By students of Sir Dr. M.S. GOSAVI COLLEGE OF PHARMACEUTICAL EDUCATION AND RESEARCH, NASHIK.

1. Optical Isomerism
Pharmaceutical organic chemistry II
continuous assessment 1
group no 7: 49-56

2. Roll no. Names Topic
49 Yash Kokate Sequence Rule
50 Aditya Kshirsagar R and S
configuration
51 Chaitanya Kulkarni Polarimeter
52 Gauri Kulkarni Optical Activity
53 Asmita kuwar Enantiomers
54 Rupali Lambole Meso compound
55 Sarthak Lokhande Symmetry of
Element
56 Akanksha Lunge Diasteroisomerism

3. Symmetry of Element
A SYMMETRY OPERATION IS AN ACTION THAT LEAVES AN OBJECT LOOKING THE SAME
AFTER IT HAS BEEN CARRIED OUT. FOR EXAMPLE, IF WE TAKE A MOLECULE OF WATER
AND ROTATE IT BY 180° ABOUT AN AXIS PASSING THROUGH THE CENTRAL O ATOM
(BETWEEN THE TWO H ATOMS) IT WILL LOOK THE SAME AS BEFORE. EACH SYMMETRY
OPERATION HAS A CORRESPONDING SYMMETRY ELEMENT, WHICH IS THE AXIS, PLANE,
LINE OR POINT WITH RESPECT TO WHICH THE SYMMETRY OPERATION IS CARRIED OUT.
THE SYMMETRY ELEMENT CONSISTS OF ALL THE POINTS THAT STAY IN THE SAME PLACE
WHEN THE SYMMETRY OPERATION IS PERFORMED. IN A ROTATION, THE LINE OF POINTS
THAT STAY IN THE SAME PLACE CONSTITUTE A SYMMETRY AXIS; IN A REFLECTION THE
POINTS THAT REMAIN UNCHANGED MAKE UP A PLANE OF SYMMETRY.

4. THERE ARE FIVE TYPE OF SYMMETRY ELEMENT:-
THE IDENTITY SYMMETRY:- THE IDENTITY OPERATION CONSISTS OF DOING NOTHING,
AND THE CORRESPONDING SYMMETRY ELEMENT IS THE ENTIRE MOLECULE. EVERY
MOLECULE HAS AT LEAST THIS ELEMENT. FOR EXAMPLE, THE CHFCLBR MOLECULE.
N-FOLD AXIS OF ROTATION:- ROTATION BY 360°/N LEAVES THE MOLECULE
UNCHANGED. THE H2O MOLECULE HAS A C2 AXIS. SOME MOLECULES HAVE MORE THAN ONE
CN AXIS, IN WHICH CASE THE ONE WITH THE HIGHEST VALUE OF N IS CALLED THE PRINCIPAL
AXIS.

5. PLANE OF SYMMETRY:- REFLECTION IN THE PLANE LEAVES THE MOLECULE LOOKING
THE SAME. IN A MOLECULE THAT ALSO HAS AN AXIS OF SYMMETRY, A MIRROR PLANE THAT
INCLUDES THE AXIS IS CALLED A VERTICAL MIRROR PLANE AND IS LABELED ΣV, WHILE ONE
PERPENDICULAR TO THE AXIS IS CALLED A HORIZONTAL MIRROR PLANE AND IS LABELED ΣH.
A VERTICAL MIRROR PLANE THAT BISECTS THE ANGLE BETWEEN TWO C2 AXES IS CALLED A
DIHEDRAL MIRROR PLANE, ΣD. Σ SYMMETRY IS INDICATED AS A PLANE ON MOLECULES;
SINCE THEY OFTEN BISECT ATOMS, WHICH SHOULD BE CLEARLY INDICATED.
CENTER OF INVERSION SYMMETRY:- INVERSION THROUGH THE CENTER OF
SYMMETRY LEAVES THE MOLECULE UNCHANGED. INVERSION CONSISTS OF PASSING EACH
POINT THROUGH THE CENTER OF INVERSION AND OUT TO THE SAME DISTANCE ON THE
OTHER SIDE OF THE MOLECULE.

6. N-FOLD AXIS OF IMPROPER ROTATION SYMMETRY:-
IMPROPER ROTATIONS ARE ALSO CALLED A ROTARY-REFLECTION AXIS. THE ROTARY
REFLECTION OPERATION CONSISTS OF ROTATING THROUGH AN ANGLE 360°/N ABOUT THE
AXIS, FOLLOWED BY REFLECTING IN A PLANE PERPENDICULAR TO THE AXIS.

