CH 15 Lecture PT 1

Carbohydrates Overview

• Focus points include:

  • Definition of carbohydrates

  • Open and closed forms of carbohydrates

  • Stereochemistry of carbohydrates

  • Nomenclature of carbohydrates

  • Glycoside formation

  • Homework assignment for Chapter 15.

Types of Carbohydrates

• Categories of Carbohydrates:

  • Monosaccharides: Simple sugars that are the building blocks of carbohydrates.

  • Oligosaccharides: Short chains of monosaccharides, typically containing 2 to 10 units.

  • Polysaccharides: Long chains of monosaccharides, can be branched or unbranched.

Stereochemistry of Carbohydrates

• Stereoisomers: Compounds with the same connectivity but different orientations in space.

  • Isomers: Same molecular formula but different structures.

  • Cis-Trans Isomers: A type of stereoisomer with different spatial arrangements.

  • Enantiomers: Non-superimposable mirror images of each other; they have identical physical properties except in their interaction with polarized light.

  • Chiral Compounds: Molecules that are not superimposable on their mirror images due to the presence of one or more chiral centers.

Enantiomers and Chirality

• Enantiomers: cannot be superimposed; differ at one or more chiral centers.

• Chiral Centers: carbon atoms bonded to four different groups or atoms

• Perspective Drawings: visual representations of stereoisomers indicating 3D orientation.

Fisher Projections

• Fisher Projections: A two-dimensional representation of a molecule.

  • Vertical lines depict bonds going into the page, while horizontal lines cross out toward the viewer.

Nomenclature System: D- and L- system

• Emil Fisher devised a nomenclature to distinguish between D and L compounds:

  • D-compounds: Have the –OH group on the right in the penultimate carbon in Fischer projection.

  • L-compounds: Have the –OH group on the left in Fischer projection.

Configurations Relative to Glyceraldehyde

• Nomenclature can relate structures back to glyceraldehyde (2,3-dihydroxypropanal).

• Examples:

  • D-glyceraldehyde: CHO at the top, two hydroxyl groups with one on the right.

  • L-glyceraldehyde: Similar structure with one hydroxyl group on the left.

Multiple Chiral Centers

• Molecules with several chiral centers can exist as pairs of enantiomers.

• Total possible stereoisomers = 2^n, where n = number of chiral centers.

Diastereomerism

• Diastereomers: Stereoisomers that are not mirror images of each other, having different physical and chemical properties.

  • Example: D-Erythrose and D-Threose.

Meso Compounds

• Meso compounds have a plane of symmetry and are superimposable on their mirror images.

• They can exhibit chiral centers yet yield optically inactive compounds.

Physical Properties of Chiral Molecules

• Optically Active Compounds: Contain chiral centers; rotate plane-polarized light, distinguishing features in optical activity.

• Measured using a polarimeter.

• Dextrorotatory (+): Rotates light clockwise.

• Levorotatory (-): Rotates light counterclockwise.

• Racemic mixture: 1:1 mix of two enantiomers that cancels light rotation resulting in an overall specific rotation of 0°.