Difficulty: Intermediate Prerequisites: Part 1 of these notes (stereoisomers, chirality, optical activity), familiarity with stereocenters and the concept of non-superimposable mirror images.
Big picture: Part 1 introduced the idea that molecules can be chiral and that enantiomers rotate light in opposite directions. But optical rotation alone does not tell you which three-dimensional arrangement a molecule has. This part introduces the naming system (R and S) that lets chemists describe the exact spatial arrangement around a stereocenter unambiguously, along with Fischer projections, a shorthand for drawing those arrangements on a flat page. These tools are essential for communicating stereochemistry in writing, on exams, and across the chemical literature.
The Cahn-Ingold-Prelog (CIP) priority rules assign R or S labels to each stereocenter based on atomic number. Fischer projections are a standardised 2D shorthand for tetrahedral carbon that let you quickly read and assign configuration without building a 3D model. Both are mechanical, rule-based skills that reward practice.
Absolute configuration
The exact three-dimensional arrangement of substituents around a stereocenter, labelled as R or S. In simple terms, it is the "name" of a specific 3D shape at a given stereocenter.
R (rectus)
Latin for "right." Assigned when the priority sequence (1 → 2 → 3) traces a clockwise arc with the lowest-priority group pointing away from you.
S (sinister)
Latin for "left." Assigned when the priority sequence (1 → 2 → 3) traces an anticlockwise arc with the lowest-priority group pointing away from you.
Cahn-Ingold-Prelog (CIP) priority rules
The set of rules used to rank substituents on a stereocenter by priority. The primary rule is: higher atomic number = higher priority. Think of it as "heavier atom wins."
Fischer projection
A standardised way of drawing a tetrahedral carbon in two dimensions. Horizontal lines represent bonds coming towards the viewer; vertical lines represent bonds going away. It lets you read stereochemistry from a flat drawing without needing a 3D model.
There is no correlation between optical rotation (+/−) and absolute configuration (R/S). You cannot look at a (+) label and conclude R, or vice versa. They are determined by entirely different methods.
R and S are assigned from the molecular structure using the CIP rules. Optical rotation is measured experimentally with a polarimeter.
Step 1: Assign priority to each substituent on the stereocenter.
Compare the atoms directly attached to the stereocenter.
Higher atomic number = higher priority.
If two substituents begin with the same atom, move outward along each chain and compare the next set of atoms until a difference is found.
Isotopes: heavier isotope gets higher priority (e.g. deuterium > hydrogen).
Step 2: Orient the molecule so the lowest-priority group (priority 4) points away from you.
Imagine looking at the stereocenter with group 4 at the back.
Step 3: Trace an arc from group 1 → 2 → 3.
Clockwise arc = R (rectus).
Anticlockwise arc = S (sinister).
Some rankings worth memorising for speed:
Br > Cl > S > F > O > N > C > H
Within carbon chains: a branch or heteroatom further along the chain can change the priority, so always trace outwards until you find the first point of difference.
Double bonds are treated as two single bonds to the same atom (a C=O is treated as if the carbon is bonded to two oxygens, and the oxygen is bonded to two carbons). This "phantom atom" rule applies to double and triple bonds alike.
Example A: A stereocenter bearing H, Br, CH₃, and NH₂.
Priority: Br (35) > N (7) > C (6) > H (1).
Orient with H pointing back. Trace Br → NH₂ → CH₃.
Determine the direction. In the source material, this gives S configuration.
Example B: A stereocenter with H, Cl, CH₃, and CH₂CH₃.
Priority at the first atom: Cl (17) > C > C > H (1).
The two carbon substituents (CH₃ and CH₂CH₃) tie at the first atom. Move outward: CH₂CH₃ has a carbon at the second position; CH₃ has only hydrogens. So CH₂CH₃ > CH₃.
Final priority: Cl > CH₂CH₃ > CH₃ > H.
Orient with H at the back, trace the arc, determine R or S.
What they represent:
A Fischer projection shows a tetrahedral (sp³) carbon as a cross.
Horizontal bonds come towards the viewer (out of the page).
Vertical bonds go away from the viewer (into the page).
The carbon at the intersection is the stereocenter.
Reading a Fischer projection:
The longest carbon chain typically runs vertically, with the most oxidised carbon (or lowest-numbered carbon) at the top.
Each intersection of a horizontal and vertical line is a tetrahedral carbon.
Rules for manipulating Fischer projections:
Allowed: rotate the entire projection by 180° in the plane of the page. This does not change the configuration.
Allowed: hold one substituent fixed and rotate the other three in a cyclic fashion (one position each). This preserves configuration.
Not allowed: rotate by 90° in the plane of the page. This inverts the configuration (swaps R to S or vice versa).
Not allowed: swap any two groups arbitrarily without checking whether it changes the configuration (each single swap inverts the stereocenter).
Converting between 3D and Fischer:
Build or draw the molecule in 3D with the horizontal bonds coming towards you.
Flatten it onto the page, maintaining the horizontal-towards/vertical-away convention.
The resulting cross is the Fischer projection.
