Primary, Secondary, Tertiary, Quaternary Terms
We use the terms primary, secondary, tertiary and quaternary at various times during the organic and biochemistry sections of the course. In each case, the terms somehow refer to the numbers 1, 2, 3 and 4, respectively. However, the exact connotations depend on the context. The purpose of this page is to bring all the meanings together, for direct comparison. The presentations here are brief; the main goal is comparison, not original presentation.
A (saturated) carbon atom in an organic molecule is classified as primary (etc) depending on how many other C atoms are attached to it. The following table illustrates this:
A carbocation (C atom carrying a positive charge) is classified the same way as a regular (neutral) carbon atom, above. That is, if the C of the carbocation is attached to one other C, then it is called a primary carbocation, etc.
The book discusses this in the context of addition reactions of alkenes; acid-catalyzed addition of HX proceeds through a carbocation intermediate. Ouellette 2/e p 120.
An alcohol is classified based on the C atom to which the -OH is attached. That is, if the alcohol -OH group is on a primary C, then the alcohol is a primary alcohol. Example: CH3CH2OH (ethanol) is a primary alcohol.
Hopefully, the classification of alcohols seems logical, following from the way C atoms are classified. But caution... the next time we use these terms is for amines, and that is a quite different story.
An amine is classified based on its N atom. If the N atom has 1 C attached, then the amine is primary, etc. Example: CH3NH2 (methylamine) is a primary amine, because the N has 1 C attached. The nature of the C itself is not relevant. Note that the classification of amines is not done by the same general procedure as the others discussed above. Amines are not classified by their C atoms, but rather by the N atom.
The same terms (primary, etc) are used to describe aspects of protein structure. These meanings have no relationship at all to the meanings above; they have no relationship to counting anything.
The terms primary, etc., are used in essentially the same way for nucleic acids as for proteins, discussed in the previous section.
The following table briefly summarizes all of the usages of the terms primary (etc) discussed above. These usages fall into three general categories. Terms shown in bold are section headings above.
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