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Fischer projection was invented by Hermann Emil Fischer in 1891, which is a two-dimensional projection of a three-dimensional organic molecule. Fischer projections were first developed for the portrayal of carbohydrates and were widely utilised by chemists, especially in organic and biochemistry. Horizontal lines represent bonds that are projecting toward the viewer from the plane of the paper, whereas vertical lines represent bonds that are projecting away from the spectator. Let’s learn more about fischer projection and discuss som important questions.
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Key terms: Projection, compound, molecules, Fischer, carbohydrates, Organic chemistry, Biochemistry, Horizontal lines
What is Fischer Projection
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Fischer, a Nobel Laureate in Chemistry in 1902, devised a systematic approach for representing chiral compounds in two dimensions, known as Fischer projections or Fischer projection formulae. Fischer projections, despite being two-dimensional structures, preserve information about the stereochemistry of molecules and, despite not being a representation of how molecules might appear in solution, are still widely used by biochemists to specify the stereochemistry of amino acids, carbohydrates, nucleic acids, triterpenoids, performance enhancing drugs, and other biologically relevant molecules.
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How to Draw Fischer Projections
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When the Fischer projections are drawn on a molecule with a single chiral centre, such as a carbon atom, the tetrahedral structure is twisted such that two groups point downward and two groups point upward. Then you draw a cross, centre the chiral centre, and arrange the molecule so that the groups sloping downward, that is, behind the plane of the paper, are connected to the ends of the vertical line, and the groups pointing upward, that is, in front of the plane of the paper, are connected to the ends of the horizontal line.
The same technique is done to each asymmetric centre in compounds with more than one chiral centre. The wedges and dashes of perspective formulas, for example, may be used to turn a Fischer projection into a three-dimensional representation, where the two horizontal bonds are shown by solid wedges and the vertical bonds are shown by dashed lines.
Conventions
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- Horizontal or vertical lines are used to represent all non-terminal bonds. Carbon atoms are sometimes not visible and are represented by the centre of crossing lines in the vertical representation of the carbon chain.
- All horizontal bonds in a Fischer projection are tilted toward the observer. This criterion can be satisfied by rotating molecules having a simple tetrahedral shape in space.
- However, there is no method to position a monosaccharide with more than three carbons in space so that all horizontal linkages are tilted toward the spectator while making a Fischer projection. The horizontal bonds with C3 are normally slanted away after rotating the molecule such that both horizontal bonds with C2 are slanted toward the spectator.
Uses of Fischer Projection
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Fischer projections are often used to depict monosaccharides in biochemistry and chemical science. They can also be utilised for amino acids or other chemical compounds, while the 2006 IUPAC guidelines discourage this.
- To distinguish between L- and D-molecules, a Fischer projection can be utilised. In a Fischer projection, the penultimate carbon of D-sugars, for example, is shown with hydrogen on the left and hydroxyl on the right by definition. The hydrogen will be on the right, while the hydroxyl will be on the left, in L-sugars.
- In non-carbohydrates, Fischer projections are prohibited; as a result, drawings can be confusing when mistaken with other styles of illustration.
Things to Remember
- All hydrogen atoms should preferably be drawn explicitly, according to IUPAC regulations; in particular, the hydrogen atoms of the end group of carbohydrates should be present. Fischer projection differs from skeleton formulas in this aspect.
- After creating the connections with C2, the molecule must be rotated 180 degrees along its vertical axis before drawing the bonds with C3.
- Lewis structures, on the other hand, do not carry any information regarding three-dimensional geometry and should not be mistaken with Fischer projections.
- The carbon atoms may not be represented by the letter itself, and the carbon atom in the centre will be depicted by the junction of the two lines
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Sample Questions
Ques. What are the use of Fischer projection? (2 marks)
Ans. Fischer projections are often used to depict monosaccharides in biochemistry and chemical science. A Fischer projection can be used to distinguish L- and D- molecules. They can also be utilised for amino acids or other chemical compounds
Ques. What do you mean by Fischer projection? (2 marks)
Ans. Every stereocenter is represented as a cross by the Fischer Projection. The horizontal line symbolises bonds that extend beyond the page's plane, whereas the vertical line depicts bonds that extend inside the page's plane.
