Chirality—The Handedness of Molecules

A left hand and its mirror image, which looks the same as the right hand. 

With one Polaroid sheet over a picture, light passes through. With a second sheet of Polaroid placed over the first so that the axes of polarization of the sheets are perpendicular, little or no light passes through. If the axes of polarization of the two sheets were parallel, light would pass through. 

The enantiomers of ibuprofen are mirror images of each other. There is only one asymmetric C atom in the molecule. Can you spot it  

As of 2006, one of the best-selling chiral dmgs, lipitor, which controls cholesterol level, is sold as a pure enantiomer. 

Strategy Recall the condition for chirality. Is the central C atom asymmetric that is, does it have four different atoms or different groups attached to it  

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Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if it is not superposable on its mirror image. Applying Thomson s term to chemistry, we say that a molecule is chiral if its two mirror-image forms are not superposable in three dimensions. The work chiral is derived from the Greek word cheir, meaning hand, and it is entirely appropriate to speak of the handedness of molecules. The opposite of chiral is achiral. A molecule that is superposable on its mirror image is achiral. 

This chapter deals almost exclusively with the concept and consequences of the handedness of molecules—chirality. Some molecules are related to each other as are your left and right hands. These are nonsuperimposahle mirror images, or enantiomers. 

Chemical Family a group of elements that share similar chemical properties and share the same column in the periodic table, for example, halogens, alkali earth Chirality condition that describes the handedness of a molecule or whether a molecule exists in forms that can be superimposed on each other Chlorofluorocarbons also called CFCs, compounds consisting of chorine, fluorine, and carbon that are responsible for stratospheric ozone destruction Coagulation precipitation or separation from a dispersed state Coefficient of Thermal Expansion measure of the rate at which a substance will expand when heated... 

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