What is a penultimate carbon?

Enumerating is done according to the priority of the functional groups. For the above molecule, number one is given to the carbon atom of the aldehyde group. It has six carbon atoms from the top to the base. The asymmetric carbon atom present here is the fifth carbon. Technically, it means next to last.

Penultimate Carbon is located on the right side of the D isomer. The identifying of this kind of molecule is started with numbering the carbon atoms. When the OH group of the penultimate carbon is positioned on the left side, it is known as the L isomer.