page 12 Flashcards
How are priorities assigned to groups around a stereogenic center for R/S configuration?
A: Groups are assigned priorities based on the atomic number of the atoms directly bonded to the stereogenic center:
Highest atomic number = Priority 1
Decreasing atomic number = Lower priorities.
Which atom receives the lowest priority in R/S configuration?
A: The atom with the lowest atomic number, such as hydrogen, typically gets Priority 4.
How are multiply bonded atoms treated when assigning priority for R and S configuration?
A: Multiply bonded atoms are treated as if they are bonded to the equivalent number of singly bonded atoms.
How is the carbon in a C=O bond treated for priority assignment?
A: The carbon is considered bonded to two oxygen atoms, and each oxygen is treated as bonded to the carbon twice.
How is the carbon in a C≡C (triple bond) treated for priority assignment?
A: Each carbon is considered bonded to three carbons, and each single bond is counted three times.
How are priorities assigned to isotopes bonded to a stereogenic center?
A: Priorities are assigned in order of decreasing mass number.
What is the order of priority for the three isotopes of hydrogen?
A:
Tritium (T): Mass number 3 (1 proton + 2 neutrons).
Deuterium (D): Mass number 2 (1 proton + 1 neutron).
Hydrogen (H): Mass number 1 (1 proton).
How does the number of protons and neutrons affect isotopic priority?
A: Higher numbers of protons and neutrons increase the mass number, resulting in higher priority.
What is the significance of assigning priority to isotopes in R and S configurations?
A: It helps in determining the correct 3D spatial arrangement for stereoisomers.
What rule determines the relative priorities for groups in stereoisomers?
A: The Cahn-Ingold-Prelog priority rules, which prioritize based on atomic number and connectivity.
What determines whether the configuration is R or S?
Clockwise rotation = R configuration
Counterclockwise rotation = S configuration
What should you do if the lowest priority group is not facing toward the back?
A: Rotate the molecule 120° around a stationary bond axis to orient the lowest priority group in the back.
How do you determine the configuration after rotation?
A:
Rotate the molecule so that the lowest priority group (4) is behind.
Trace the direction from priority 1 → 2 → 3.
Clockwise = R isomer.
Counterclockwise = S isomer.
What is the maximum number of stereoisomers for a molecule with
n stereogenic centers?
A: The maximum number of stereoisomers is
2^n
.
How do you determine all possible stereoisomers for a compound with two stereogenic centers?
A:
Draw one stereoisomer by arranging substituents around the stereogenic centers.
Draw its mirror image.
Continue by arranging substituents differently to find all combinations.
What happens when you switch the positions of two groups (e.g., H and Br) on one stereogenic center of a molecule?
A: It creates a new stereoisomer, labeled C in this example, which is different from the original isomers (A and B)
What is the relationship between C and its mirror image, D?
A: C and D are enantiomers.
What are stereoisomers that are not mirror images of one another called?
A: Diastereomers.
What is the relationship between A and C in the example of 2,3-dibromopentane?
A: A and C are diastereomers.
What distinguishes enantiomers from diastereomers?
A:
Enantiomers are non-superimposable mirror images.
Diastereomers are stereoisomers that are not mirror images.
