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carvone

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?

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    $\begingroup$ Both CH (which are actually both CH2, but that doesn't matter) are different in what else is attached to them. Imagine a simpler molecule, with the following four substituents: H, Cl, CH3, and C2H5. You'll see right away that they are different. But wait, how are CH3 and C2H5 different if they both start with CH2? Well, just like that. $\endgroup$ – Ivan Neretin May 7 at 11:03
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    $\begingroup$ I edited the CH to CH2 (whoops), I think about benzene every time I see a cyclic molecule. $\endgroup$ – Rafael Franco May 7 at 11:10
  • $\begingroup$ So one of the CH2 groups have a CO group attached, and then the next carbon is just a C(after the CO), does this matter at all? or what matters is what is immediately next to that CH2 and everything else down the chain is not relevant $\endgroup$ – Rafael Franco May 7 at 11:20
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    $\begingroup$ Yes it does matter. Everything down the chain is relevant. You can imagine two chains longer than these, which are very similar and only differ at the very end, and yes, they will be different. $\endgroup$ – Ivan Neretin May 7 at 11:34
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    $\begingroup$ Related: chemistry.stackexchange.com/q/39536/7951 $\endgroup$ – Loong May 7 at 13:11
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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!

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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.

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  • $\begingroup$ I'm afraid that doesn't actually answer the question, I don't think OP has any problem identifying what surrounds the asymmetric carbon (next neighbour), but rather why it is asymmetric in the first place. $\endgroup$ – Martin - マーチン May 8 at 15:57
  • $\begingroup$ I had actually meant to post this as a comment rather than an answer. $\endgroup$ – Michael Lautman May 8 at 18:53

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