Cahn-Ingold-Prelog Rules – Need Help?

Okay, I am doing a question for Organic Chemistry, and I need some help.

Assign priorities in the following set of substituents according to Cahn-Ingold-Prelog rules.

-H

-CH3

-OH

-CH2OH

We have to rank them, starting with the highest priority? Thanks!!!

Im Sorry, its

-OH

-Cl

-CH2SH

-CH2OH

3 Answers

  • CIP rules rank groups based on their atomic number. The atom with the lowest atomic number is the lowest group and the atom with the highest number is the highest priority. H is lowest, OH is highest. Those two are easy. For the other two groups, if there is no difference in priority between the first atoms connected to the chiral center (they are both carbon), then you look at the next atoms in the group. One has 3 H atoms and the other has two H atoms and one oxygen atom. The oxygen is higher. So the order for these four groups is

    H < CH3 < CH2OH < OH

    Note: you should realize that a single hydrogen on the chiral center will always be lowest priority.

  • Cahn Ingold Prelog

  • Highest is -OH, followed by CH2OH, CH3 then H

    You look at the atoms which are directly attached to the chiral carbon and the one with the highest atomic n.o. is the highest priority. So with OH the O has a M.Wt of 16amu so it is first.

    When the chiral carbon has 2 carbons attached, you look at the next one along. So with CH3 and CH2OH, the C in CH2OH has an oxygen directly attached whereas the C in CH3 only has a hydrogen attached. Oxygen has a higher M.Wt than hydrogen therefore CH2OH is higher priority than CH3.

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