7. CHIRAL MOLECULE:-
A CHIRAL MOLECULE HAS A MIRROR IMAGE THAT CANNOT LINE UP WITH IT PERFECTLY- THE
MIRROR IMAGES ARE NON SUPERIMPOSABLE. THE MIRROR IMAGES ARE CALLED
ENANTIOMERS
PROPERTIES OF CHIRAL:-
CHIRAL COMPOUND IS A COMPOUND THAT IS OPTICALLY ACTIVE AND CAN ROTATE LIGHT
ALTHOUGH EVERYTHING HAS A MIRROR IMAGE, MIRROR IMAGE MAY OR MAY NOT BE
SUPERIMPOSABLE
CHIRAL COMPOUNDS ARE NON-SUPERIMPOSABLE MIRROR IMAGE LIKE OUR HANDS.
EXAMPLE:-

8. ACHIRAL MOLECULE:-
A MOLECULE OR OBJECT THAT IS SUPERIMPOSABLE ON ITS MIRROR IMAGE IS SAID TO BE
ACHIRAL. IN AACHIRAL MOLECULE – CARBON ATOM CONTAIN AT LEAST TWO SAME
MOLECULE ON THEIR HAND.

9. Optical Activity
A BRIEF HISTORY
 OPTICAL ACTIVITY WAS FIRST OBSERVED BY THE FRENCH PHYSICIST JEAN-BAPTISTE BIOT.
 HE CONCLUDED THAT THE CHANGE IN DIRECTION OF PLANE-POLARIZED LIGHT WHEN IT
PASSED THROUGH CERTAIN SUBSTANCES WAS ACTUALLY A ROTATION OF LIGHT, AND THAT
IT HAD A MOLECULAR BASIS.
 HIS WORK WAS SUPPORTED BY THE EXPERIMENTATION OF LOUIS PASTEUR.
 PASTEUR OBSERVED THE EXISTENCE OF TWO CRYSTALS THAT WERE MIRROR IMAGES IN
TARTARIC ACID, AN ACID FOUND IN WINE.
 THROUGH METICULOUS EXPERIMENTATION, HE FOUND THAT ONE SET OF MOLECULES
ROTATED POLARIZED LIGHT CLOCKWISE WHILE THE OTHER ROTATED LIGHT COUNTER
CLOCKWISE TO THE SAME EXTENT.
 HE ALSO OBSERVED THAT A MIXTURE OF BOTH, A RACEMIC MIXTURE (OR RACEMIC
MODIFICATION), DID NOT ROTATE LIGHT BECAUSE THE OPTICAL ACTIVITY OF ONE
MOLECULE CANCELLED THE EFFECTS OF THE OTHER MOLECULE. PASTEUR WAS THE FIRST
TO SHOW THE EXISTENCE OF CHIRAL MOLECULES.

10. INTRODUCTION
 LIGHT WAVES ARE OSCILLATING ELECTRIC AND MAGNETIC FIELDS
 IN ORDINARY LIGHT, THE OSCILLATION IS RANDOMLY ORIENTED IN AN
INFINITE NUMBER OF PLANES. WHEN ORDINARY LIGHT IS PASSED
THROUGH A POLARIZER, ALL PLANES OF OSCILLATION ARE FILTERED OUT
EXCEPT ONE, RESULTING IN PLANE-POLARIZED LIGHT.

11.  A BEAM OF PLANE-POLARIZED LIGHT, WHEN PASSED THROUGH A SAMPLE
OF A CHIRAL COMPOUND, INTERACTS WITH THE COMPOUND IN SUCH A WAY
THAT THE ANGLE OF OSCILLATION WILL ROTATE. THIS PROPERTY IS
CALLED OPTICALACTIVITY.
 THE MAGNITUDE OF THE OBSERVED OPTICAL ACTIVITY IS DEPENDENT ON
TEMPERATURE, THE WAVELENGTH OF LIGHT USED, SOLVENT,
CONCENTRATION OF THE CHIRAL SAMPLE, AND THE PATH LENGTH OF THE
SAMPLE TUBE (PATH LENGTH IS THE LENGTH THAT THE PLANE-POLARIZED
LIGHT TRAVELS THROUGH THE CHIRAL SAMPLE).

