A sp³ hybridized carbon atom is the one which forms 4 single bonds with other atoms.
In the structure of Bilobalide, we can see 12 sp³ hybridized carbon atoms which are attached to other carbon, hydrogen and oxygen atoms.
In order to classify carbon atoms as 1°, 2°, 3°, or 4° we need to take a look at the atoms to which these carbon atoms are attached.
A 1° carbon atom is the one which is attached to only 1 carbon atom.
In the diagram, we can see 3 methyl groups ( -CH3) being attached to only 1 carbon atom. There is one more primary carbon atom.
There are 4 primary carbon atoms in Bilobalide.
A 2° carbon atom is the one which is attached to 2 other carbon atoms.
In Bilobalide we can see 4 secondary carbon atoms.
A 3° carbon atom is the one which is attached to 3 other carbon atoms.
In Bilobalide we can see only 1 tertiary carbon atom.
A 4° carbon atom is the one which is attached to 4 other carbon atoms.
In Bilobalide we can see 3 quaternary carbon atoms.
Please see the attached image for classification.
