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Credits What will you learn Play/pause Restart Lets Learn! Interactivity options Sliders(IO1)‏ / Input Boxes(IO2)‏ /Drop down(IO3)‏ (if any)‏ Sequence Rules Notations Test your understanding (questionnaire)‏ Animation Area Lets Sum up (summary)‏ Want to know more… (Further Reading)‏ Output result of interactivity (if any)‏ Instructions/ Working area

Conversion of Fischer projection to R-S configuration This Learning object shows you how to convert the Fischer projection of a given molecule to its R-S configuration. Author: Rachit Agarwal Department of chemistry, IIT Bombay

Learning objectives: 1 • After interacting with this Learning Object, the learner will be able to: • convert any molecule given in Fisher projection to R-S • configuration 2 3 4 5

Definitions of the components: Stereocenter or stereogenic center: It is any point in a molecule, though not necessarily an atom, bearing groups, such that an interchanging of any two groups leads to a stereoisomer (the atoms making up the isomers are joined up in the same order but have a different spatial arrangement.) Chirality center: It is a stereocenter consisting of an atom holding a set of atoms or groups of atoms in a spatial arrangement which is not superimposable on its mirror image. A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer. Fischer projection: Fischer Projections are abbreviated structural forms that allow one to convey valuable stereochemical information to a chemist without them having to draw a 3D structural representation of a molecule. These representations are only used for molecules that contain stereogenic centers, which are then represented as simple crosses. They can be derived by considering the more accurate 3D representation using wedges and assuming the convention that horizontal lines represent bonds coming out of the plane of the paper and vertical lines represent bonds going behind the plane of the paper. 1 2 3 4 5

Sequence rules: 1 • There are certain rules that are to be followed while assignment of the R/S to Fischer projection • To assign the priority of the atoms around the chiralcenter • Look at the atoms directly attached to the chiral atom, the higher atomic no gets higher priority. In case of isotopes, higher atomic number gets higher priority. • If directly attached atoms are same then go for the next atom attached to this atom , and higher atomic no gets higher priority • In Fischer projection the atoms/bonds which are in the horizontal • line should come out of the plane while those are in vertical line • should be going inside the plane • If the lowest priority group is in the vertical plane then R remains R S remains S and If the lowest priority group in the horizontal plane then S becomes R , R becomes S. • While checking the direction of the curl around the atom, the lowest priority is not considered. 2 3 4 5

Notations: 1 • S stands for sinister which means the anti clock wise rotation of • the curl around the chiral center if the lowest priority group is • away from viewer • R stands for rectus which means the clock wise rotation of the • curl around the chiral center if the lowest priority group is away • from viewer 2 Arrow showing the sense of the rotation of the curl (anti clock wise)‏ 3 Arrow showing the sense of the rotation of the curl (clock wise)‏ R / S are simple notation and are used at each cross section. (carbon atom)‏ 4 Numerical 1,2,3,4 are used for the notation of each atom around a particular cross section (carbon atom) 5

Step 1: Molecule in a Fisher projection 1 2 3 4 5

Step 2: Naming the chiralcenters 1 2 C1 3 C2 4 5

Step 3: Assignment of C1 1 2 C1 3 C2 Magnified view 4 5

Step 4: Assignment of C1 1 2 C1 3 C2 4 5

Step 5: Assigning priority numbers 1 2 C1 3 C2 4 5

Step 5: Answer and feedback 1 2 C1 3 C2 4 5

Step 6: Removing the lowest priority group 1 2 C1 3 C2 4 5

Step 7: Giving the direction of rotation and assignment of R/S 1 2 C1 3 C2 4 5

Step 8: Assignment of C2 1 2 C1 3 C2 Magnified view 4 5

Step 9: Assignment of C2 1 2 C1 3 C2 4 5

Step 10: Assigning priority numbers 1 2 C1 3 C2 4 5

Step 11: Answer and feedback 1 2 C1 3 C2 4 5

Step 12: Removing the lowest priority group 1 2 C1 3 C2 4 5

Step 13: Giving the direction of rotation and assignment of R/S 1 2 C1 3 C2 4 5

Step 14: Combining both the chiral centres 1 2 3 4 5

Questionnaire: Assign R and S to the following molecules by typing “R” or “S” appropriately in the boxes given on the fischer projection of the molecule. a) Check

Questionnaire: Assign R and S to the following molecules by typing “R” or “S” appropriately in the boxes given on the fischer projection of the molecule. b) Check

Questionnaire: Assign R and S to the following molecules by typing “R” or “S” appropriately in the boxes given on the fischer projection of the molecule. c) Check

Summary 1) In Fischer A Fischer projection is a two-dimensional representation of the sp3-hybridized carbon(s) in a molecule that allows the configuration (i.e., the stereochemistry) of a tetrahedral carbon to be shown without the use of wedges and dashes. 2) In every Fischer projection the two horizontal bonds are understood to be projecting out of the plane toward you and the two vertical bonds are projecting into the plane away from you. 3) Assigning R- and S-Configurations to Stereocenters in Fischer Projections: a) Start out by determining the relative priorities of the four substituents attached to the stereocenter by using atomic number priorities as usual. b) Remember that to correctly assign a configuration at a stereocenter you need to orient the structure so that the lowest priority group is pointing away from you. c) The lowest priority group (# 4) is on a vertical bond; the circle connecting the groups with priorities 1 to 2 to 3 goes clockwise, so the stereocenter in this Fischer projection has an R- configuration. d) If the lowest priority group (# 4) is on a horizontal bond, then the structure as drawn has the lowest priority group oriented toward you. Therefore, your perspective on the structure is the opposite of what you need to assign the configuration directly. In a case like this, draw a circle to connect the groups with priorities 1 to 2 to 3 as before but now reverse the configuration that you get from directly reading the Fischer projection.

Links for further reading Books: 1) Ernest L. Eliel and Samuel H. Wilen, Stereochemistry of organic compounds, John Wiley & Sons (2003) 2) Jonathan Clayden, Nick Greeves, Stuart Warren, and Peter Wothers - Organic Chemistry, Oxford University Press (2001) 2) Solomons, Fryhle: Organic Chemistry, 8th Edition, Wiley-India (2009) Weblinks: http://cobalt.rocky.edu/~barbaroj/fischer-projections.pdf

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