# Why isn't cyclopropane considered unsaturated?

Looking up "cycloalkanes" on wikipedia (link), I found this:

In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are monocyclic saturated hydrocarbons.

Of course, that implies that cyclopropane is saturated. Later I looked up "saturated and unsaturated compounds" on wikipedia (link). Here's the definition they mentioned:

In chemistry, a saturated compound is a chemical compound (or ion) that resists the addition reactions, such as hydrogenation, oxidative addition, and binding of a Lewis base.

Now the definition mentions that a saturated compound "resists addition reactions". I am aware that cyclopropane does undergo addition reactions, as mentioned here (link):

So finally, to me it seems these two wikipedia pages are contradicting each other. My question is - why is cyclopropane considered saturated if the above definitions are correct?

• Saturated simply means that all the bonds are single bonds. Unsaturated would mean that the compound contains at least one double or triple bond. // The quirk here is that the molecular formula is $\ce{C4H8}$ for both cyclopropane (saturated) and propene(unsaturated).
– MaxW
Apr 12 '20 at 10:21
• @MaxW So does that mean the wikipedia definition is wrong? Apr 12 '20 at 10:50
• Addition is meant without breaking the structure. Apr 12 '20 at 10:51
• @Poutnik Hmm, I'm still a little skeptical about this. It still is an addition reaction, isn't it? (of course, it involves breaking a bond, but then all addition reactions are required to involve that) Apr 12 '20 at 10:57
• Breaking a bond is not the same as breaking a structure. Apr 12 '20 at 10:59

In short, there are two definitions with separate purpose.

IUPAC defines alkanes as

"Acyclic branched or unbranched hydrocarbons having the general formula $$\ce{C_nH_{2n+2}}$$, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms."

and notes notes about cycloalkanes, contrasting e.g., to unsaturated cycloalkenes:

"Saturated monocyclic hydrocarbons (with or without side chains), e.g. cyclobutane. Unsaturated monocyclic hydrocarbons having one endocyclic double or one triple bond are called cycloalkenes and cycloalkynes, respectively. Those having more than one such multiple bond are cycloalkadienes, cycloalkatrienes, etc. The inclusive terms for any cyclic hydrocarbons having any number of such multiple bonds are cyclic olefins or cyclic acetylenes."

Thus, as in the earlier comment by @MaxW, cyclopropane equally is a saturated alkane because each carbon atom is connected with the next carbon atom only by a $$\ce{C-C}$$ single bond. Or, as MSU condenses,

"Alkanes and cycloalkanes are termed saturated, because they incorporate the maximum number of hydrogens possible without breaking any carbon-carbon bonds."

This set of definitions is not the one serving to determine organic structures from spectra, e.g. in Field's training book, where

"[...] the molecular formular is reduced to $$\ce{C_nH_m}$$ and the degree of unsaturation is given by $$\mbox{Degree of Unsaturation} = n - \frac{m}{2} + 1$$ The degree of unsaturation indicates the number of $$\pi$$ bonds or rings that the compound contains."

(4th edition, p. 4; emphasis added by mine.)

• Thanks, I see where I went wrong (the quote from MSU clears it). Apr 13 '20 at 9:57

Saturation has its IUPAC definition, but it also has its historical roots.

You can add hydrogen to some hydrocarbon up to some point where it is considered "saturated".

If you start from benzene, you can add 3 molecules of hydrogen and consider the resulting substance saturated, but if you try harder, you can add 2 more hydrogen atoms. What is saturated, then? (This observation predates the knowledge of the benzene structure.)

I don't think it has something to do with cyclopropane in particular. Indeed, cycloalkanes are naturally unsaturated, but they are associated with alkanes, which are saturated. The fact that the article specifically mentioned "monocyclic" offered enough details about the degree of unsaturation. However, just for the fact that they are the result of cyclization of saturated compounds, many sources assign cycloalkanes a somewhat saturated character.

TL;DR Cycloalkanes are the saturated hydrocarbons from the family of monocyclic compounds (which may contain other unsaturation-causing factors)

• I think you might be confusing the hydrogen deficiency index (for which a monocyclic alkane would have a value of one) with the degree of unsaturation. The terms are often used interchangeably in NMR calculations; however, I make sure to only use the term hydrogen deficiency index to avoid confusion with the term unsaturation, which is in fact different. Apr 12 '20 at 23:14