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This is the structure of carvone, with the chiral carbon noted by *.
I don't understand why this is a chiral carbon, to my understanding a chiral carbon is a carbon with 4 different chemical groups attached to it.
In the molecule, the chiral carbon is surrounded by $\ce{H}$, $\ce{C(CH3)=CH2}$ and two $\ce{CH2}$ groups.
Why are both $\ce{CH2}$ groups different and thus making that carbon chiral?
If you consider the carbon with as star (C*), it bears a C(CH3)(=CH2) group and an H group (not shown). But it is also part of a ring and in that case, we actually follow the exact same rules!
From C*, if we go to the left or the right, we get a CH2 group. So let's get further: on the left we get a CH group with a double bond to another carbon, while on the right, we get a C atom with a double bond to an oxygen and a single bond to a carbon atom.
Because you get different groups if you follow the ring from C* to the left or the right, it means that C* is chiral!
This is just a general note on handling this type of question. Many students struggle with line diagrams when they are first introduced. This is because a lot of important information is implied by the drawing, rather than showing it explicitly.
Take the time to redraw the molecule, clearly showing all of the missing atoms (especially the hydrogen atoms). This should make things clear for you.
It is well worth your time to redraw the molecule until you are very familiar with line drawings.
