Determine the Absolute Configuration of Fischer Projection Examples
Organic Chemistry → Stereochemistry → Isomers → Stereoisomers → Enantiomers → Stereocenters

The McLaurin Method

(aka: The Rock-On Method)

The Fastest, Easiest, Simpliest and most Realistic Method for Assigning Absolute Configuration

Determine the Absolute Configuration of the Fischer Projection Practice Problems

 

 

 

Overview of Fischer Projections:

1. Fischer Projections follow strict rules in order to determine the R and S configurations of stereocenters.


2. Fischer Projections only consists of vertical and horizontal lines (a shortcut that represents the 3-dimensional structure).


3. When a horizontal line intersects with a vertical line, the intersection of the vertical and horizontal lines, if represented with a dot, represents a stereogenic

    (chiral) carbon (that is, the carbon is a sp3 hybridized, bonded to four different elements and or groups).


4. Vertical lines represent elements going into the plane of the paper, denoted by a dashed or dotted wedge in 3-dimensional molecular representations.


5. Horizontal lines represent the elements going out of of the plane of the paper, denoted with a dark solid wedge in 3-dimensional molecular representations.


6. College chemistry students learn from the professor's lectures and the textbooks, of 'Exchange Rules' and/or 'Opposite Configurations'. The beginning

    Organic Chemistry student is told by the textbooks and professors, to work with a model set, therefore, working on their skill set to mentally visualize a

    3-dimensional molecule, in addition to, mentally rotate the molecule.

 

I personally, never used a model set when I took Organic Chemistry in the years of 1982-1983, and it was very difficult for me, to mentally visualize and
rotate the molecule. In addition, I missed 80% of the questions on assigning RS configurations to stereocenters, yet still made A's in Organic Chemistry.
The University where I took Organic Chemistry was tuff. The University graded all their classes on a seven (7) point scale. And, with Organic Chemistry,
there was no extra credit, ten (10) quizzes per semester and no dropped quizzes, three (3) exams and no dropped exams and no curve. You either made
the grade or you did not!  This method, I developed, is the best method you will ever learn, and you will remember how it works, years later!

 

7. With this method, the student does NOT have to adhere to the rules of Fischer Projections, exchanging groups or of opposite configurations. The method

    only requires that the Fischer projection be stated as a Fischer projection!

8. In order to determine the RS Configuration(s) (Absolute Configuration), all college Organic Chemistry textbooks, state that the student must use

    exchange rules and/or opposite configurations. Exchange rules and/or opposite configurations sounds confusing, and they are for a significant percentage

    of students!

9. When the student is told the molecule (chemical structure) is a Fischer Projection, student's use this method WITHOUT using exchange rules and/or

    opposite configurations.

10. Student's do not need to remember what the vertical lines and horizontal lines represent to get the correct answer every time, simply and quickly!

11. The following Fischer projection examples, demonstrate how easy and simple it is to determine RS Configuration's (Absolute Configuration) of Fischer

      Projections with 1 or more stereocenters.

 

 

 Review of the Fischer Projection molecular representation:

 

 

 

 Fischer1

 

Figure-1

 

 

 

Important Note:

Fischer Projections use vertical and horizontal lines to represent the bonds going into the plane of the paper and going out of the plane of the paper, respectively.

If the Fischer Projection was drawn as a 3-dimensional molecular representation, vertical lines would be dashed and horizontal lines would be a solid wedge.

 

 

 

Example A - Fischer Projection: (R)-1-fluoroethanol

 

 

 

Fischer2

 

 

 

Example B - Fischer Projection: (1R,2S)-1,2-difluoroethane-1,2-diol

 

 

 

Fischer3

 

 

 

Fischer5

 

 

 

Important Note:

Every stereocenter of a Fischer Projection must be treated separately,
and it's configuration has to be determined separately!
The name of the above molecule (chemical structure) is: (1R,2S)-1,2-difluoroethane-1,2-diol

 

 

 

Example C - Fischer Projection: (1R,2R)-2,3-dibromobutane

 

 

 

 Fischer6

 

 

 

Fischer7

 

 

 

Fischer8     Fischer9     

 

 

 

Fischer10

 

 

 Therefore, the name of the Fischer Projection:

(1R,2R)-2,3-dibromobutane

The first (1st) stereocenter (Carbon 2 - C2) has an R configuration
The second (2nd) stereocenter (Carbon 3 - C3) has an R configuration

 

 

 

Example D - Fischer Projection: (2R,3R)-3-bromo-2-fluorobutan-2-ol

 

 

 

