Lecture Outline Mirror-Image Stereoisomers

Name ______________________________________ Section 7.1 of the text introduces the concept of chirality ("handedness") in organic molecules. Section 7.2 states that a molecule having one "chirality center" — a tetrahedral atom with four different things attached — must be chiral (has two non-superimposable mirror image forms). Many molecules have more than one — most are chiral, some are achiral — we'll get to that in time. In class I will use the more traditional term stereocenter or "sc", or more correctly "tetrahedral stereocenter", rather than chirality center. (The TAs knows these by yet another name — "tetrahedral asymmetric stereocenter" or "t.a.s.") Read the beginning of section 7.2 and work problem 7.2 (In particular, be sure you understand parts d, e, and f). 1 We're looking for tetrahedral atoms with 4 different things attached. Mark each stereocenter in the following molecules with an asterisk (*). Some may have no sc, some may have more than one. Cl O Br Br CH3 F Cl 2 Consider the central Cs of the two structures below. The one on the left is a stereocenter; the one on the right is not. What's up with that? (i.e. what's the key difference?) Br Br Lecture outline A compound that can exist as two non-superimposable mirror-image stereoisomers is chiral. The mirror-image forms of a chiral compound are called enantiomers. They are, in effect “right-” and “left-handed” versions of a molecule. Compound that cannot exist in mirror-image forms are achiral. In other words, although everything has a reflection (except a vampire), the mirror image of an achiral molecule and the original structure are absolutely identical, i.e. superimposable. H C e.g., 1-bromo-1-fluoroethane, Br CH3 , is chiral F Draw the reflection of the structure shown. mirror plane F H Br CH3 The two molecules above are enantiomers. Cl chlorofluoromethane, H2C , is achiral mirror plane F Draw the reflection of the structure shown. Make models and confirm that the two are identical. F H H Cl A molecule with ONE stereocenter ("sc") (or "chirality center" in the text) — a tetrahedral atom with 4 different things attached — must be chiral. (Why?) — a molecule with 2 or more scs may be chiral or achiral (see below). — most (but not all) chiral molecules have at least one sc. (see the exceptions on pp 246 and 247 of the text) While recognizing stereocenters can provide a hint as to whether a compound is chiral or achiral, keep in mind that by definition, a compound is achiral if it is superimposable on its reflection (not if it lacks an sc — text section 7.10 shows several examples of molecules that are chiral but have no sc), and chiral if it's not. So we need to construct the mirror image of a molecule and then see if we can match it up with the original — or pick the mirror in just the right place that the reflection is automatically superimposed on the original structure! In other words, if we can find an internal mirror plane, the molecule's reflection through it is just the molecule itself! Cool. More generally, if a molecule contains a plane of symmetry (aka, an internal mirror plane) or an inversion center in any reasonable conformation, it must be achiral mirror plane F Cl H H Cl C H Cl C H CH3 CH3 Notice that one of these has no sc; the other has two. Each of the two structures below has an inversion center — a point such that every atom has a counterpart on the opposite side of the point. (If you “turned the structure inside-out” by passing it through the point, you would get the same structure back again.) The presence of an inversion center also makes a molecule achiral. Try drawing the reflection of each of these — you’ll see that it’s the same as the original structure! (When looking for symmetry elements we can consider all rings to be flat.) O CH3 CH3 H Cl C CH3 O H CH3 C Cl Planes of symmetry and inversion centers are symmetry elements. Either of these alone is sufficient to make a molecule achiral. Now look at the molecule below. This is chiral — verify this by drawing its reflection. But it clearly has some sort of symmetry, i.e. another kind of symmetry element... hmmmm.... Cl Note that this compound does not have a plane of symmetry, nor does it have an inversion center. But there's clearly some kind of symmetry there... what's the symmetry element? Cl Look for planes of symmetry or inversion centers and label each structure below chiral or achiral. Br Cl Br Br Br Cl Br Cl Cl OH O CH3 O Cl Cl Cl Et OCH3 As a check, (1) try drawing the reflection of each molecule you labelled achiral — demonstrate that it’s the same as the original structure. Then (2) mark stereocenters — are the molecules with no scs chiral or achiral? How about the ones with one sc? Two scs?