12. ROTATION OF LIGHT
 AN ENANTIOMER THAT ROTATES PLANE-POLARIZED LIGHT IN THE POSITIVE
DIRECTION, OR CLOCKWISE, IS CALLED DEXTROROTARY [(+), OR D-], WHILE
 THE ENANTIOMER THAT ROTATES THE LIGHT IN THE NEGATIVE DIRECTION,
OR COUNTER CLOCKWISE, IS CALLED LEVOROTARY [(-), OR L-].
 WHEN BOTH D- AND L- ISOMERS ARE PRESENT IN EQUAL AMOUNTS, THE
MIXTURE IS CALLED A RACEMIC MIXTURE.

13. SPECIFIC ROTATION
 THE STANDARD MEASUREMENT FOR ROTATION FOR A SPECIFIC CHEMICAL
COMPOUND IS CALLED THE SPECIFIC ROTATION,
 IT IS DEFINED AS AN ANGLE MEASURED AT A PATH LENGTH OF 1 DECIMETRE
AND A CONCENTRATION OF 1G/ML.

14. DIFFERENT ENANTIOMERS OF A COMPOUND WILL ALWAYS ROTATE PLANE-
POLARIZED LIGHT WITH AN EQUAL BUT OPPOSITE MAGNITUDE. (S)-
IBUPROFEN, FOR EXAMPLE, HAS A SPECIFIC ROTATION OF
+54.5O (DEXTROROTATORY) IN METHANOL, WHILE (R)-IBUPROFEN HAS A
SPECIFIC ROTATION OF -54.5O.
THERE IS NO RELATIONSHIP BETWEEN CHIRAL COMPOUND'S R/S DESIGNATION
AND THE DIRECTION OF ITS SPECIFIC ROTATION. FOR EXAMPLE,
THE S ENANTIOMER OF IBUPROFEN IS DEXTROROTATORY, BUT
THE S ENANTIOMER OF GLYCERALDEHYDE IS LEVOROTATORY.

15. Polarimeter
 POLARIMETRY IS A SENSITIVE, NONDESTRUCTIVE TECHNIQUE FOR
MEASURING THE OPTICAL ACTIVITY EXHIBITED BY INORGANIC
AND ORGANIC COMPOUNDS. A COMPOUND IS CONSIDERED TO BE
OPTICALLY ACTIVE IF LINEARLY POLARIZED LIGHT IS ROTATED
WHEN PASSING THROUGH IT. THE AMOUNT OF OPTICAL ROTATION
IS DETERMINED BY THE MOLECULAR STRUCTURE AND
CONCENTRATION OF CHIRAL MOLECULES IN THE SUBSTANCE.
EACH OPTICALLY ACTIVE SUBSTANCE HAS ITS OWN SPECIFIC
ROTATION AS DEFINED IN BIOTS LAW

16. APPLICATION OF POLARIMETER
 PHARMACEUTICAL INDUSTRY
POLARIMETRY DETERMINES PRODUCT PURITY BY MEASURING SPECIFIC
ROTATION AND OPTICAL ROTATION OF:
AMINO ACIDS.
ANTIBIOTICS.
DEXTROSE. STEROIDS.
 POLARIMETRY FOR FLAVOR, FRAGRANCE, AND ESSENTIAL OIL INDUSTRY
UTILIZES POLARIMETRY FOR INCOMING RAW MATERIALS INSPECTION OF
CAMPHORS.
GUMS.
ORANGE OIL.
CITRIC ACID.

17. FUNCTION OF POLARIMETER
 OPTICALLY ACTIVE SUBSTANCES CHANGE THE DIRECTION OF THE LIGHT
WAVE. OPTICALLY ACTIVE ARE CHIRAL SUBSTANCES WHICH HAVE
MOLECULAR SYSTEMS WHOSE ASYMMETRY RESULTS ARE IN HANDEDNESS.
THESE CHIRAL SUBSTANCES HAVE A PAIR OF NONSUPERIMPOSABLE
MIRROR-IMAGE SHAPES. EXAMPLES OF OPTICALLY ACTIVE MEDIAARE
SUGAR, LACTIC ACID
 POLARIMETERS ARE USED TO DETERMINE THE RELATIONSHIP BETWEEN
THE POLARIZATION STATES OF INCIDENT BEAMS AND OUTPUT BEAM.
POLARIMETERS ARE COMPOSED OF A POLARIZATION GENERATOR
CONSISTING OF ALL ELEMENTS NEEDED TO PRODUCE A BEAM OF A KNOWN
POLARIZATION STATE, AND A POLARIZATION ANALYZER TO PERFORM A
PARTICULAR POLARIZATION COMPONENT IN THE OUTPUT LIGHT BEAM.