Fischer11

 

 

 

Fischer12          Fischer13

 

 

 

Fischer14

 

 

Therefore, the name of the Fischer Projection:

(2R,3R)-3-bromo-2-fluorobutan-2-ol

The first (1st) stereocenter (carbon 2 - C2) has an R configuration
The second (2nd) stereocenter (carbon 3 - C3) has an R configuration

 

 

 

Example E - Fischer Projection: (2R,4S)-4-bromo-2-ethyl-2-hydroxy-4-methyl-5-oxopentanoic acid

 

 

 

 Fischer15

 

 

 

Fischer16                     Fischer17          

 

 

 

Fischer18

 

 

 

Therefore, the name of the Fischer Projection is:

(2R,4S)-4-bromo-2-ethyl-2-hydroxy-4-methyl-5-oxopentanoic acid

The first (1st) stereocenter (carbon 2 - C2) has an R configuration and
The second (2nd) stereocenter (carbon 4 - C4) has an S configuration

 

 

 

Example F - Fischer Projection: (2S,3R,4S)-3-amino-4-bromo-4-chloro-3-fluoro-2-hydroxypentanoic acid

 

 

 

 Fischer19

 

 

 

Fischer20                    Fischer21

 

 

 

Fischer22

 

 

Therefore, the name of the Fischer Projection:

(2S,3R,4S)-3-amino-4-bromo-4-chloro-3-fluoro-2-hydroxypentanoic acid

The first (1st) stereocenter (carbon 2 - C2) has an S configuration
The second (2nd) stereocenter (carbon 3 - C3) has an R configuration
The third (3rd) stereocenter (carbon 4 - C4) has an S configuration

 

 

 

Advantages of the using the McLaurin Method for all Fischer Projections:

1. The method does not require 'Exchange Rules'.


2. The method does not require 'Opposite Configuration'.


3. The student does not have to remember whether the vertical or the horizontal lines, mean out of or into the plane of the paper!


4. The method is the fastest, easiest and simplest method you will ever learn, concerning the naming of stereocenters (Absolute Configuration/RS

    Configuration of Chiral Centers/Stereoisomers).


5. The method determines the 'Absolute Configuration' (RS Configuration) of one (1) or more stereocenters, whether it be a Fischer Projection or any

    other molecular representation, that indicates some sort of 3-dimensional direction of an element on the stereogenic carbon.


6. This method mimic's the tetrahedral shape of the stereocenter better than any published method. Therefore, helping the beginning student with their

    skill of mentally visualizing the 3-dimensional chemical structure (stereoperception), and rotating it to the desired position.

 

Acknowledgments


I would like to acknowledge and thank the following for their time and effort: Denise for her help with her editing skills, Dr. and Mrs. Greever (Organic Chemists)

for making Organic Chemistry interesting and learnable when I was an undergraduate Organic Chemistry student in 1982-1983, the girls of Physical Therapy

(Zofia (Chairman of the Physical Therapy Department) Julie, Debbie, Sharon and Gloria) for making me realize my academic success and excellence is the

number one priority, aside from my nearly four (4) consecutive years of work with patients (not customers or clients) in the Physical Therapy Department

(while going to college full-time during the fall, spring and summer semesters, of 1980-1984). In addition, I would like to thank very much, the Chairman (Dean)

of the University of Mississippi Medical School, for providing me the inspiration and wisdom, at the end of every semester, to continue my undergraduate

studies to be a future physician, during the years of 1981-1984, in addition to the time Dr. Corbett (Dean of the Basic Sciences - Ross Medical School -

1994-1996) provided me, with my unannounced 20-30 minute visits to his office in the administration building, about every week or two, during my two years

of basic sciences in medical school. Dr. Corbett, for two years, never said he was too busy to talk with me. Dean Corbett and his wife, even asked me over

to their house, for supper many times! And last but least, a special thanks to Avril Lavigne for being instrumental and thus providing me with the key to the

development of this method and my parents for which none of this would have ever happened if it was not for their unyielding positive support,

that no other could equal.

 

Why was the Method developed?

When I took Organic Chemistry in 1982-83, I missed 80-85% of the questions related to determining the configuration(s) of enantiomers (that is,

RS configurations).  Of which, I still have my Organic Chemistry quizzes and exams from 1982-83. Twenty-one (21) years later, I wrote a review book on

Organic Chemistry and when I got to Stereochemistry, I remembered the trouble I had with determining/assigning RS configurations. As a result, I decided

to try to develop a method that would make it as easy as possible for students to determine/assign the correct configuration.

