Question

How would the molecule below be named based on IUPAC naming?

Answer 1

The IUPAC name is (`R`)-2-methyl-5-(prop-1-en-2-yl)cyclohex-2-en-1-one.

Explanation:

Step 1. Start with the ring system

The carbonyl carbon is `"C1"`.

Then you number the ring carbons counterclockwise, giving the double-bonded carbons priority in numbering.

The base name is cyclohex-2-en-1-one

Step 2. Name the substituents

We have a methyl group at `"C2"` and a prop-1-en-2-yl group at `"C5"`.

The name is now 2-methyl-5-(prop-1-en-2-yl)cyclohex-2-en-1-one

Step 3. Assign stereochemistry

`"C5"` is a chiral centre.

All we can say is that the `"H"` atom is priority 4.

`"C4, C5"`, and `"C2"` of the propenyl group are tied for priorities 1, 2, and 3.

We must go one atom further out to break the tie.

`"C4"` and `"C6"` are each attached to a `"C"` and two `"H"` atoms, so they are still tied (`"C,H,H"`).

The double bond counts as two `"C"` atoms, so the propenyl carbon is attached to `"C, C"`, and `"C"` (`"C,C,C"`).

This breaks a tie. The propenyl carbon is priority 1.

We must go still further out from `"C4"` and `"C6"`.

This breaks the tie! (`"O,O,C"`) is higher priority than (`"C,C,H"`)

The 1→2 →3 direction is counterclockwise (`S`).

However, the `"H"` atom is "up", so we reverse the assignment to (`R`).

The name of the compound is
(`R`)-2-methyl-5-(prop-1-en-2-yl)cyclohex-2-en-1-one.

Its common name is (-)-carvone.