18. If a tube with no optically active substance is placed
between two parallel polarisation filters, the oscillation
plane is retained. Light will be visible on the screen at
the end. If the tube is filled with an optically active
substance, e.g. B. a solution of granulated sugar a
change becomes visible
the operation of an optically active substance has the
following effect: the light at the back of the screen
becomes much less visible. This happens because the
oscillation plane of the light has been changed. The
reason: sugar solution, as an optically active substance,
has rotated the polarisation direction. The angle of
rotation depends on the length of the path in the
solution and on the concentration of the solution itself
and it also depends on the type of sugar.

19. If the tube is filled with granulated sugar (sucrose), the
angle of rotation is dextrorotatory (clockwise). (+)
If the tube is filled with pure fructose, the angle of
rotation is levorotatory (counter-clockwise).(-)

20. THE POLARIMETER IS MADE UP OF TWO NICOL PRISMS (THE POLARIZER AND
ANALYZER). THE POLARIZER IS FIXED AND THE ANALYZER CAN BE ROTATED.
THE PRISMS MAY BE THOUGHT OF AS SLITS S1 AND S2. THE LIGHT WAVES MAY
BE CONSIDERED TO CORRESPOND TO WAVES IN THE STRING THE POLARIZER
S1 ALLOWS ONLY THOSE LIGHT WAVES WHICH MOVE IN A SINGLE PLANE. THIS
CAUSES THE LIGHT TO BECOME PLANE POLARIZED. WHEN THE ANALYZER IS
ALSO PLACED IN A SIMILAR POSITION IT ALLOWS THE LIGHT WAVES COMING
FROM THE POLARIZER TO PASS THROUGH IT. WHEN IT IS ROTATED THROUGH
THE RIGHT ANGLE NO WAVES CAN PASS THROUGH THE RIGHT ANGLE AND THE
FIELD APPEARS TO BE DARK. IF NOW A GLASS TUBE CONTAINING AN
OPTICALLY ACTIVE SOLUTION IS PLACED BETWEEN THE POLARIZER AND
ANALYZER THE LIGHT NOW ROTATES THROUGH THE PLANE OF POLARIZATION
THROUGH A CERTAIN ANGLE, THE ANALYZER WILL HAVE TO BE ROTATED IN
SAME ANGLE.

21. Enantiomers
A STEREOISOMERS WHICH ARE NONSUPERIMPOSABLE MIRROR
IMAGES OF EACH OTHER ARE CALLED ENANTIOMERS.
EXAMPLE:

22. PROPERTIES OF ENANTIOMERS :-
 ENANTIOMERS HAVE CHIRAL CENTRES
 THESE TWO FORMS NON-IDENTICAL MIRROR IMAGES ❖THEY HAVE
PHYSICAL PROPERTIES ( SUCH AS DENSITY, BOILING POINT, MELTING POINT,
REFRACTIVE INDEX , ETC.) AND CHEMICAL PROPERTIES.
 THEY DIFFER ONLY ON DIRECTION OF ROTATION ON PLANE POLARIZED
(PPL), GIVE RISE TO D AND L COMPOUNDS.
 ENANTIOMERS EXHIBITS OPTICAL ACTIVITY AND ARE ALSO CALLED OPTICAL
ISOMERS.

23. Diasteroisomerism
 DIASTEREOMERS ARE DEFINED AS COMPOUNDS WHICH HAVE THE
SAME MOLECULAR FORMULA AND SEQUENCE OF BONDED ELEMENTS
BUT WHICH ARE NONSUPERIMPOSABLE, NON-MIRROR IMAGES.
 HENCE, THEY OCCUR WHEN TWO OR MORE STEREOISOMERS OF A
COMPOUND HAVE DIFFERENT CONFIGURATIONS AT ONE OR MORE (BUT
NOT ALL) OF THE EQUIVALENT (RELATED) STEREOCENTERS AND ARE
NOT MIRROR IMAGES OF EACH OTHER.
 WHEN TWO DIASTEREOISOMERS DIFFER FROM EACH OTHER AT ONLY
ONE STEREOCENTER THEY ARE EPIMERS.
 EACH STEREOCENTER GIVES RISE TO TWO DIFFERENT
CONFIGURATIONS AND THUS TYPICALLY INCREASES THE NUMBER OF
STEREOISOMERS BY A FACTOR OF TWO.

