I was reading an article about acetaminophen and it says its chemical name is 4'-hydroxyacetanilide.

What does the 4 signify and most importantly, what does the apostrophe mean? First semester chem student so if you can answer in layman terms that would be great.

  • $\begingroup$ 4: fourth atom (usually but not necessarily carbon). Prime (not apostrophe): concerning the secondary structure not the primary one. Should be explained in every organic chemistry textbook in the nomenclature section. $\endgroup$
    – Jan
    Nov 12, 2017 at 14:11
  • $\begingroup$ @Jan What do you mean by secondary and primary structure? Thanks $\endgroup$
    – user54747
    Nov 24, 2017 at 4:57
  • $\begingroup$ It’s a bit hard to explain concisely. In your example, you have an amide (acetanilide). The carbonyl side will be referred to with only the numbers (e.g. 2-hydroxyacetanilide) while the amino side’s carbons are referenced with primed numbers (4'-hydroxy). Another example would be 4,4'-dimethylbisphenyl; here, an arbitrary phenyl group is the primary and the other arbitrary one is the secondary. $\endgroup$
    – Jan
    Nov 24, 2017 at 6:59
  • $\begingroup$ See also chemistry.stackexchange.com/questions/61155/… $\endgroup$
    – dat
    Feb 12, 2020 at 17:43

1 Answer 1


4 means the fourth atom and the prime symbol specifies the part of the molecule (one ' means the secondary structure), so 4'-hydroxyacetanilide means you have acetanilide with a hydroxide group on the fourth atom of the secondary structure.
Naming organic compounds works like this:

  1. Find the longest continueous chain of carbon atoms. Called the parent chain this chain establishes the base alkane name for the compound.
  2. Number the carbon atoms in the parent chain beginning with the end closest to any substituents.
  3. Using the correct substituent name (methyl, ethyl, chloro,etc.), and the carbon atom number on the parent chain.
  4. If a particular substituent occurs more than once, use a prefix (di-, tri-, tetra-) to indicate the number of those substituents.
  5. List the alkyl substituents in alphabetical order.

Source: http://intro.chem.okstate.edu/1014/lecture/organic.html

Here are the IUPAC guidelines about the nomenclature of organic compounds: http://old.iupac.org/reports/provisional/abstract04/BB-prs310305/Chapter1.pdf

  • $\begingroup$ thank you ! What exactly do you mean by secondary and primary structure? Also, when you say 4th atom, how do you tell what the 4th atom is? $\endgroup$
    – user54747
    Nov 13, 2017 at 8:35
  • $\begingroup$ @user54747 The quote should explain how to find the fourth atom. As for the secondary structure...To be honest I don't know it myself as I only added information given in a comment. $\endgroup$
    – GreenSmurf
    Nov 13, 2017 at 20:14
  • $\begingroup$ Assume you have a biphenyl (Google it) with a bromine in the 3-position (aka, meta) in one ring and a chlorine at the 4-position (aka, para) in the other, its name is 3-bromo-4'-chloro-1,1'-biphenyl. The primary structure is the one without primes; the secondary with primes. The 1,1' nomenclature is the point of attachment of the two benzene rings. $\endgroup$
    – user55119
    Nov 27, 2017 at 19:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.