5
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Consider, for example:

3-ethyl-2-methylpentane

From what I've learned in chemistry class, this molecule would be unofficially named 3-isopropylpentane.

What would the official IUPAC name for this molecule be? And what's the general way to name these types of molecules?

EDIT: For this specific molecule, the naming can be simplified by looking at the main carbon chain differently. For a more general case, consider heptane instead of pentane, in this case there's one chain that you're stuck with.

another molecule

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    $\begingroup$ 3-ethyl-2-methylpentane, also it would be propan-2-ylpentane and learn the IUPAC rules that's how to name all molecules. $\endgroup$ – AS_1000 Sep 20 '17 at 17:50
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    $\begingroup$ @AS_1000 Wouldn't the name "3-ethyl-2-methylpentane" suggest that there's 2 branches on the pentane chain, an ethyl on C3 and a methyl on C2 ? That's a different molecule. Also the name " propan-2-ylpentane" doesn't specify which of the pentane carbons is the propyl attached to.. $\endgroup$ – Chady Sep 20 '17 at 17:56
  • $\begingroup$ @Chady are you sure it's a different molecule? Draw it out. $\endgroup$ – orthocresol♦ Sep 20 '17 at 18:01
  • $\begingroup$ @Chady AS_1000 is correct. Once you have attached the isopropyl group, there is more than one possible chain of length 5, and one of those has a methyl at carbon 2 and an ethyl at carbon 3. $\endgroup$ – Tyberius Sep 20 '17 at 18:01
  • $\begingroup$ @AS_1000, orthocresol, Tyberius, I see it now, it didn't occur to me to look at the chain differently. $\endgroup$ – Chady Sep 20 '17 at 18:06

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