Most of my goals in the endeavor are as follows:

 

1. I decided that the following list contained most of the appropriate expectations of students.

     a. The method has to be as easy, fast and simple, as any method that has ever been published, if not better than all methods that have been published.

     b. The method should be used in the classroom during quizzes and exams, even on the medical college admission test (MCAT).

     c. The method should be as to scale as a 'Real' molecule (as similar to the model set as possible).

     d. The method should demonstrate the four (4) elements bonded to the chiral carbon.

     e. The method should help students develop their 'stereoperception' (the ability to visualize the 3-dimensional spatial arrangement of the molecule at and around

         the stereocenter and be able to mentally rotate the molecule), thus the use of the 'Pinky' finger, Index finger, Thumb and Arm, with the hand acting as the

         chiral carbon.

     f. The method should work with all molecular representations that show some 3-dimensional direction, for example, a dash or wedge, as well as, work with

        Fischer Projections.

     g. The method should only require the student to use one (1) hand for all the stereocenters of each chiral molecule, even if the molecule has two or more

         stereocenters.

     h. The method has to determine the correct configuration 100% of the time.

     i. The method should be easy to learn, use and remember.

     j. The method should be FREE for all students, faculty and others.

 

2. In addition, the method should not do the following.

     a. The method should not be a 'Trick of the Trade', for example, exchanging elements (groups). For this to happen in real life, bonds are broken and made,

         involving energy. In real life, this exchanging of elements (groups) does not occur. In addition, exchanging of elements does not help the student develop

         their 'Stereoperception', mental visualization of 3-dimensional spatial arrangement of the molecule(s) (chiral molecules).

     b. The method should not require two (2) hands to determine/assign an RS configuration, or multiple configurations on the same molecule (increases

         confusion).

     c. The method should not require the student to be a contortionist, in order to use the method.

     d. The method should not provide the opposite answer (aka: opposite configuration)!

     e. The method should not have been published in the Journal(s) by anyone, ever before.

 

*Historical Origin of Stereochemistry (1):

It was . . . Louis Pasteur's separation of a racemic form of tartaric acid in 1848 that led to the discovery of the phenomenon called enantiomerism. Pasteur, consequently, is considered to be 'The Founder of the field of Stereochemistry'. Pasteur's discovery of enantiomerism and his demonstration that the optical activity of the two forms of tartaric acid was a property of the molecules themselves led, in 1874, to the proposal of the tetrahedral structure of carbon by van't Hoff and Le Bel.

In 1877, Hermann Kolbe (of the University of Leipzig), one of the most eminent organic chemists of the time, wrote the following:

Not long ago, I expressed the view that the lack of general education and of thorough training in chemistry was one of the causes of the deterioration of chemical research in Germany . . . Will anyone to whom my worries seem exaggerated please read, if he can, a recent memoir by a Herr van't Hoff on "The Arrangements of Atoms in Space," a document crammed to the hilt with the outpourings of a childish fantasy . . . This Dr. J. H. van't Hoff, employed by the Veterinary College of Utrecth, has, so it seems, no
taste for accurate chemical research. He finds it more convenient to mount his Pegasus (evidently taken from the stables of the Veterinary College) and to announce how, on his bold flight to Mount Parnassus, he saw the atoms arranged in space.

Kolbe, nearing the end of his career, was reacting to a publication of a 22-year-old Dutch scientist. This publication had appeared earlier, in September 1874, and in it, van't Hoff had argued that the spatial arrangement of four groups around a central carbon atom is tetrahedral. A young French scientist, J. A. Le Bel, independently put forth the same idea in a publication in November 1874. Within 10 years after Kolbe's comments however, abundant evidence had accumulated that substantiated the "childish fantasy" of van't Hoff. Later in his career (in 1901), and (also) for other work, van't Hoff was named the first recipient of the Nobel Prize in Chemistry.

Together, the publications of van't Hoff and Le Bel marked an important turn in a field of study that is concerned with the structures of molecules in three dimensions: Stereochemistry

Literature

1. Solomons, T. W. G.; Fryhle, C. B. Organic Chemistry, 9th ed.; Wiley: New York, 2008; pp. 188-189, 214.

 

 

Future Chemistry and Organic Chemistry Websites

StereoChemistry.net
StereoIsomer.com
StereoIsomers.net

ChemistryTerminology.com

 

 

NOTE: This method is copyrighted with the Library of Congress located in an Organic Chemistry Review Book, I wrote years ago.

 

Written permission needs to be obtained from
Tim McLaurin for reproduction and duplication.

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