Ques. What are some instances of representations of Fischer projections? (2 marks)
Ans. Fischer projections are effective in showing monosaccharides (e.g., glucose and fructose) as well as amino acids (e.g., alanine) since they feature several stereocenters or carbons with unique linkages. The various monosaccharides are quite similar, with the exception of the position of the stereocenters.
Ques. How Do You Create a Fischer Projection? (2 marks)
Ans. The longest chain is depicted vertically in a Fischer projection. The horizontal lines represent hydrogen, hydroxyl, and amino group linkages. The four bonds to chiral carbon form a cross, with the carbon atom at the crossroads of the horizontal and vertical lines.
Ques. What distinguishes Fischer projection from Haworth projection of carbohydrates? (2 marks)
Ans. The fischer projection and the Haworth projection are both used to show carbohydrate stereochemistry. Haworth projections are a type of chemical notation that is used to describe sugar rings. In Haworth projections, the groups on the right side of a Fischer projection are identical to those below the plane of the ring. While Fischer projections are commonly used to portray sugars in their open-chain forms, Haworth projections are frequently employed to depict sugars in their cyclic forms.
Ques. What are some convections of Fischer projection? (2 marks)
Ans. The carbon chain is oriented so that the first carbon (C1) is at the top.
All horizontal bonds in a Fischer projection are pointed toward the spectator. To achieve these criteria, you may simply rotate molecules in space with a basic tetrahedral shape.
There is no method to position the molecule in space so that all horizontal bonds are tilted towards the viewer when using the Fischer projection for a monosaccharide with more than three carbons.
Ques. What are the rules to remember while making a Fischer projection? (3 marks)
Ans. Following rules should be kept in mind while making fischer projection.
- Since the "up" and "down" elements of the bonds remain constant, a Fischer projection may be rotated 180 degrees without losing its meaning.
- A Fischer projection cannot be rotated by more than 90 degrees. A rotation of this kind often alters the arrangement of the enantiomer. Simply swap the right and left horizontal bonds to discover the enantiomer of a molecule shown as a Fischer projection.
- To establish whether a molecule in Fischer projection is a meso compound, draw a horizontal line across its centre and see if it is symmetric around that line.
Ques. How Do You Tell the Difference Between D- and L-Sugars? (3 marks)
Ans. The differences between D-and L- sugars are as follow:
- Locate the aldehyde functional group at the sugar's terminal end. This carbon is counted as a single unit. If the sugar in question is a ketohexose (e.g., fructose).
- Find the ketone functional group and count 2 carbons.
- Number the remaining carbons from 2 to 6 for aldohexose and 2 to 5 for aldopentose in chronological sequence.
- Find the fifth carbon (for aldohexose) or fourth carbon (for aldopentose). This is chiral carbon, which is bound to four distinct groups.
- It is a D-sugar if the hydroxyl group on the 5th (4th) carbon is to the right of the molecule. It is L-sugar if the hydroxyl group on the 5th (4th) carbon is to the left of the molecule.
Ques. What can you accomplish with a Fischer Projection and what can't you do with one? (3 marks)
Ans. When drawing up a Fischer projection from a different angle, you can rotate the molecule by 180 degrees but not by 90 degrees.
The reason for this is because it is the same molecule, the absolute arrangement of the chirality centres must be preserved. Only if the horizontal groupings remain horizontal and the vertical groups remain vertical is this possible. When you rotate the molecule 90 degrees, the horizontal groups become vertical, indicating that they are now pointing away from you in Fischer projections. As a result, you're displaying the enantiomer if the molecule's absolute configuration changes from S to R.
Ques. How do you draw a Fischer projection's enantiomer and diastereomer? (3 marks)
Ans. Fischer projections make drawing distinct stereoisomers simple. If you're requested to draw the enantiomer of the following molecule with three chiral centres, for example, you may create an imaginary mirror plane and then draw the molecule's reflection, which is obtained by simply switching the two groups of a chiral centre. Assign the absolute configuration of the chirality centres to establish that these two are enantiomers. From R to S and S to R, they're all reversed. If you need to design a diastereomer of the molecule, only one chirality centre must be switched, or all but one must be switched. To put it another way, they should be non-mirror image stereoisomers. Fischer projections are particularly beneficial for drawing carbohydrates because they include numerous chiral centres, which take longer to draw.
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