The Reason for Handedness in Molecules Chirality

Molecules that are not superimposable with their mirror images and thus exist in two enantiomeric forms are said to be chiral (ky-ral, from the Greek cheir, hand ). You can t take a chiral molecule and its enantiomer and place one on the other so that all atoms coincide. 

The most common, although not the only, cause of chirality in an organic molecule is the presence of a carbon atom bonded to four different groups— for example, the central carbon atom in lactic acid. Such carbons are currently referred to as chirality centers, although numerous other terms such as asymmetric center and stereogenic center have been also used, Note that chirality is a property of the entire molecule, whereas a chirality center is a structural feature within the molecule that gives rise to chirality. 

The achiral propanoic acid molecule versus (he chiral i NOT j 

Propanoic acid has a plane of symmetry that makes one side of the 0 L T  

Detecting chirality centers in a complex molecule takes practice because it s not always immediately apparent that four different groups are bonded to a given carbon. The differences don t necessarily appear right next to the chirality center. For example, 5-bromodecane is a chiral molecule because four different groups are bonded to C5, the chirality center (marked by an asterisk)  

The achiral propanoic acid molecule versus the chiral lactic acid molecule. Propanoic acid has a plane of symmetry that makes one side of the molecule a mirror Image of the other side. Lactic acid, however, has no such symmetry plane. 

Propanoic acid (achiral) Lactic acid (chiral) 

Test your knowledge of Key Ideas by using resources in ThomsonNOW or by answering end-of-chapter problems marked with A.. 

Thomsen Click Organic Interactive to practice identifying chirality centers in organic molecules. 

How can you predict whether a given molecule is or is not chiral A molecule is not chiral if it has a plane of symmetry. A plane of symmetry is a plane that cuts through the middle of a molecule (or any object) in such a way that one half of the molecule or object is a mirror image of the other half. 

FIGURE 5.3 The meaning of symmetry plane, (a) An object like the flask has a symmetry plane cutting through it, making right and left halves mirror images. 

A molecule that has a plane of symmetry in any conformation must be identical to its mirror image and hence must be nonchiral, or achiral. Thus, propanoic acid, CH3CH2CO2H, has a plane of symmetry when lined up as shown in 

As Other possible examples, look at methylcyclohexane and 2-methylcyclo-hexanone. Methylcyclohexane is achiral because no carbon atom in the molecule is bonded to four different groups. You can immediately eliminate all -CH2-carbons and the -CH3 carbon from consideration, but what about Cl on the ring The Cl carbon atom is bonded to a -CH3 group, to an -H atom, and to C2 and C6 of the ring. Carbons 2 and 6 are equivalent, however, as are carbons 

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Enantiomers and the Tetrahedral Carbon 307 The Reason for Handedness in Molecules Chirality Optical Activity 312 Specific Rotation 313... 

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