Organic Chemistry

Organic Chemistry - Ch 11 534 Daley & Daleymake molecular models of the examples. Using molecular modelshelps to clarify the molecular relationships discussed in this chapter.11.1 Symmetry and AsymmetryA symmetric moleculecan be superimposedon its mirror image. Anasymmetric moleculecannot besuperimposed on itsmirror image.All three-dimensional objects are either symmetric orasymmetric. When the structure of an object allows you tosuperimpose it upon its mirror image, that object is symmetric, oridentical, to its mirror image. The rectangular box on the left in Figure11.1 is superimposible upon its mirror image, so the box is symmetric.However, if you take the same box and paint an X in one corner, youcan no longer superimpose it on its mirror image. The box is thenasymmetric. Note that the difficulty in superimposing the box with theX onto its mirror image is the impossibility of placing one X on top ofthe other X.MirrorsFigure 11.1. The box on the left is superimposible on its mirror image. Painting an Xin one corner, as shown on the right, makes the box nonsuperimposible on its mirrorimage.A molecule possesseschirality if it cannot besuperimposed on itsmirror image.The nonsuperimposibility characteristic of an object upon itsmirror image, a trait common to all asymmetric objects, is calledchirality. The term chiral comes from the Latin word chiro or chirthat means “hand” or “handedness.” The most obvious example ofchirality is your own hands. Examine them. A typical human handhas four fingers and a thumb. Although all hands have these samebasic structural components, hands are asymmetric. That is, a lefthand is a nonsuperimposible mirror image of a right hand. Placingyour palms together makes this mirror image relationship clearlyvisible. Many other common objects also have chirality: golf clubs, thethreaded base of a light bulb, shoes, and even this book.www.ochem4free.com 5 July 2005