24. DIASTEREOMERS DIFFER FROM ENANTIOMERS IN THAT THE LATTER ARE
PAIRS OF STEREOISOMERS THAT DIFFER IN ALL STEREOCENTERS AND ARE
THEREFORE MIRROR IMAGES OF ONE ANOTHER.

25. CHARACTERISTICS OF DIASTEREOMERS
 DIASTEREOMERS HAVE DIFFERENT PHYSICAL PROPERTIES SUCH AS MELTING
POINTS, BOILING POINTS, DENSITIES, SOLUBILITIES, REFRACTIVE INDICES,
DIELECTRIC CONSTANTS AND SPECIFIC ROTATIONS. ENANTIOMERS HAVE
SIMILAR PHYSICAL PROPERTIES EXCEPT THE OPPOSITE SIGN OF SPECIFIC
ROTATION.
 DIASTEREOMERS OTHER THAN GEOMETRICAL ISOMERS MAY OR MAY NOT BE
OPTICALLYACTIVE.
 DIASTEREOMERS SHOW SIMILAR, BUT NOT IDENTICAL CHEMICAL PROPERTIES.
THE RATES OF REACTIONS OF THE TWO DIASTEREOMERS WITH A GIVEN
REAGENT PROVIDED BY THE REAGENT IS NOT RAPIDLYACTIVE.
 ON ACCOUNT OF DIFFERENCES IN THEIR PHYSICAL PROPERTIES,
DIASTEREOMERS CAN BE SEPARATED FROM ONE ANOTHER THROUGH
TECHNIQUES LIKE FRACTIONAL CRYSTALLIZATION, FRACTIONAL DISTILLATION,
CHROMATOGRAPHY ETC. THE DIFFERENCE FROM ENANTIOMERS WHICH CAN’T
BE SEPARATED BY THESE TECHNIQUES

26. ERYTHRO AND THREO DIASTEREOMERS
WE KNOW STEREOISOMERS THAT ARE NOT MIRROR IMAGES ARE CALLED
DIASTEREOMERS. THEYARE EITHER GEOMETRICAL ISOMERS OR COMPOUNDS
CONTAINING TWO OR MORE CHIRAL CENTRES. A DIASTEREOMER IS CALLED
ERYTHRO IF ITS FISCHER PROJECTION SHOWS SIMILAR GROUPS ON THE SAME SIDE
OF THE MOLECULE. IT IS CALLED THREO IF SIMILAR GROUPS ARE ON THE
OPPOSITE SIDES OF THE FISCHERS PROJECTION.

27. Meso compounds
 A MESO COMPOUND OR MESO ISOMER IS A NON-OPTICALLY ACTIVE
MEMBER OF A SET OF STEREOISOMERS, AT LEAST TWO OF WHICH
ARE OPTICALLY ACTIVE.
 THIS MEANS THAT DESPITE CONTAINING TWO OR MORE
STEREOGENIC CENTERS, THE MOLECULE IS NOT CHIRAL. A MESO
COMPOUND IS "SUPERPOSABLE" ON ITS MIRROR IMAGE (NOT TO BE
CONFUSED WITH SUPERIMPOSABLE, AS ANY TWO OBJECTS CAN BE
SUPERIMPOSED OVER ONE ANOTHER REGARDLESS OF WHETHER
THEY ARE THE SAME).
 TWO OBJECTS CAN BE SUPERPOSED IF ALL ASPECTS OF THE
OBJECTS COINCIDE AND IT DOES NOT PRODUCE A "(+)" OR "(-)"
READING WHEN ANALYZED WITH A POLARIMETER.

