Question

What is the order from lowest to highest priority of -F, -CH3, -COOH, -CH2F according to the IUPAC approved sequence rules?

Select one:
a. -F, -CH3, -CH2F, -COOH
b. -F, -CH2F, -CH3, -COOH
c. -CH3, -CH2F, -COOH, -F
d. -CH3, -COOH, -CH2F, -F

Answer 1

The correct ranking is (d) `"CH"_3<"COOH"<"CH"_2"F"<"F"`.

Explanation:

This is formally called the Cahn-Ingold Prelog ranking. It's used to keep track of how groups attached to a chiral center or stereocenter or a non-rotatable bond (like a double bond) are oriented relative to each other.

The Cahn-Ingold-Prelog rules go like this:

Rule 1. The highest ranked group has the highest atomic number, or the highest mass number if atomic numbers are equal but you have different isotopes, in the atom immediately attached to the stereocenter or nonrotatable bond. Thus fluorine in one of the grouis listed ranks above carbon in the other three.

Rule 2. When two groups have the same immediately attached atom, we look for the highest atomic number or mass number among isotopes with one intermediate atom between them and the stereocenter or double bond. Here the `"COOH"` group has two oxygen atoms attached to the carbon that's attached to the stereocenter, which is a higher atomic number than the hydrogen atoms in `"CH"_3` but lower than fluorine in `"CH"_2"F"`.

In applying Rule 2 to the given groups, the fact that fluorine has higher atomic number than oxygen has greater importance than the carboxyl group having two oxygen atoms, but if there were no higher atomic number atom then two oxygen atoms wpuld rank above just one.

Rule 3. If there is still a "tie", we similarly look at atoms with two intermediate atoms, then three intermediate atoms, etc., but here all the groups are separated after just Rules 1 and 2.

So the ranking here us `"CH"_3<"COOH"<"CH"_2"F"<"F"`. Rule 2 separates the first three of these groups but Rule 1, which comes first, places the single fluorine atom at the top.

See the article referenced above for a fuller explanation of the rules and how they work.