Question

The observed rotation of 2.0 g of a compound in 50 mL of solution in a polarimeter tube 20 cm long is +138°. What is the specific rotation of the compound?

Short answer

If the 4^th priority group is at vertical and horizontal position.

Step-by-step solution

Definition of specific rotation

The observed rotation can be used to calculate the specific rotation at a given temperature and wavelength using the formula [a] = a//xc where [a] is the specific rotation, T is the temperature in degrees Celsius, l is the wavelength of the incident light (when the sodium D-line is used, l is indicated as D), an is the observed rotation, l is the length of the sample tube in decimetres, and c is the sample concentration in grammes per 100 millilitres.

Rules for assigning absolute configuration (R and S) to wedge-dash compounds for a, b and c

• Assign the four substituents’ priorities, with #1 being the highest and #4 being the lowest. The atomic number is used to determine priority.

• Draw a circle from point #1 to point #2 to point #3.

• Determine the #4 priority group's orientation.

• (#4 group looking away from you) or is at dash- a clockwise circle represents the R configuration, while anticlockwise circle represents the S configuration.

• (#4 group pointing at you) or is at wedge- a clockwise circle corresponds to the S configuration, while a anticlockwise circle belongs to the R configuration.

Rules for assigning absolute configuration (R and S) to horizontal-vertical compounds for d).

• Using the CIP priority system, assign priority numbers to the four ligands (groups) bound to the chiral centre.

• If the 4^th priority group is at vertical position- The lowest priority ligand is pointing away from the viewer if it is on a Vertical bond then clockwise direction of arrows gives R configuration and anticlockwise gives S configuration.

• If the 4^th priority group is at horizontal position-The lowest-priority ligand is pointed toward the viewer if it is on a horizontal bond then clockwise direction of arrows gives S configuration and anticlockwise gives R configuration.