Question

According to the Cahn-Ingold-Prelog system, we assign priorities (1, 2, 3, or 4) to the atoms directly bonded to the stereogenic center in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1). Suppose, there is a tie - as would be the case with the chiral center of 2-Butanol - where two different groups - methyl and ethyl are attached to the chiral carbon. So there is a tie between the Carbons of each groups. In that case, how is the tie resolved?

• A. We assign a higher priority to whichever group is heavier
• B. We assign a higher priority to the more reactive group
• C. We assign priority based on the atomic number of the atoms bonded to these atoms (which are tied). One atom of higher atomic number determines a higher priority.
• D. None of the above

Answer 1

The correct option is C We assign priority based on the atomic number of the atoms bonded to these atoms (which are tied). One atom of higher atomic number determines a higher priority.

Let us draw the structure of 2-Butanol.

Carbon number 2 is a chiral center since four different groups are attached to it – the hydroxyl group, the methyl group, the ethyl group and a Hydrogen atom.
Now let us look at the compound without numbering the longest chain (to avoid confusion with the priorities).

As you can see, there are two Carbons attached to the chiral center. How do we break the tie?

If two atoms on a stereogenic center are the same, assign priority based on the atomic number
of the atoms bonded to these atoms. One atom of higher atomic number determines a higher
priority.

The Carbon of the ethyl group is bonded to 3 Hydrogen atoms. So let us write down HHH beside that carbon. Now, the other carbon atom is bonded to another Carbon atom and two other Hydrogens. So we write CHH (decreasing atomic numbe) beside the carbon of the Ethyl group - like this:

The Carbon of CHH is the firstpoint of difference whenwe compare “CHH” with “HHH”. And atomic number of C > Atomic number H. Hence the ethyl group gets a higher priority than methyl group. So the priority is as shown: