In class today, I gave you a special name for 1,2-diols: glycols. Since a 1,2-diol requires a minimum of two carbon atoms, the simplest compound in this class is ethylene glycol, HOCH2CH2OH.
It turns out that one glycol has been in the news a lot recently. During lunch I skimmed through a recent article on hair straighteners that appeared in C&ENews, the weekly news magazine of the American Chemical Society. The article had this to say:
“Straightening techniques such as Brazilian Blowout originated in Brazil, where they’re called escova progressiva
(progressive blow-dry). They provide a few months’ worth of
straightening before wearing off. That staying power comes from fusing
additional amino acids from keratin to the hair fiber. A cross-linking
reagent, such as a solution of formaldehyde gas in water that can go by
the name of formalin or methylene glycol, accomplishes the fusion.”
Methylene glycol, HOCH2OH, contains only one carbon so it is a 1,1-diol. Because this name seems to break the rule that I had stated in class (glycol = 1,2-diol = vic-diol), I decided to do a little more leg work. First, I checked our textbook. Loudon, p. 323, states that glycols contain two hydroxyl groups on adjacent carbons. The Encyclopaedia Britannica gives a looser definition, saying that a glycol is any diol in which the hydroxyl groups are attached to different carbons. Common sense application of these definitions seems to suggest that “methylene glycol” is a misuse of the term “glycol” because this one-carbon compound cannot contain “adjacent” or “different” carbons. This probably won’t keep people from using this term, however, because it sounds very scientific and it is fairly well entrenched.
So why was I reading an article about hair straighteners and why is one “glycol” so newsworthy?
It turns out that the news story has a local twist. Oregon’s OSHA visited several dozen hair salons around the state and tested bottles of a hair straightener called Brazilian Blowout. They were looking for formaldehyde, H2C=O, and the levels they found were sufficiently alarming (formaldehyde is a cancer-suspect agent) that the story even made it to National Public Radio. After all, would you want your hair to be dipped in a cancer-suspect agent? Maybe of greater concern, if you worked in a hair salon, would you want to spend your working day being exposed to an agent like this?
I can’t address the health issues of formaldehyde/methylene glycol, but the chemical connection between these compounds is something that I have been teaching students about for over two decades so I was fascinated by some of the claims made in a blog post by Doug Schoon on a web site called Personal Care: Information Based on Scientific Facts. FYI – this web site claims to separate “truth” from “scare” when it comes to personal care products by providing visitors with “scientific facts”.
The Personal Care web site lists Mr. Schoon as one of its “experts” and his biography impressively states that he is “an internationally-recognized scientist, author and educator with over 30 years experience in the cosmetic, beauty and personal care industry. He is a leading authority, known for his technical and regulatory work that has helped shape the beauty industry. He is Co-Chair of the Nail Manufacturers Council (NMC), and as Creative Nail Design’s (CND) Chief Scientist was head of the R&D laboratory, QA, and Field Testing/Evaluation departments for almost 20 years.”
Here is what he wrote (in part) about “methylene glycol” vs. “formaldehyde” on Oct 14, 2010 (“Doug Schoon Responds to Misleading Claims by Oregon OSHA That “Methylene Glycol” Is a Synonym for “Formaldehyde”):
“Methylene Glycol and Formaldehyde are very different, both chemically and physically! Methylene Glycol is a liquid; Formaldehyde is a gas. … in 1972, both Methylene Glycol and Formaldehyde were assigned different CAS registry numbers indicating the American Chemical Society also believes these are different and unique chemical substances. Chemists with an understanding of organic chemistry will agree, whatever their opinion about these substances, that Methylene Glycol and Formaldehyde are two completely different chemicals.”
Because I count myself as a “chemist with an understanding of organic chemistry,” I thought it might be interesting to compare my actual understanding to Mr. Schoon’s claims about my understanding:
- First, I understand that a molecule of methylene glycol is quite different from a molecule of formaldehyde. A molecule of methylene glycol is HOCH2OH. A molecule of formaldehyde is CH2=O. Different compositions, different formulas. They are not even isomers.
- How about the claim that “Methylene Glycol is a liquid”? I have to disagree with that. To my knowledge, no one has ever prepared or worked with a liquid sample of methylene glycol. The compound is generated when formaldehyde gas is added to water and exists as one component of this aqueous solution. I don’t know of any way to separate methylene glycol from the other components of this solution (see below), so I don’t think it can exist as a pure liquid. Mr. Schoon may have come by this notion by checking the boiling temperature listed for methylene glycol in CAS (Chemical Abstracts Service). However, this boiling temperature is a computer-generated prediction and not an experimental measurement. CAS does not list an experimental value for the boiling temperature of methylene glycol. Given the fact that methylene glycol is an easily prepared substance with a long long history, the lack of an experimental value strongly suggests that the substance does not exist as a pure liquid. Sorry Mr. Schoon.
- His next claim concerns how the CAS (not the ACS!) assigns registry numbers to substances. His argument is that different CAS registry numbers imply “different and unique chemical substances,” but this is simply untrue. Registry numbers are code numbers for identifying substances in the CAS database. While different substances cannot share the same registry number, it is quite common for different registry numbers to be assigned to the same substance. Methylene glycol provides a perfect example. One registry number for methylene glycol, HOCH2OH, is 463-57-0. Another registry number for methylene glycol is 596111-13-6. And there are several more registry numbers for this substance as well. The distinguishing features captured by these registry numbers turns out to be the isotopic composition of the substance, something that has almost no effect on its chemical or physical properties. 463-57-0 refers to methylene glycol without isotopes specified. 596111-13-6 refers to methylene glycol that contains C-13 (note: 463-57-0 would contain some of this too). Registry numbers prove nothing and I would have expected someone with Mr. Schoon’s background to know better.
- The real test, however, lies with his final claim, namely, that “chemists with an understanding of organic chemistry will agree that Methylene Glycol and Formaldehyde are two completely different chemicals.” We can argue about what “chemical” might mean in this context, but for me it is quite simple: if I am exposed to methylene glycol, will I be exposed to formaldehyde? The answer to this is unequivocally YES. As I noted above (and as stated in the C&ENews article), the easiest way to make methylene glycol is to add formaldehyde to water. This equilibrium, Formaldehyde + Water = Methylene Glycol, is completely reversible at room temperature and methylene glycol spontaneously decomposes to make formaldehyde + water if one attempts to remove either formaldehyde or water from a solution of methylene glycol. The three ingredients necessarily go together. I can think of no simpler way to expose a person to formaldehyde than to expose them to a methylene glycol solution.
The bottom line: methylene glycol may have a different molecular structure from formaldehyde, but it readily forms formaldehyde and must be considered a cancer-suspect agent in its own right. To say methylene glycol is not a “synonym” for formaldehyde is like saying that Clark Kent is not a “synonym” for Superman because Clark Kent wears glasses and Superman wears a cape. Unfortunately, web sites that claim to separate “truth” from “scare”, and to provide “scientific facts” about important topics like formaldehyde exposure, have no incentive to actually get things right. Just the opposite. Let the reader beware.
Luc did some additional legwork on ‘glycol’ and dug up the following IUPAC definition of ‘glycol’: “Dihydric alcohols, also known as diols, in which the two hydroxy groups are on different carbon atoms, usually but not necessarily adjacent.” (from http://www.chem.qmul.ac.uk/iupac/class/)