26.07.2021 Views

General Chemistry Principles, Patterns, and Applications, 2011

General Chemistry Principles, Patterns, and Applications, 2011

General Chemistry Principles, Patterns, and Applications, 2011

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

the solute is said to be optically active. A clockwise rotation is called dextrorotatory (from the<br />

Latin dextro, meaning “to the right”) <strong>and</strong> is indicated in the name of the compound by (+), whereas a<br />

counterclockwise rotation is called levorotatory (from the Latin levo, meaning “to the left”) <strong>and</strong> is<br />

designated (−). As you will soon discover, this designation is important in underst<strong>and</strong>ing how chiral<br />

molecules interact with one another.<br />

Note the Pattern<br />

Chiral molecules are optically active; achiral molecules are not.<br />

The magnitude of the rotation of plane-polarized light is directly proportional to the number of chiral<br />

molecules in a solution; it also depends on their molecular structure, the temperature, <strong>and</strong> the wavelength<br />

of the light. Because of these variables, every chiral compound has a specific rotation, which is defined as<br />

the amount (in degrees) by which the plane of polarized light is rotated when the light is passed through a<br />

solution containing 1.0 g of solute per 1.0 mL of solvent in a tube 10.0 cm long. A chiral solution that<br />

contains equal concentrations of a pair of enantiomers is called a racemic mixture. In such a solution, the<br />

optical rotations exactly cancel one another, so there is no net rotation, <strong>and</strong> the solution is optically<br />

inactive. The categories of stereoisomers are summarized in Figure 24.9 "Classification of Stereoisomers".<br />

Figure 24.9 Classification of Stereoisomers<br />

In both types of stereoisomer—geometric <strong>and</strong> optical—isomeric molecules have identical<br />

connectivity, but the arrangement of atoms in space differs. Cis <strong>and</strong> transisomers exhibit different<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

2223

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!