Cahn-Ingold-Prelog Priority Rules in Chemistry

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The goal of this lesson is to understand the Cahn-Ingold-Prelog priority rules in chemistry and how these rules are used to prioritize groups of atoms bonded to a stereocenter in a molecule.

Who Gets to Go First?

When you were a young child playing with your friends and there came a time to take turns with a specific toy, how did you decide who got to go first? Maybe you went by who was older or whose birthday month came first in the year. Perhaps you took the approach of drawing straws or sticks to see who got to go first. Or, maybe a parent had to intervene and assign the order of who got to play with the toy when and in what order!

Regardless of how the pecking order was ultimately established, you would probably agree that children benefit from sets of rules. Did you know that some organic compounds need rules to decide who goes first too? You see, when the stereochemistry of an organic compound is being determined, a set of rules called the Cahn-Ingold-Prelog priority rules is used to do this. In our lesson we are going to be learning those rules and seeing how they work. Let's see who gets to go first!

Definition of the Cahn-Ingold-Prelog Priority Rules

The purpose of the Cahn-Ingold-Prelog priority rules (CIP priority rules) is so that chemists can correctly and unambiguously name a specific stereoisomer of a molecule. Stereoisomers are compounds that have the same chemical formula, the same atom connectivity, but differ in their three-dimensional orientation.

By using the CIP priority rules, it's possible to correctly prioritize each atom or group of atoms bonded to a specific carbon atom (stereocenter) within a molecule so that chemists can assign a designator to that specific carbon. Something that's important for us to realize, however, is that in order for a carbon to be a stereocenter within a compound it has to have four different atoms or groups of atoms bonded to it.

Using the CIP Priority Rules

Luckily for us as long as we can count and use the periodic table of the elements, the CIP priority rules are quite easy to navigate by following a few simple steps:

1. Look up the atomic numbers of each of the atoms bonded to the stereocenter carbon of interest. The atom with the highest number always receives the top priority number (1 in most cases) and each remaining atom is ordered accordingly based on their atomic numbers (priority numbers 2-4).

2. If there is a tie between two or more atoms, we must consider atoms at a two-bond distance from the stereocenter to break the tie.

3. If there is still a tie, then we must consider atoms at a three-bond distance from the stereocenter until the tie is broken.

Examples of Using the CIP Priority Rules

Consider as an example a carbon atom that is bonded to a fluorine atom, a hydrogen atom, a bromine atom, and a hydroxyl (-OH) group. First and foremost notice that this carbon would be classified as a stereocenter since it's bonded to four different groups of atoms. Let's see if we can use the CIP priority rules to assign a priority number to each of the substituents.

A stereocenter carbon atom bonded to four different substituents.

Let's start by giving the atomic number for each atom directly bonded to the carbon:

  • Fluorine: 9
  • Bromine: 35
  • Hydrogen: 1
  • Oxygen: 8

According to the CIP priority rules atoms with a higher atomic number receive highest priority and we rank others accordingly. With that being said, the bromine atom would be priority group 1, the fluorine atom would be priority group 2, the oxygen (of the hydroxyl group) would be priority group 3, and finally the hydrogen would be priority group 4. Easy enough right?

Let's look at a slightly more complicated case. Notice that the carbon atom bearing the chlorine in 2-chlorobutane is the stereocenter in this compound since it's the only carbon that's bonded to four different substituents. If we step out one atom at a time from the stereocenter we hit a chlorine, a hydrogen, and two carbon atoms. The chlorine atom will receive the top priority (number 1) since the atomic number is greater than both hydrogen and carbon.

2-Chlorobutane contains one stereocenter.

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