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A chiral carbon atom is defined as

"a carbon atom with four different groups attached to it."

I am aware that the open-chain form of glucose has four chiral carbon atoms.

However, I do not undertstand why that is, since the alleged chiral atoms do not have four different substituents: For example, the 3rd carbon atom is bound to one hydrogen atom, one hydroxylgroup and two neighbouring carbon atoms.

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    $\begingroup$ 2 same atoms does not mean 2 same atomic groups. $\endgroup$
    – Poutnik
    Commented Feb 21, 2023 at 6:58
  • $\begingroup$ Okay, but why is the last carbon atom at the bottom NOT considered chiral then? $\endgroup$
    – Johanna
    Commented Feb 21, 2023 at 7:01
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    $\begingroup$ Exception: 2 same atoms means 2 same groups if the atom is the whole group. $\endgroup$
    – Poutnik
    Commented Feb 21, 2023 at 7:03
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    $\begingroup$ Groups R-COOH and R-CH2OH are not the same, even if both are attached to R via C atom. Or, do you think they are the same? $\endgroup$
    – Poutnik
    Commented Feb 21, 2023 at 7:09
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    $\begingroup$ @Johanna Similar to an onion, starting from the central atom to check if it is a chiral one / a stereogenic centre (and if so, on can assign R, or S configuration), one works toward more remote "shells". User @user55119 compiled instructive answers; just two of them here, and here. $\endgroup$
    – Buttonwood
    Commented Feb 21, 2023 at 13:45

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