CIP assignment procedure (summary):
Rank substituents: highest atomic number = priority 1.
Place priority 4 at the back.
Trace 1 → 2 → 3.
Clockwise = R. Anticlockwise = S.
Fischer projection convention:
(away from you)
|
(towards you) ── C ── (towards you)
|
(away from you)
Horizontal = towards viewer. Vertical = away from viewer.
R/S naming is the universal language for describing drug stereochemistry. Pharmaceutical patents, regulatory filings, and clinical trial data all use R/S labels to specify which enantiomer is the active ingredient. The blockbuster heartburn drug esomeprazole (Nexium) is the S-enantiomer of omeprazole, and its patent and marketing hinged entirely on that single letter.
Fischer projections are heavily used in biochemistry to draw amino acids and sugars. The D/L system for carbohydrates and amino acids is read directly from a Fischer projection, and you will encounter it throughout biochemistry coursework.
"R always means (+) and S always means (−)." This is the single most common mistake. R/S is determined by atomic-number-based priority rules. (+)/(−) is measured experimentally. They are independent of each other.
"You can rotate a Fischer projection any way you like." You cannot. A 90° rotation in the plane of the page swaps the configuration. Only 180° rotations and cyclic three-group rotations are safe.
"If two substituents start with the same atom, they have the same priority." They do not. You must continue outward along the chain until you find the first point of difference. CH₂CH₃ outranks CH₃ because at the second atom, CH₂CH₃ has a carbon where CH₃ has hydrogen.
"Double bonds are just ignored in CIP rankings." They are not ignored. A double bond to an atom is treated as two single bonds to that atom (the phantom-atom rule). This frequently changes the priority order.
⚠️ Assigning R/S is tested on nearly every stereochemistry exam. Practise the three-step procedure until it is automatic.
⚠️ CIP tiebreaking (moving outward along chains) is a favourite exam trap. If the first atoms match, keep going.
⚠️ The phantom-atom rule for double and triple bonds is commonly tested. Know how to expand C=O, C=C, and C≡N for CIP ranking.
⚠️ Fischer projection manipulation (which rotations are allowed, which swap configuration) appears frequently. Know that 90° rotation = configuration change.
⚠️ Converting between 3D wedge-dash drawings and Fischer projections is a core skill. Practise both directions.
True or false: A molecule labelled (R) must be dextrorotatory (+). A: False. R/S and (+)/(−) are determined independently. An R stereocenter can be either (+) or (−).
True or false: In a Fischer projection, horizontal bonds point towards the viewer. A: True.
Fill in the blank: When the arc from priority 1 → 2 → 3 is anticlockwise (with priority 4 at the back), the configuration is ______. A: S (sinister).
Fill in the blank: In CIP rules, a C=O double bond is treated as if the carbon is bonded to two ______ and the oxygen is bonded to two ______. A: Two oxygens; two carbons (phantom atoms).
True or false: Rotating a Fischer projection 90° in the plane preserves the configuration. A: False. A 90° rotation inverts the configuration.
Q: A stereocenter has the following four substituents: -OH, -H, -CH₃, -NH₂. Assign priorities using CIP rules.
A: Priority is based on atomic number of the directly attached atom. O (8) > N (7) > C (6) > H (1). So priority order is: 1 = OH, 2 = NH₂, 3 = CH₃, 4 = H.
Q: You determine the CIP priorities and orient the molecule with priority 4 at the back. The arc from 1 → 2 → 3 is clockwise. What is the configuration?
A: R (rectus). Clockwise = R.
Q: In CIP ranking, how do you handle -CH₂OH versus -CH₂CH₃? Both start with a carbon.
A: The first atoms are both carbon, so it is a tie. Move to the next set of atoms. -CH₂OH has O, H, H at the second position. -CH₂CH₃ has C, H, H. Oxygen (8) outranks carbon (6), so -CH₂OH has higher priority.
Q: You are given a Fischer projection and asked to determine R or S. The lowest priority group (H) is on a horizontal bond (pointing towards you). You trace 1 → 2 → 3 and get a clockwise arc. What is the actual configuration?
A: Because H is on a horizontal bond (towards you, not away), the answer you get from tracing is the opposite of the real configuration. The clockwise arc would normally indicate R, so the actual configuration is S. When the lowest-priority group is not at the back, you must either re-orient the molecule or flip your answer.
Q: How many distinct stereoisomers exist for a molecule with three stereocenters and no meso forms?
A: 2³ = 8 stereoisomers.
R/S assignments carry through into reaction mechanisms. In SN2 reactions (Chapter 7), the stereocenter undergoes inversion of configuration, meaning R becomes S and vice versa. In SN1 reactions, a planar carbocation intermediate forms, leading to racemisation (a mixture of R and S). Understanding CIP rules is therefore a prerequisite for predicting the stereochemical outcome of substitution reactions.
Fischer projections come back in full force in biochemistry when discussing carbohydrate chemistry (D- and L-sugars) and amino acid configuration (L-amino acids in biology). The horizontal/vertical convention becomes second nature once you reach those topics.
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