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Three-Dimensional Representations

Assigning Priorities

Determining Stereochemical Relationships

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Three-Dimensional Representations: Fischer Projection Examples

Consider the following Fischer Projections and count how many carbons are in the longest chain of the molecule, and how many alcohols are oriented upward and how many alcohols are oriented downward.

When finding the longest chain, recall that a carbon chain with a functional group - an alcohol or halogen, etc. - should be counted as the "major" chain. In this case there are six carbons in the longest chain, two from the ethyl group on the top carbon, one from the carboxylic acid group on the bottom carbon, and three connecting carbons in the backbone of the Fischer Projection. Since all of the horizontal lines represent an orientation upward and all of the vertical lines represent an orientation downward, two alcohols are oriented upward and no alcohols are oriented downward.

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