Chirality in Glucose: Chiral Carbons and Stereoisomers

Chiral Carbons of Glucose

• Glucose has 4 chiral carbons: C2, C3, C4, and C5.
• The presence of a single chiral carbon gives rise to stereoisomerism.
• A carbon atom is chiral if it is attached to four different groups.
• If any two groups are identical, the carbon is not chiral.

Chirality

• Compounds can have more than one chiral carbon.
• The maximum number of stereoisomers is 2n, where n is the number of chiral carbon atoms.
• Glucose has 2^4 or 16 stereoisomers.

Fischer Projections

• Fischer Projections depict three-dimensional shapes for chiral molecules.
• The chiral carbon is represented by the intersection of two lines.
• Fischer Projections of carbohydrates have the carbonyl (C=O) group at the top.
• The two horizontal bonds are coming toward the viewer out of the plane in which they are drawn.
• The bottom carbon group is projecting away from the viewer.

D and L Enantiomers

• The hydroxyl group on the chiral carbon farthest from the C=O group determines whether the carbohydrate is D or L.
• D-Glucose and L-Glucose are enantiomers.
• The physical properties of D and L enantiomers are generally the same.
• The exception is the way solutions of the two compounds affect polarized light.

Enantiomer Properties

• Specific Rotation [α] is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength, and concentration.
• Optical Activity is the property of rotating the plane of polarized light.
• Measurements of optical activity are useful for differentiating between enantiomers.
• Levorotatory enantiomers rotate polarized light to the left (-), while dextrorotatory enantiomers rotate polarized light to the right (+).
• Racemic Mixture is composed of dextrorotatory and levorotatory forms of a compound in equal proportion.

Importance of Carbohydrates

• Carbohydrates provide energy through their oxidation.
• They supply carbon for the synthesis of cell components.
• They serve as a stored form of chemical energy.
• They form a part of the structural elements of some cells and tissues.

Classification of Carbohydrates

• Carbohydrates are classified according to size: monosaccharides, disaccharides, and polysaccharides.
• Monosaccharides are single polyhydroxy aldehyde or ketone units.
• Disaccharides are composed of two monosaccharide units.
• Polysaccharides are very long chains of linked monosaccharide units.

Stereochemistry

• Stereoisomers are compounds with the same structural formula but different spatial arrangements of atoms.
• Enantiomers are stereoisomers that are non-superimposable mirror images.
• Diastereomers are stereoisomers that are non-superimposable non-mirror images.
• Many carbohydrates exist as enantiomers.

Chirality

• An object or system is chiral if it is distinguishable from its mirror image.
• A chiral object cannot be superimposed on its mirror image.
• A chiral carbon is one that has four different groups attached to it.
• If any two groups are identical, the carbon is not chiral.

Meso Compounds

• A meso compound is a molecule with multiple chiral centers that is superimposable on its mirror image.
• Meso compounds have a minimum of two chiral carbons, with at least one on each side of the internal mirror plane.
• Meso compounds are achiral and cannot have an enantiomer.
• Meso compounds are optically inactive.

Learn about chiral carbons in glucose, stereoisomerism, and the maximum number of stereoisomers in a compound with multiple chiral carbon atoms.