28. FOR EXAMPLE,: TARTARIC ACID CAN EXIST AS ANY OF THREE
STEREOISOMERS DEPICTED BELOW IN A FISCHER PROJECTION. OF THE FOUR
COLORED PICTURES AT THE TOP OF THE DIAGRAM, THE FIRST TWO REPRESENT
THE MESO COMPOUND (THE 2R,3S AND 2S,3R ISOMERS ARE EQUIVALENT),
FOLLOWED BY THE OPTICALLY ACTIVE PAIR OF LEVOTARTARIC ACID (L-(R,R)-
(+)-TARTARIC ACID) AND DEXTROTARTARIC ACID (D-(S,S)-(-)-TARTARIC ACID).
THE MESO COMPOUND IS BISECTED BY AN INTERNAL PLANE OF SYMMETRY
THAT IS NOT PRESENT FOR THE NON-MESO ISOMERS (INDICATED BY AN X).
THAT IS, ON REFLECTING THE MESO COMPOUND THROUGH A MIRROR PLANE
PERPENDICULAR TO THE SCREEN, THE SAME STEREOCHEMISTRY IS
OBTAINED; THIS IS NOT THE CASE FOR THE NON-MESO TARTARIC ACID, WHICH
GENERATES THE OTHER ENANTIOMER. THE MESO COMPOUND MUST NOT BE
CONFUSED WITH A 50:50 RACEMIC MIXTURE OF THE TWO OPTICALLY-ACTIVE
COMPOUNDS, ALTHOUGH NEITHER WILL ROTATE LIGHT IN A POLARIMETER.

30. IT IS A REQUIREMENT FOR TWO OF THE STEREOCENTERS IN A MESO
COMPOUND TO HAVE AT LEAST TWO SUBSTITUENTS IN COMMON (ALTHOUGH
HAVING THIS CHARACTERISTIC DOES NOT NECESSARILY MEAN THAT THE
COMPOUND IS MESO). FOR EXAMPLE, IN 2,4-PENTANEDIOL, BOTH THE SECOND
AND FOURTH CARBON ATOMS, WHICH ARE STEREOCENTERS, HAVE ALL FOUR
SUBSTITUENTS IN COMMON.
SINCE A MESO ISOMER HAS A SUPERPOSABLE MIRROR IMAGE, A COMPOUND
WITH A TOTAL OF N CHIRAL CENTERS CANNOT ATTAIN THE THEORETICAL
MAXIMUM OF 2N STEREOISOMERS IF ONE OF THE STEREOISOMERS IS MESO.

31. BEFORE APPLYING THE R AND S NOMENCLATURE TO A STEREOCENTER, THE
SUBSTITUENTS MUST BE PRIORITIZED ACCORDING TO THE FOLLOWING
RULES:
RULE 1:
FIRST, EXAMINE AT THE ATOMS DIRECTLY ATTACHED TO THE STEREOCENTER
OF THE COMPOUND. A SUBSTITUENT WITH A HIGHER ATOMIC NUMBER TAKES
PRECEDENCE OVER A SUBSTITUENT WITH A LOWER ATOMIC NUMBER.
HYDROGEN IS THE LOWEST POSSIBLE PRIORITY SUBSTITUENT, BECAUSE IT
HAS THE LOWEST ATOMIC NUMBER.
 WHEN DEALING WITH ISOTOPES, THE ATOM WITH THE HIGHER ATOMIC
MASS RECEIVES HIGHER PRIORITY.
 WHEN VISUALIZING THE MOLECULE, THE LOWEST PRIORITY SUBSTITUENT
SHOULD ALWAYS POINT AWAY FROM THE VIEWER (A DASHED LINE
INDICATES THIS).
Sequence Rule

32. RULE 2:
IF THERE ARE TWO SUBSTITUENTS WITH EQUAL RANK, PROCEED ALONG THE
TWO SUBSTITUENT CHAINS UNTIL THERE IS A POINT OF DIFFERENCE. FIRST,
DETERMINE WHICH OF THE CHAINS HAS THE FIRST CONNECTION TO AN ATOM
WITH THE HIGHEST PRIORITY (THE HIGHEST ATOMIC NUMBER). THAT CHAIN
HAS THE HIGHER PRIORITY.
RULE 3:
IF A CHAIN IS CONNECTED TO THE SAME KIND OF ATOM TWICE OR THREE
TIMES, CHECK TO SEE IF THE ATOM IT IS CONNECTED TO HAS A GREATER
ATOMIC NUMBER THAN ANY OF THE ATOMS THAT THE COMPETING CHAIN IS
CONNECTED TO.
IF NONE OF THE ATOMS CONNECTED TO THE COMPETING CHAIN(S) AT THE
SAME POINT HAS A GREATER ATOMIC NUMBER: THE CHAIN BONDED TO THE
SAME ATOM MULTIPLE TIMES HAS THE GREATER PRIORITY
IF HOWEVER, ONE OF THE ATOMS CONNECTED TO THE COMPETING CHAIN HAS
A HIGHER ATOMIC NUMBER: THAT CHAIN HAS THE HIGHER PRIORITY.

