so we would say that the X = H compound is achiral and the X = OH compound is chiral.Grossman, CHE 230 5.4 HHXXHHXXHHXXachiral(R) (S) Note that the word "chiral" refers to a single object or molecule, while the word "enantiomeric" refers to the
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