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Cahn-Ingold-Prelog (CIP) Rules

The Cahn-Ingold-Prelog (CIP) rules are a set of guidelines used to assign priorities to groups attached to a stereocenter (in R/S nomenclature) or a double bond (in E/Z nomenclature). These rules are essential for determining the absolute configuration of chiral molecules and the geometry of double bonds.

Key Principles of CIP Rules:

  1. Priority Assignment:
  • Priorities are assigned based on the atomic number of the atoms directly attached to the stereocenter or double bond.
  • Higher atomic number = higher priority.
  1. Tie-Breaking Rules:
  • If two atoms are the same, compare the next set of atoms in the group.
  • Continue outward until a difference is found.
  1. Double and Triple Bonds:
  • Treat double or triple bonds as if each bond is connected to a separate atom.
  • Example: (C=O) is treated as (C) bonded to two (O) atoms.

Steps to Apply CIP Rules:

1. Assign Priorities:

  • Identify the atoms directly attached to the stereocenter or double bond.
  • Assign priorities based on atomic number (higher atomic number = higher priority).

2. Break Ties:

  • If two atoms are identical, move to the next set of atoms in the group and compare their atomic numbers.
  • Continue this process until a difference is found.

3. Handle Double/Triple Bonds:

  • Treat double or triple bonds as if each bond is connected to a separate atom.
  • Example: (C=O) is treated as (C) bonded to two (O) atoms.

Example 1: Assigning Priorities for R/S Nomenclature

Consider the molecule 2-chlorobutane:
[
CH_3-CH(Cl)-CH_2-CH_3
]

  • The chiral center is (C_2), bonded to:
  1. (Cl) (chlorine)
  2. (CH_3) (methyl)
  3. (CH_2CH_3) (ethyl)
  4. (H) (hydrogen)

Step-by-Step Priority Assignment:

  1. Atomic Numbers:
  • (Cl) (17) > (C) (6) in (CH_3) and (CH_2CH_3) > (H) (1).
  1. Tie-Breaking for (CH_3) and (CH_2CH_3):
  • (CH_3): Carbon is bonded to (H, H, H).
  • (CH_2CH_3): Carbon is bonded to (C, H, H).
  • (C) (in (CH_2CH_3)) has a higher atomic number than (H) (in (CH_3)).
  1. Final Priorities:
  • (Cl) (1) > (CH_2CH_3) (2) > (CH_3) (3) > (H) (4).

Example 2: Assigning Priorities for E/Z Nomenclature

Consider the molecule 2-butene:
[
CH_3-CH=CH-CH_3
]

  • The double bond is between (C_2) and (C_3).
  • Each carbon of the double bond has two groups:
  • (C_2): (CH_3) and (H)
  • (C_3): (CH_3) and (H)

Step-by-Step Priority Assignment:

  1. Atomic Numbers:
  • For (C_2): (C) (6) in (CH_3) > (H) (1).
  • For (C_3): (C) (6) in (CH_3) > (H) (1).
  1. Final Priorities:
  • For (C_2): (CH_3) (1) > (H) (2).
  • For (C_3): (CH_3) (1) > (H) (2).

Special Cases:

1. Double or Triple Bonds:

  • Treat double or triple bonds as if each bond is connected to a separate atom.
  • Example: (C=O) is treated as (C) bonded to two (O) atoms.

2. Cyclic Compounds:

  • Treat the ring as a single group and assign priorities based on the atoms directly attached to the stereocenter or double bond.

Summary of CIP Rules:

  • Priority Assignment: Based on atomic number.
  • Tie-Breaking: Compare the next set of atoms in the group.
  • Double/Triple Bonds: Treat as if each bond is connected to a separate atom.

Practice Problem:

Assign priorities to the groups attached to the chiral center in the following molecule:
[
CH_3-CH(OH)-CH_2Cl
]

  • The chiral center is (C_2), bonded to:
  1. (OH) (hydroxyl)
  2. (CH_3) (methyl)
  3. (CH_2Cl) (chloromethyl)
  4. (H) (hydrogen)

Solution:

  1. Atomic Numbers:
  • (O) (8) in (OH) > (Cl) (17) in (CH_2Cl) > (C) (6) in (CH_3) > (H) (1).
  1. Tie-Breaking for (CH_3) and (CH_2Cl):
  • (CH_3): Carbon is bonded to (H, H, H).
  • (CH_2Cl): Carbon is bonded to (Cl, H, H).
  • (Cl) (in (CH_2Cl)) has a higher atomic number than (H) (in (CH_3)).
  1. Final Priorities:
  • (OH) (1) > (CH_2Cl) (2) > (CH_3) (3) > (H) (4).

The CIP rules are fundamental for understanding stereochemistry and assigning configurations in organic chemistry.

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