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Can someone explain why the ibuprofen medication has the chemical name as (RS)-2-[4-(2-methylpropyl)phenyl]propanoic acid?

Ibuprofen structure

What does 2,4,2 refer to in the chemical name?

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    $\begingroup$ By the way: the name “ibuprofen” is related to the obsolete name “2-(4-isobutylphenyl)propanoic acid” for 2-[4-(2-methylpropyl)phenyl]propanoic acid. $\endgroup$
    – user7951
    Nov 26, 2016 at 21:17

2 Answers 2

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If we want to name chemicals we need a systematic approach with good rules so whoever names the compound come up with the same name and it also has to be unambiguous. There are two major naming systems, one by IUPAC and one by Chemical abstracts. Both work very similarly.

So how do we come up with that name and what are those numbers? First we find a starting point, we call this the group with the highest priority, which is the acid on the right in this case. Now I won't get into detail here why we do things a certain way, this would take a lot of time and there are resources explaining all the rules.

In this case we start off with the acid and here we got, as our "base system", an acid with a chain out of 3 carbons (including the one of the acid group) so that's a propionic acid. (the purple structure in the picture below) Now attached to this is a phenyl- ring (blue) but we need to write down where it is attached. We start counting at the acid so it's position two, that's the first 2 in the name. On that phenyl- ring we got another group, and this time we count from the position where it's attached to our previous group. So we end up with position 4. And on that 4 position there's the red group, a propyl group and again here on two position there's another methyl.

Ibuprofen

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The outer two refers to the fact that the (4-(2-methylpropyl)phenyl) group is on the second carbon of the propanoic acid. The four tells us that the methylpropyl group is across from the first carbon in the benzene ring (which is the one attached to the propanoic acid group). Finally, the second two refers to the fact that the methyl group on the methylpropyl substituent on the benzene ring is two away from the benzene ring. Alternatively, this compound could be called (RS)-2-(4-(isobutyl)phenyl)propanoic acid, or even (RS)-2-(para-(isobutyl)phenyl)propanoic acid.

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