In Chemistry, molecules exist in a three dimensional plane but to represent them on paper is a daunting task. In 1891, Emil Fischer devised a method by which three dimensional objects could be represented by two dimensional objects by projection. In this article, the definition, convention and uses of a Fischer projection.
Fischer projection is defined as the method of presenting three-dimensional organic molecules in two-dimensional structures on a two-dimensional plane such as a paper. Horizons and vertical lines are used to represent the bonds and the intersection of a horizontal and a vertical line represents the central carbon atom. Isomers of different sugars in organic chemistry are the most common representations of Fischer projection.
There are a few rules to be followed while the Fischer projection is to be used:
The carbon atoms in a chain are represented on a vertical line and the non terminal atoms are represented by horizontal and vertical lines. The carbon atoms may not be represented by the letter itself and the intersection of the two lines will depict the carbon atom at the center.
All the horizontal lines of a Fischer projection are projected towards the viewer and all the vertical lines are projected away from the viewers.
According to IUPAC rules, all the hydrogen atoms must be drawn explicitly, especially the hydrogen atoms at the end of the group in a carbohydrate.
The Fischer projection looks like a skeletal structure in this regard with the depiction of hydrogen atoms.
Uses and examples
Fischer projections are used to represent molecules and compounds in biochemistry and organic chemistry, especially structures of monosaccharides and amino acids. Since 2006, IUPAC has discouraged representation of amino acids by means of Fischer projection. Monosaccharides have numerous carbon centers with unique bonds on all four sides. Using Fischer projection, the structure of monosaccharides can be visualized easily.
Apart from showing structures of organic compounds, Fischer projection can also be used to differentiate between L-sugars and D-sugars. L-sugars are depicted by hydrogen atoms on the right and hydroxyl groups on the left, and D-sugars are presented by hydrogen atoms on the left and the hydroxyl groups on the right.
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More About Fischer Projection
We know that the objects around us are in a three dimensional or 3D state. However, depicting any 3D structure on papers permits two dimensional (2D) representations only. The transition from a 3D state to a 2D state leads to some distortions of the depicted object. Such distortions are typical with chemical molecules in the 3D form, and we have to show them in the 2D state by using some method. The Fischer projection is one of the techniques of showing three-dimensional chemical molecules on paper.
The Fischer projection is a method of representing three-dimensional structures or organic molecules in a two-dimensional setup. Emil Fischer devised the groundbreaking plan in 1891. Horizontal and vertical lines are used to describe the 3D state of the molecules, with the horizontal lines representing attachments pointing out of the papers towards us, and vertical lines representing attachments pointing out the back of the paper, away from us. The intersection of the two lines depicts the central carbon. They are a convenient way to represent chiral molecules and distinguish between pairs of enantiomers. They are most commonly used to show isomers of sugars.
Fischer projections were initially proposed for representing carbohydrates and were used by chemists, particularly in organic chemistry and biochemistry.
Conventions of Fischer Projection
There are specific Fischer projection rules that you must acknowledge before working with the system.
The non-terminal bonds of the molecule are depicted as horizontal and vertical lines. The carbon chain is defined vertically. At times, the carbon atoms may not be shown and represented by the centre of crossing lines.
The orientation of the carbon chain is such that the first carbon (C1) is at the top.
In a Fischer projection, all horizontal bonds are directed towards the viewer. You can easily rotate molecules with a simple tetrahedral geometry in space to meet this condition.
When we use the Fischer projection for a monosaccharide with more than three carbons, there is no way to orient the molecule in space so that all horizontal bonds are slanted towards the viewer.
A Fischer projection is not an accurate representation of the actual 3D configuration of a chemical molecule. It is an altered version of the molecule, ideally twisted at multiple levels along its backbone.
According to IUPAC rules, you must explicitly draw all hydrogen atoms. The hydrogen atoms of the end group of carbohydrates should be particularly present. Hence, in this regard, the Fischer structure is different from skeletal projection.
Thus, it is essential to keep in mind the above mentioned Fischer projection rules before using them practically.
Fischer Projection and Chemical Formula
A fischer projection formula is a convention used to depict a stereo formula in two-dimensional spaces without destroying the stereochemical information, the absolute configurations, at chiral centres.
To convert a stereo formula into a Fischer projection, we can apply the following procedure.
You have to hold the molecule so that the chiral centre is on the plane of the paper. The two bonds coming out of the plane of the paper should be on a horizontal plane. The two remaining bonds going into the plane of the paper are on a vertical plane.
The second step is to push the two bonds coming out of the paper’s plane onto the plane of the paper.
The third step is to pull the two bonds into the paper's plane onto the paper's plane.
Lastly, you should omit the chiral atom symbol for convenience.
Fischer Projection Examples
Fischer projections are generally used for depicting monosaccharide and amino acids. They are suitable for representing monosaccharide because they have numerous stereocenters or carbons with four unique bonds. All monosaccharides are relatively similar and have different orientations along the stereocenter. The Fischer projection enables us to visualize the direction of each monosaccharide easily.
Uses of Fischer Projection
Fischer projections are most commonly used in biochemistry and organic chemistry. You can also use them to show amino acids and other organic molecules, but this is discouraged by IUPAC recommendations since 2006.
You can also use a Fischer projection to differentiate between L-and D- molecules. For example, by definition, in a Fischer projection, the penultimate carbon of D-sugars is depicted with hydrogen on the left and hydroxyl on the right. Likewise, the L sugars will be shown with hydrogen on the right and hydroxyl on the left.
Fischer projections in non-carbohydrates are discouraged; as such, drawings are ambiguous when confused with other types of illustration.
The topic of Fischer projection may seem very tricky to grasp. However, the concept does get manageable with thorough understanding, regularly solving questions and numerical, practising papers and proper revision. Before trying to understand the whole idea, we must learn the basics of the projection and then move to its rules, Fischer projection examples, structure, usage, etc.
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