Cork polycyclic aromatic hydrocarbons

In the spirit of both Christmas and recycling, I decided to build a Christmas tree by gluing together wine corks accumulated during this year. In addition, if you needed one, the intermediates of this process make rather cute models of polycyclic aromatic hydrocarbons.

(a) benzene

Colourful Compound Interest

I discovered Andy Brunning’s Compound Interest last year and got absolutely hooked on it – and I don’t even teach chemistry! If, perchance, you do teach chemistry and don’t yet know what CI is all about, then you probably should check it out. (And if you want to use the material in the classroom, you can download the high-resolution PDF files.) The topics range from general chemistry to material science, chemical warfare and everyday compounds. You’ve got answers to many questions you always wanted to ask but never had time to find out for yourself, like, “is it worth (not) to refrigerate tomatoes?”. The Undeserved Reputations section is a perfect antidote to the “oh my God, our food is still full of chemicals” stream of rubbish published by your Facebook friends.

Here are ten the top ten some of my CI favourites.

Metal Ion Flame Test Colours Chart

1. This is how (I’d like to think) I’ve got interested in chemistry. We used to have a gas hob in our kitchen. I loved the fact that the flame was blue. One day, my brother told me that you can make the flame bright orangey-yellow if you sprinkle it with table salt or bicarbonate of soda. “Why?”, I asked. “Sodium”, was the answer. Unsatisfactory as it was, it stayed in my memory. Yes, chemistry won’t be of any interest to me if not for flame and colours.



Colours of Transition Metal Ions in Aqueous Solution

2. When they are not busy burning or, better still, exploding stuff, your archetypal chemists are often imagined (and therefore portrayed; or is it the other way round?) as hiding behind the test tubes filled with colourful solutions. Which is just as well. The test tubes filled with colourless solutions would be really boring.



What Causes the Colour of Gemstones?

3. Who didn’t dream of finding a treasure, that is, a pirate’s chest filled with gold and jewels? Wait. I still dream of that. I remember how surprised I was when, back in elementary school, I read in some book that ruby and sapphire are basically the same mineral corundum, the only difference is in a type of impurity. Well it’s quite an important difference then. Without impurities, most gemstones would be colourless.



The Chemistry of The Colours of Blood

4. My interest in bioinorganic chemistry (even though at the time I didn’t know at it was called that) was also awakened in school, when I learned that some animals have blue blood. I also discovered that, contrary to what anatomy textbooks show, veins do not carry blue blood in humans. I am not sure if I was relieved or disappointed. Later, already in the university, I read about a Soviet-developed fluorocarbon-based blood substitute nicknamed “Blue Blood”. Fascinating stuff.



The Chemicals Behind the Colours of Autumn Leaves

5. I remember, as a child, reading, or rather browsing, an illustrated book about plants (translated from English), with many beautiful colour photographs. “This apple is yellow because of anthocyanin”. Next page: “This apple is yellow because of carotene”. Next page: “This apple is green because of chlorophyll”. The realisation dawned that, apple-wise, being green is not only necessary but sometimes sufficient.

   But what about leaves? When autumn comes, chlorophyll starts to break down and we get to see other pigments in them. Apart from caroteinoids and flavonoids, there are also coloured chlorophyll degradation products, termed “rusty pigments”.



The Chemistry of Stain Removal

6. Sometimes, however, we want to get rid of all these beautiful colours. The infographic shows the chemical methods of achieving that, although I am not sure that “enzymatic stains” is a correct name for stains caused by blood or grass (yes, haem and chlorophyll again!).



The Atmospheres of the Solar System

7. Alchemists associated seven metals with seven planets (which included the sun and the moon). At the time, it seemed to be quite reasonable. Now that nobody expects Mercury to be made of mercury (and, for that matter, Pluto to be made of plutonium), precious little is known about composition of these planets. About their atmospheres, we’ve learned a bit more. Hey, isn’t it amazing that Mercury’s atmosphere has by far highest percentage of molecular oxygen (42%) compared to any other atmosphere in Solar system? We still won’t be able to breathe there though, because its atmosphere is way too thin (its surface pressure is less than 10⁻¹⁴ bar).



The Metals in UK Coins

8. Compared to gemstones, coins are so much duller, especially now that we don’t come across either gold or silver coins any longer. Continuing the alchemical tradition, we can say that modern British coins of 20 pence and higher are mostly from Venus (that is, copper), while 1 p, 2 p, 5 p and 10 p coins are mostly from Mars (i.e. iron). Of course, you can find much more metal variety in commemorative coins.



The Metal Reactivity Series

9. In contrast to their salts, aqueous complexes and gemstones, pure metals do not offer a great variety of colours. Copper is red, gold is yellow and caesium is yellowish; the rest are coming in many shades of grey. But their chemical behaviour is wildly different, as this infographics shows. You don’t need a sophisticated lab equipment or fancy reagents, just water and some (diluted) acids. If there’s no reaction whatsoever, you’ve got a precious metal. Easy!



