25.2 Representing Carbohydrate Stereochemistry: Fischer Projections
3 min read•may 7, 2024
are a handy way to show 3D carbohydrate structures in 2D. They use horizontal and vertical lines to show bond directions, with the carbon chain vertical and the most oxidized carbon at the top. This makes it easy to see the layout of hydroxyl groups and other substituents.
These projections help us understand carbohydrate . By looking at the orientation of hydroxyl groups, we can determine D or L configurations and identify different types of isomers like and . This knowledge is crucial for understanding carbohydrate structure and function.
Fischer Projections and Carbohydrate Stereochemistry
Fischer projections of carbohydrates
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Top images from around the web for Fischer projections of carbohydrates
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- represent the three-dimensional structure of molecules on a two-dimensional surface
- Horizontal lines depict bonds pointing towards the viewer (coming out of the page)
- Vertical lines depict bonds pointing away from the viewer (going into the page)
- When drawing Fischer projections of carbohydrates, orient the carbon chain vertically with the or group at the top
- Place the most oxidized carbon (aldehyde or ketone) at the top of the projection
- Place the least oxidized carbon (often a CH2OH group) at the bottom
- Place substituents (hydroxyl groups) on either side of the carbon chain
- Groups on the right side of the carbon chain are oriented towards the viewer
- Groups on the left side of the carbon chain are oriented away from the viewer
- Examples of carbohydrates commonly represented using Fischer projections include , , and
- These are examples of , the simplest form of carbohydrates
Stereochemistry from Fischer projections
- Determine the configuration at each chiral center by the orientation of the substituents
- If the hydroxyl group is on the right side of the carbon chain, the configuration is D
- If the hydroxyl group is on the left side of the carbon chain, the configuration is L
- Determine the overall configuration of the carbohydrate by the configuration at the chiral center furthest from the aldehyde or ketone group
- If the hydroxyl group at the chiral center furthest from the aldehyde or ketone is on the right, the sugar is a sugar (D-glucose)
- If the hydroxyl group at the chiral center furthest from the aldehyde or ketone is on the left, the sugar is an sugar (L-glucose)
- The configuration at each chiral center contributes to the overall stereochemistry of the carbohydrate molecule
- Enantiomers have opposite configurations at all (D-glucose and L-glucose)
- have opposite configurations at some, but not all, chiral centers (D-glucose and D-galactose)
- Epimers are a specific type of diastereomers that differ in configuration at only one chiral center
Optical activity and isomerism in carbohydrates
- Carbohydrates with chiral centers exhibit , the ability to rotate plane-polarized light
- Different types of isomers in carbohydrates:
- Enantiomers: mirror images with opposite configurations at all chiral centers
- Diastereomers: non-mirror image stereoisomers
- Epimers: diastereomers differing at only one chiral center
- : cyclic forms of monosaccharides that differ in configuration at the anomeric carbon (C1)
Conversion of carbohydrate representations
To convert a Fischer projection to a (cyclic structure):
- Cyclize the structure by forming a bond between the oxygen of the hydroxyl group on carbon 5 and the aldehyde carbon (carbon 1)
- Groups that were on the right side of the Fischer projection (towards the viewer) will point downwards in the Haworth projection
- Groups that were on the left side of the Fischer projection (away from the viewer) will point upwards in the Haworth projection
To convert a Haworth projection to a Fischer projection:
- Open the cyclic structure by breaking the bond between the oxygen and carbon 1
- Rotate the structure so that the carbon chain is vertical with the aldehyde or ketone group at the top
- Groups that were pointing downwards in the Haworth projection will be on the right side of the Fischer projection
- Groups that were pointing upwards in the Haworth projection will be on the left side of the Fischer projection
- Interconvert between chair conformations and Fischer projections by following a similar process
- Cyclize the Fischer projection to form a Haworth projection
- Convert the Haworth projection to a by orienting the groups axially or equatorially
Key Terms to Review (23)
Aldehyde: An aldehyde is a class of organic compounds containing a carbonyl group (C=O) where the carbon atom is bonded to one hydrogen atom and one alkyl or aryl group. Aldehydes are important functional groups in organic chemistry and are involved in various reactions and synthesis pathways.
Anomers: Anomers are the two possible stereoisomers that can exist at the anomeric carbon of a monosaccharide, where the hydroxyl group can be in either the alpha (α) or beta (β) configuration. This concept is crucial in understanding the classification of carbohydrates, their stereochemical representation, and the reactions they undergo.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Chair conformation: The chair conformation is a three-dimensional shape that cyclohexane (a six-carbon ring) can adopt, characterized by its stability due to minimized steric hindrance and torsional strain. It resembles a chair, with alternating carbon atoms serving as the "seat" and "legs" or "backrest" of the chair.
