Difficulty: Intermediate | Prerequisites: Part 1 (stereoisomers, chirality, stereocenters), CIP priority rules from E/Z notation
This section covers two things: a naming system for telling one enantiomer from another (R/S configuration), and the physical property that distinguishes them in the lab (optical activity). R/S labels are the language of absolute configuration. Optical rotation is how you detect chirality experimentally using a polarimeter. Both are tested heavily and both are needed for the enantiomeric excess calculations that come next in Part 3.
The R/S system assigns an absolute label to each stereocenter by arranging substituents in order of CIP priority and reading the rotation from highest to lowest priority as clockwise (R) or counterclockwise (S). Chiral molecules rotate plane-polarised light; enantiomers rotate it equally but in opposite directions. A racemic mixture shows zero net rotation.
R configuration
The absolute configuration at a stereocenter where the priority sequence from highest to lowest runs clockwise, with the lowest-priority group pointing away from you (in the back/dashed position). Think of it as: R for rectus (Latin for "right"), and the priorities circle to the right.
S configuration
The absolute configuration at a stereocenter where the priority sequence from highest to lowest runs counterclockwise, with the lowest-priority group pointing away. Think of it as: S for sinister (Latin for "left"), and the priorities circle to the left.
Specific rotation, [alpha]
The constant of proportionality that relates observed optical rotation to concentration and path length. Defined as the observed rotation at a concentration of 1 g/mL and a path length of 1 dm. Units: degrees mL g^-1 dm^-1. In simple terms, it is the standardised measure of how much a compound rotates light, corrected for how much compound and how long a tube you used.
Observed rotation (alpha)
The actual angle (in degrees) that the plane of polarised light is rotated after passing through a sample. Related to specific rotation by: alpha = [alpha] * c * l.
Optically active
A substance that rotates the plane of polarised light. All chiral compounds in pure form are optically active.
Optically inactive
A substance that does not rotate polarised light. This includes achiral compounds, meso compounds, and racemic mixtures.
Dextrorotatory (+, d)
Rotates plane-polarised light clockwise (to the right). Indicated by a (+) sign before the specific rotation.
Levorotatory (-, l)
Rotates plane-polarised light counterclockwise (to the left). Indicated by a (-) sign before the specific rotation. From the Latin laevus, meaning "left."
Polarimeter
The instrument used to measure optical rotation. It passes plane-polarised light through a sample and measures the angle by which the polarisation plane is rotated.
Step-by-step method:
Identify the stereocenter (the carbon with four different groups).
Assign CIP priorities to the four substituents (1 = highest, 4 = lowest) using atomic number. Higher atomic number = higher priority. Break ties by moving outward along the chain.
Orient the molecule so that the lowest-priority group (priority 4) points away from you, into the page (the dashed bond position).
Read the remaining three groups from highest to lowest priority (1, 2, 3).
Clockwise = R
Counterclockwise = S
Shortcut when the lowest-priority group is not in the back: assign R or S as if it were, then flip the answer. Swapping any two substituents on a stereocenter switches R to S or S to R.
Swapping any two groups on a stereocenter produces the mirror image at that centre, which switches R to S (or S to R). This is a useful shortcut for drawing enantiomers: take any one stereocenter and swap two groups.
For a molecule with multiple stereocenters, swapping all stereocenters gives the enantiomer; swapping only some gives a diastereomer.
Optically active compounds rotate plane-polarised light. A polarimeter works in two steps:
First, a reference condition is set: polarised light passes through an empty (or optically inactive) sample tube, and the analyser is rotated to find the dark field at 0 degrees.
Then the sample is placed in the tube. If optically active, it rotates the plane of polarisation by some angle alpha. The analyser is rotated to re-establish the dark field, and the angle of rotation is read.
The sign of rotation tells you the direction: (+) is dextrorotatory (clockwise), (-) is levorotatory (counterclockwise).
Critical point: there is no predictable relationship between R/S configuration and the sign of optical rotation. An R enantiomer can be (+) or (-). The only way to know is to measure it.
The observed rotation alpha depends on concentration and path length:
alpha = [alpha] x c x l
Where:
alpha = observed rotation (degrees)
[alpha] = specific rotation (degrees mL g^-1 dm^-1)
c = concentration (g/mL)
l = path length (dm)
For a pure liquid, c is taken as the density.