33. EXAMPLE:
A 1-METHYLETHYL SUBSTITUENT TAKES PRECEDENCE OVER AN ETHYL
SUBSTITUENT. CONNECTED TO THE FIRST CARBON ATOM, ETHYL ONLY HAS
ONE OTHER CARBON, WHEREAS THE 1-METHYLETHYL HAS TWO CARBON
ATOMS ATTACHED TO THE FIRST; THIS IS THE FIRST POINT OF DIFFERENCE.
THEREFORE, 1-METHYLETHYL RANKS HIGHER IN PRIORITY THAN ETHYL, AS
SHOWN BELOW:
AFTER ALL YOUR SUBSTITUENTS HAVE BEEN PRIORITIZED IN THE CORRECT
MANNER, YOU CAN NOW NAME/LABEL THE MOLECULE R OR S.

34. R & S Configuration
R,S CONFIGURATION SYSTEM INDICATES THE ARRANGEMENTS OF ATOMS
AROUND THE CHIRALITY CENTRE. THE CAHN-INGOLD-PRELOG
SYSTEM(CIP SYSTEM) IS A SET OF RULES THAT ALLOWS US TO DEFINE THE
STEREOCHEMICAL CONFIGURATION OF ANY STEREOCENTRE, USING THE
DESIGNATION ‘R’ (FROM THE LATIN RECTUS MEANING RIGHT-HANDED) OR
‘S’ (FROM THE LATIN SINISTER MEANING LEFT-HANDED). PRIORITY FIRST
ASSIGNED ON THE BASIS OF THE ATOM THAT IS DIRECTLY ATTACHED TO
THE CHIRALITY CENTER. THE ATOM WITH LOWEST ATOMIC NO. IS GIVEN
THE LOWEST PRIORITY. THE ATOM WITH HIGHEST ATOMIC NO. IS GIVEN
THE NEXT HIGHER PRIORITY.
THE ORDER OF PRIORITY IS AS FOLLOWS :
I>BR>CL>SO3H>F>OCOCH3>OH>NO2>NH2>COOCH3>COOH>CONH2>COCH3
>CHO>CH2OH>CN>C6H5>C2H5>CH3>D>H.
TO ASSIGN WHETHER A CHIRAL CENTER IS R OR S, WE NEED TO FOLLOW
THREE STEPS.

35. STEP 1: PRIORITIZING THE SUBSTITUENTS
NUMBER EACH OF THE SUBSTITUENTS ON THE CHIRAL CENTER CARBON
USING THE CAHN–INGOLD–PRELOG SYSTEM.
ACCORDING TO THE CAHN–INGOLD–PRELOG PRIORITIZING SCHEME, THE
HIGHEST PRIORITY GOES TO THE SUBSTITUENT WHOSE FIRST ATOM HAS THE
HIGHEST ATOMIC NUMBER. (FOR EXAMPLE, BR WOULD BE HIGHER PRIORITY
THAN CL, BECAUSE BR HAS A LARGER ATOMIC NUMBER.)

36. STEP 2: PUTTING THE NUMBER-FOUR SUBSTITUENT IN THE BACK
THE NEXT STEP IS TO ROTATE THE MOLECULE SO THAT THE NUMBER-FOUR
SUBSTITUENT IS POINTED TOWARD THE BACK, AS SHOWN HERE. FOR MANY
PEOPLE, THIS IS THE MOST DIFFICULT STEP BECAUSE TO ROTATE THE
MOLECULE, IT REQUIRES VISUALIZING THE MOLECULE IN THREE
DIMENSIONS.

37. STEP 3: DRAWING THE CURVE
NOW THAT THE NUMBER-FOUR PRIORITY SUBSTITUENT IS IN THE BACK, DRAW
A CURVE FROM THE FIRST-PRIORITY SUBSTITUENT THROUGH THE SECOND-
PRIORITY SUBSTITUENT, TO THE THIRD-PRIORITY SUBSTITUENT. IN THE
FOLLOWING FIGURE, THE CURVE GOES CLOCKWISE, SO THE MOLECULE HAS R
STEREOCHEMISTRY, AS SHOWN HERE