Chair Conformation: The chair conformation is a stable three-dimensional arrangement adopted by cyclohexane and other six-membered ring compounds. This specific configuration minimizes steric strain and allows for the most stable positioning of substituents on the ring.
Chiral Centers: Chiral centers are atoms within a molecule that have four different substituents attached, resulting in a non-superimposable mirror image. This asymmetry gives rise to the concept of chirality, which is essential in understanding optical activity, meso compounds, and the stereochemistry of various organic reactions and biomolecules.
D-: D- is a prefix used in organic chemistry to denote the stereochemistry of a carbohydrate molecule. It indicates the orientation of the hydroxyl group (-OH) on the carbon atom farthest from the carbonyl group in a Fischer projection of the carbohydrate.
Diastereomers: Diastereomers are a type of stereoisomer that have the same molecular formula and connectivity, but differ in their three-dimensional arrangement of atoms in space. They are not mirror images of each other and do not exhibit the property of chirality.
Enantiomers: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms, resulting in a unique handedness or chirality.
Epimers: Epimers are a type of stereoisomers that differ in the configuration of only one stereocenter, or chiral carbon, within a molecule. This subtle difference in the spatial arrangement of atoms can have significant implications in the context of carbohydrate chemistry and stereochemistry.
Fischer projections: Fischer projections are a two-dimensional drawing method used in organic chemistry to represent the three-dimensional spatial arrangement of atoms around chiral centers, primarily in carbohydrates. This method uses horizontal lines to represent bonds projecting forward (out of the plane) and vertical lines for bonds projecting backward (into the plane).
Fischer Projections: Fischer projections are a way of representing the three-dimensional stereochemistry of organic molecules, particularly carbohydrates, on a two-dimensional plane. They are named after the German chemist Emil Fischer, who developed this method of depicting the spatial arrangement of atoms in a molecule.
Fructose: Fructose is a monosaccharide, or the simplest form of carbohydrate, that is naturally found in fruits, honey, and some vegetables. It is one of the three dietary sugars, along with glucose and galactose, and is known for its unique properties and role in various metabolic processes.
Galactose: Galactose is a monosaccharide, or simple sugar, that is a C-4 epimer of glucose. It is an important component of lactose, the primary sugar found in mammalian milk, and is also produced in the body during the metabolism of lactose.
Glucose: Glucose is a simple sugar, or monosaccharide, that serves as the primary source of energy for the body's cells. It is a key component in various metabolic processes and plays a central role in carbohydrate chemistry and biochemistry.
Haworth Projection: The Haworth projection is a two-dimensional representation of the cyclic structure of monosaccharides, which helps visualize the stereochemistry and orientation of the sugar ring and its substituents. This projection is particularly useful in understanding the anomeric configuration and reactions of carbohydrates.
Ketone: A ketone is a functional group in organic chemistry that consists of a carbonyl group (a carbon-oxygen double bond) bonded to two alkyl or aryl groups. Ketones are widely encountered in various organic chemistry topics, including the hydration of alkynes, oxidative cleavage of alkynes, organic synthesis, oxidation and reduction reactions, and the chemistry of aldehydes and ketones.
L-: The L- prefix in organic chemistry refers to the stereochemical configuration of a compound, specifically the orientation of the hydroxyl group (-OH) on the carbon atom furthest from the carbonyl group in a Fischer projection. The L- designation indicates that the hydroxyl group is on the left side of the carbon chain when viewed in the standard Fischer projection orientation.
Monosaccharides: Monosaccharides are the most basic units of carbohydrates, serving as the building blocks for more complex carbohydrate structures. They are simple sugars that cannot be broken down into smaller sugar molecules through hydrolysis.
Optical Activity: Optical activity is the ability of certain molecules to rotate the plane of polarized light as it passes through a solution containing those molecules. This phenomenon is directly related to the concept of chirality, where molecules can exist in two non-superimposable mirror-image forms, known as enantiomers.
Pyranose: Pyranose is a cyclic structure formed by monosaccharides, where the sugar ring contains five carbon atoms and one oxygen atom. This ring structure is a key feature of carbohydrates and plays a crucial role in understanding their stereochemistry, cyclic structures, and the classification of essential monosaccharides and disaccharides.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
β Diketone: A β-diketone is an organic compound containing two ketone groups separated by a carbon atom, which is the beta (β) position relative to each ketone group. These molecules are characterized by the presence of hydrogen atoms on the carbon between the two carbonyl (C=O) groups, making them acidic and prone to enolate ion formation.