Enantiomers have specific rotations of equal magnitude and opposite sign. If the R enantiomer has [alpha] = +15 degrees, the S enantiomer has [alpha] = -15 degrees.
This follows directly from the mirror-image relationship. A 50:50 (racemic) mixture of R and S has [alpha] = 0 degrees. Any mixture not at 50:50 will show a net rotation proportional to the excess of one enantiomer.
R/S configuration is preserved through a reaction if the bonds to the asymmetric carbon are not broken. If a reaction adds or removes groups elsewhere in the molecule without touching the stereocenter, R stays R and S stays S. However, the priority numbering might change (because a new substituent may change the CIP ranking), so the label can appear to flip even when the spatial arrangement has not changed.
Students often assume that R = (+) and S = (-). There is no such relationship. R/S is a naming convention based on spatial arrangement; (+)/(-) is a measured physical property. You must measure or be told the sign of rotation.
Students often confuse specific rotation [alpha] with observed rotation alpha. Specific rotation is the standardised constant; observed rotation depends on concentration and path length.
Students sometimes think that swapping the lowest-priority group into the back position changes the configuration. It does not change the molecule, only your viewing angle. If you cannot orient the molecule properly, assign R/S with the group where it is and then flip the answer.
Students often forget that changing a substituent elsewhere in the molecule can change the CIP priority ranking, which changes the R/S label even though the spatial arrangement at the stereocenter has not changed. This is called stereochemical correlation.
R/S assignment problems are guaranteed on the exam. Practise until the four-step method is automatic.
Expect questions asking you to determine the specific rotation of a mixture given the ratio of enantiomers, or vice versa. Know the formula alpha = [alpha] x c x l cold.
A common exam trap: giving you a molecule where the lowest-priority group is not on the dashed bond, so you need to either mentally rotate the molecule or assign R/S and flip.
Stereochemical correlation questions ask whether R/S changes in a reaction. The answer depends on whether the bonds to the stereocenter were broken.
True or false: R configuration always corresponds to dextrorotatory (+) rotation. Answer: False. There is no predictable relationship between R/S and the direction of optical rotation.
Fill in the blank: The formula relating observed rotation to specific rotation is alpha = ______. Answer: [alpha] x c x l
True or false: Swapping any two groups on a stereocenter switches R to S (or vice versa). Answer: True.
Fill in the blank: A compound that rotates polarised light to the left is called ______. Answer: Levorotatory (or designated with a (-) sign).
Q: A stereocenter has substituents with CIP priorities 1, 2, 3, 4. When viewed with group 4 pointing into the page, groups 1-2-3 run counterclockwise. What is the configuration?
A: S.
Q: The specific rotation of pure (R)-2-bromobutane is +23.1 degrees. What is the specific rotation of pure (S)-2-bromobutane?
A: -23.1 degrees. Enantiomers have equal and opposite specific rotations.
Q: A pure compound has [alpha] = -53 degrees. You measure an observed rotation of -47.7 degrees on a sample. What does this tell you about the sample?
A: The observed rotation is less than the pure value, suggesting the sample is not enantiopure. Calculating: 47.7/53 = 0.9, so the enantiomeric excess is 90%.
Q: A reaction converts (R)-3-chloropentan-1-ol into (R)-3-chloropentanoic acid without breaking any bonds to the asymmetric carbon. Does the R label necessarily stay R?
A: The spatial arrangement at the stereocenter is preserved, but the R/S label may change if the oxidation alters CIP priority rankings. This is stereochemical correlation: same arrangement, potentially different label.
R/S configuration uses the same CIP priority rules as E/Z notation for alkene stereochemistry (Chapter 4). Optical activity connects to spectroscopy methods you will encounter later. The concept of stereochemical correlation becomes essential in reaction mechanisms (Chapters 7-11), where you need to track whether stereochemistry is retained, inverted, or lost.
R configuration, S configuration, rectus, sinister, CIP priority rules, Cahn-Ingold-Prelog, specific rotation, observed rotation, optical activity, optically active, optically inactive, dextrorotatory, levorotatory, (+), (-), d, l, polarimeter, polarised light, polarimetry, stereochemical correlation, absolute configuration