Organic Alkeyne Notes

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  • Topic: Chemistry, Stereochemistry, Chirality
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CHEM 3305 - 001 Organic Chemistry 1
Dr Michael Findlater

Spring Semester, 2013


• Stereochemistry refers to the three-dimensional structure of a molecule.

• As a consequence of stereochemistry, apparently minor differences in 3D structure can result in vastly different properties.

• We can observe this by considering starch and cellulose, which are both composed of the same repeating unit.

Stereochemistry of Starch and Cellulose
Figure 5.2

• In cellulose, the O atom joins two rings using equatorial bonds.

• In starch, the O atom joins two rings using one equatorial and one axial bond.

• Due to these differences in stereochemistry, humans can metabolize starch for energy but we cannot digest cellulose.

Constitutional Isomers
• Isomers are different compounds with the same molecular formula.

• The two major classes of isomers are constitutional isomers and stereoisomers.

• Constitutional/structural isomers have: • different IUPAC names • same or different functional groups • different physical properties • different chemical properties

• Stereoisomers:

• Differ only in the way the atoms are oriented in space.

• Have identical IUPAC names (except for a prefix like cis or trans).

• Always have the same functional group(s).

• Differ in configuration (its particular three-dimensional arrangement).

Constitutional and Stereoisomers
Figure 5.3

Non-superimposable Mirror Images
• Although everything has a mirror image, mirror images may or may not be superimposable. • Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable. • A molecule (or object) that is not superimposable on its mirror image is said to be chiral.

Superimposable (Achiral) Objects
• Other molecules are like socks. • Two socks from a pair are mirror images that are superimposable. • A sock and its mirror image are...
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