Field of Science

One ring to bind them all: things you might not know about benzene

Space filling model.  c. Michelle Francl
UPDATE:  The Sceptical Chemist has a quiz up on benzene structural formulae based on the article in Nature Chemistry.  

Next year is the 150th anniversary of Kekulé's paper proposing the cyclic nature of the benzene 'nucleus.' and the 190th anniversary of Michael Faraday's paper on bi-carburet of hydrogen, more familiarly known as benzene.  In Berlin in 1890, on the 25th anniversary, German chemists threw the Benzolfest in honor of Kekulé.

I wrote a short 'fest' for Nature Chemistry, which appears in the January issue($).  I learned much about organic chemistry's iconic molecule, and herewith are 10 pieces of trivia that did not make it into the final piece.

1. There are 217 skeletal isomers of benzene, including the iconic hexagon, more than 300 if you count stereoisomers. [Patrick W. Fowler, MATCH Commun. Math. Comput. Chem. 63 (2010) 333-346].

 2. Benzene tastes sweet, with a hint of almond, and can be tasted in water starting at about 0.5 ppm, or roughly 1 drop in 40 gallons.

 3. An alternate synthesis (circa 1850) claims to passes benzoic acid through a “red hot gun” barrel packed with pumice. There's no evidence that it actually works.

 4. There is a tie knot that is hexagonal, perfect for tie-wearing chemistry conference attendees.  It was discovered by mathematicians, but you can still wear it.

 5. Dewar never suggested benzene's preferred structure was bicyclic. He used the structure, along with the more conventional structure, to demonstrate the potential of his brass molecular models. ["On the oxidation of phenyl alcohol, and a mechanical arrangement adapted to illustrate structure in the non-saturated hydrocarbons," Proceedings of the Royal Society of Edinburgh 6: 82–86 (1867), you can find it on Google Books]

6. The extraction of flowers of benzoin - benzoic acid - was first reported by Michel de Nostredame, otherwise known as Nostradamus, in the 16th century.

7  Kekulé actually didn't didn't present his revolutionary theory in January of 1865, his colleague Adolphe Wurtz read the paper in his absence.

8.  As late as the 1920s, major figures in organic chemistry did not think benzene or its derivatives were planar.  Sixty four years after Kekulé's paper was submitted (almost to the day), crystallographer Kathleen Lonsdale submitted her paper showing unequivocally that hexamethyl benzene, and presumably therefore benzene, was planar. "Thus the 'diamond' [puckered] type of benzene ring is shown to be wholly inadmissible." (Emphasis is Lonsdale's.) [K. Lonsdale, Proc. Roy. Soc. A, 494-515, 1929.]

9. In the days before ChemDraw, there were stamps.

10.  Hexagons were not the only proposed structural formula for benzene.  Loschmidt suggested the representation at the right.

If you would like a reprint of the essay, send me an email.

Tea transitions

Tea bag is paper. Vapor is from phase
transition of water
Introductory chemistry at my college has a broad audience.  My students are not just potential chemistry major and pre-meds, but I teach geologists, art historians, psychologists and even students who simply want to know more about how the hidden world of atoms and molecules works.

I tell them my job is to help them transition from being science students to scientists -- or rather into people who can critically assess and use the science information they encounter.  Can they filter the junk from the reality?

Enter the Food Babe, whose stirs up real science with fake science and then peppers it with a healthy dose of panic to produce posts that are at a perfect level for my students to read and try to tease out fact from fiction.1  I couldn't do a better job if I made them up.

This example is about my favorite beverage:  tea.  Are plastic tea bag safe, wonders the Food Babe?

To quote:
"Another temperature consumers need to worry about in tea is the “glass transition” temperature. Here’s the science behind the glass transition temperature or, Tg, and why it becomes dangerous according to The Atlantic: 
“That is the temperature at which the molecule in certain materials such as polymers begin to break down. As a rule, the Tg of a material is always lower than the melting point. In the case of PET and food grade nylon (either nylon 6 or nylon 6-6), all have a Tg lower than the temperature of boiling water. For example, while the melting point of PET is 482 degrees Fahrenheit, the Tg is about 169 degrees. Both nylons have a lower glass transition temperature than PET. (Remember that water boils at 212 degrees.) This means the molecules that make up these plastic tea bags begin to break down in hot water.” (Emphasis is the Food Babe's.)

Since we just finished looking at ways to display information about the phases of materials, this is a great piece to dissect.

Phase diagrams are a compact way to show not only the state a material might be in under a given set of pressure and temperature conditions, but the astute reader can pull some information out about the energies required to get a material to change state, and even something about the forces that hold a material together.

The iconic phase diagram in general chemistry is the one at the right for carbon dioxide.  The diagrams are not difficult to read.  The solid lines indicate conditions where two phases are present, in equilibrium.  So at normal atmospheric pressure (1 atm) and -78.5oC, solid CO2 ("dry ice") and gaseous CO2 are in equilibrium with each other.  At normal pressure, if you heat solid CO2 above  -78.5oC, it will change from a solid directly into a gas without melting, it's icy cold, but doesn't produce a liquid when it warms.  It's a "dry" ice.

When a material changes phase, it's important to understand that the core identity of the molecule doesn't change. That is, the structure of the individual molecules does not change, molecules do not 'break down' during a phase change. The CO2 molecules in the solid are just like the CO2 molecules in the gas.  What changes in a phase transition is the arrangement of the molecules, their relationships to each other.  In the gas phase, the molecules are widely separated and moving randomly, while in the solid they are arranged in a crystalline symmetrical array, and don't move much.

There are more possible phases for a material than the familiar solid, gas or liquid.  Many materials have several different ways for the molecules to pack themselves into a solid.  (Water has more than a dozen different types of ice, with different densities and other macroscopic properties.)  Some solid forms are crystalline, with all the molecules neatly arrayed.  Others show less order on the molecular level. These forms we call amorphous solids.  Some amorphous solids are called glasses.  At the glass transition temperature, a nice crystalline solid can change into a less ordered glass form.  For a polymer like the PET, the glass transition temperature is when (on the molecular level) the molecules become less rigidly ordered.  On the level we can observe, it means the polymer becomes less brittle, more malleable.  Softer.

It does not mean that the (presumably dangerous) molecules break down and leach into your tea water.  If there is a danger lurking in your fancy PET tea bag (and there might be) - this isn't it.

1.  The classic example is about airplane travel and gases, where she tries to get people incensed about the 'fact' that the cabin air contains nitrogen because the airlines are too cheap to use pure oxygen, and about pressurized cabins.  It's not only wacky fake science, but a bit scary because she suggests medication doses and timing will have to be adjusted due to the nitrogen content.  (Do not take medical advice from the Food Babe.  She doesn't even know what is in the air she breathes.)

You can't find the airplane air post on her blog anymore, like her infamous microwave post ("For the experiment pictured above, microwaved water produced a similar physical structure to when the words “satan” and “hitler” were repeatedly exposed to the water.") it's been taken down. She's blocked caching, but you can read it here.