Chemistry - Is benzene a polymer of ethyne?

Solution 1:

The scientist who coined the term polymer, Jöns Jacob Berzelius, used the word to refer to different substances which had the same empirical formula. In this sense, benzene is a polymer of ethyne (acetylene), because benzene $\ce{C6H6}$ and ethyne $\ce{C2H2}$ have the same empirical formula, namely $\ce{CH}$. In other words, all substances with molecular formula $\ce{(CH)_x}$ were polymers of the smallest unit in the family (in this case, $\ce{C2H2}$, because $\ce{CH}$ molecules are only known as extremely fleeting species on Earth, though they are abundant in space).

However, that was back in the 1830s, when we had no clue about atomic and molecular structure (in 1808, one of the eminent chemists of the time, John Dalton, defended $\ce{HO}$ as the formula for water). The meaning of the term has evolved over time to accommodate our increase in knowledge. The most current IUPAC recommendations on polymer nomenclature (Compendium of Polymer Terminology and Nomenclature - IUPAC Recommendations 2008) states the definition of polymer as:

A polymer is a substance composed of macromolecules.

And, fortunately, they also define a macromolecule as:

Molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass.

Note 1: In many cases, especially for synthetic polymers, a molecule can be regarded as having a high relative molecular mass if the addition or removal of one or a few of the units has a negligible effect on the molecular properties. This statement fails in the case of certain properties of macromolecules which may be critically dependent on fine details of the molecular structure, e.g., the enzymatic properties of polypeptides.

Note 2: If a part or the whole of the molecule has a high relative molecular mass and essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass, it may be described as either macromolecular or polymeric, or by polymer used adjectivally.

Note 3: In most cases, the polymer can actually be made by direct polymerization of its parent monomer but in other cases, e.g., poly(vinyl alcohol), the description ‘conceptual’ denotes that an indirect route is used because the nominal monomer does not exist.

As you can see, there is quite a bit of detailing to try to keep things consistent. Observe that the definition speaks of high relative molecular mass, and this point is expanded upon in Note 1. With these considerations, benzene is not a polymer - it is certainly a very small molecule, and adding or removing a portion of it (presumably cyclooctatetraene $\ce{C8H8}$ and cyclobutadiene $\ce{C4H4}$, respectively) would result in a substance with drastically different properties.

In some sense, Berzelius wasn't really wrong to call benzene a polymer, because for all he knew back then, benzene could well have the formula $\ce{C20000H20000}$ (actually, it was not widely believed that "large" molecules could exist at all, with some resistance to the idea existing as late as the 1920s-1930s). He did the best with what knowledge was available to him. But now we can do better, and it just is more convenient and self-consistent to not label benzene as a polymer.

This isn't to say that, however, that benzene and ethyne are completely chemically unrelated. There is in fact a close relationship, in that ethyne can efficiently undergo a [2+2+2] cycloaddition reaction to form benzene. This is not exploited industrially, however, as there are much cheaper (and safer) ways of making it.

Solution 2:

Short answer: Yes and No, depending if we consider polymer(1) or polymer(2).

Long answer: The first step in decision "Is X A or not ?" must be clarification what we mean by "A".

Benzene is a cyclic trimer of ethyne. Trimers are a special case of polymers(1) as repeated monomer structures, being a superset of oligomers and polymers(2). Benzene is a polymer(1) with n=3.

OTOH, we consider polymers(2) as multiple repetitions of monomer unit, where repetition count $n \gg 1$. Such polymers(2) occur rather with a range of $n$ and no particular $n$ has a special, privileged status. Like a polymer plastic material may have $n$ in range 2000-4000.

In such a sense, benzene is not a polymer(2). Neither we say $\ce{N2O4}$ as the dimer of $\ce{NO2}$ is polymer(2), but we can still say it is a polymer(1).