Analytical Chemistry – Infrared (IR) Spectroscopy

10. Did I tell you that my first love, as far as the world of analytical chemistry is concerned, was vibrational spectroscopy? If not, I’m telling you now. I’ve never got to do any experiment worthy of a publication, because if I did, believe me, it would have been awesome. This infographics reminded me of happy days of my studenthood when I knew and cared more about amide bands (bless them) than about money or my future career.

Periodic Videos

It’s been a while since I posted anything on this blog, but now I’m back.

This is a very cool collection of videos, “a lesson about every single element on the periodic table”. Featuring Professor and a really awesome reaction, here’s one about one of my favourite elements. Yes, iron is in my blood! (In yours too.)

Binary pnictogen halides

Here’s a tricky (trick?) question from the final exam of MITx course Introduction to Solid State Chemistry:

Which compound has the higher boiling point, phosphorus trifluoride (PF₃) or phosphorus pentafluoride (PF₅)?

Naturally, you are supposed to figure this out from the first principles, or rather, from some principles taught in this course, not from Wikipedia (or “by googling”, as some put it).

(a)	(b)

The problem is, the “right” answer, PF₃, is actually, factually wrong. Even though this question cost only two points (of 150), a rather animated debate followed the exam.

Those who defended the “right” but factually wrong answer (a) were proposing that what the problem was testing our thinking rather than actual knowledge, and our thinking should have been along the lines of VSEPR model. VSEPR rules correctly predict PF₃ to be trigonal pyramidal and PF₅ to be trigonal bipyramidal. The dipole moment of a polar molecule PF₃ should make it less volatile than apolar PF₅. Those who chose the “wrong” but factually correct answer (b) were arguing that polarisability of larger PF₅ is higher than that of PF₃ and therefore the London dispersion forces in PF₅ would beat dipole-dipole interactions in PF₃. The (a) party were saying that making the answer you’d get by applying principles different to the one you’d get by “googling” is a good protection against cheating. The (b) party were retorting that this is a silly way of protection, that the question asked was what has the higher boiling point, not what could be expected to have the higher boiling point, and that expecting students to come up with the factually wrong answer is not exactly pedagogical.

Truth to be told, the methods of estimating boiling or melting points of materials were simply not a part of this course. The only thing one could do was to determine whether the molecule has a non-zero dipole moment. But there is no way to figure out which effect will be stronger, the increase in dispersion forces or dipole-dipole interactions.

One would think that the physical properties of such simple compounds as binary halides of Group 15 elements (pnictogens) are studied well and long ago. Not really. I tried to compile a table of dipole moments and melting/boiling points for pnictogen tri- and pentahalides, MX₃ and MX₅, using various resources [1—5]. As you can see, there are still many gaps.

Trihalide	μ (D)	mp (°C)	bp (°C)	Pentahalide	mp (°C)	bp (°C)
NF3	0.234	–207	–129	—
PF3	1.03	–151.5	–101.8	PF5	–93.7	–84.5
AsF3	2.59	–6.0	62.8	AsF5	–79.8	–52.8
SbF3	?	290	345	SbF5	8.3	141
BiF3	?	649	900	BiF5	154.4	230
NCl3	0.6	–40	71	—
PCl3	0.97	–93.6	76.1	PCl5	167	160s
AsCl3	2.15	–16.0	130.8	AsCl5	–50	?
SbCl3	2.75	73.4	223	SbCl5	4	140
BiCl3	4.6	233.5	441	BiCl5	?	?
NBr3	?	?	?	—
PBr3	?	–41.5	173.2	PBr5	<100d	106d
AsBr3	1.66	31	221	AsBr5	?	?
SbBr3	2.47	96	288	SbBr5	?	?
BiBr3	3.6	219	462	BiBr5	?	?
NI3	?	–20s	?	—
PI3	~0	61.1	227	PI5	41	?
AsI3	?	141	400	AsI5	?	?
SbI3	1.58	170.5	401	SbI5	79	401
BiI3	?	408.6	~542	BiI5	?	?

d, decomposition
s, sublimation

What, if any, trends can we see?

• The dipole moment of MX₃ grows larger down the group of the central atom M, e.g. μ(NF₃) < μ(PF₃) < μ(AsF₃), and grows smaller down the group of ligand atom X, e.g. μ(SbCl₃) > μ(SbBr₃) > μ(SbI₃).
• As the sizes of both central atom and ligands go up, so do the melting and boiling points.
• As dipole moments go up, so do the melting and boiling points.

Something curious happens, though, when one crosses the phosphorus—arsenic borderline. AsF₃ has a dipole moment of 2.59 debye. As expected, both mp and bp of AsF₃ are, respectively, higher than those of AsF₅. PF₃, however, has much lower moment of 1.03 D. Both mp and bp of PF₃ are, respectively, lower than those of PF₅. Similarly, mp of AsCl₃ is higher than mp of AsCl₅, whereas mp of PCl₃ is lower than mp of PCl₅. Similarly... but no, there are too many gaps in the table “down there”.

Which shows, by the way, that “googling” does not help if the data is not available. For the future, the course authors may consider asking a very similar question about a pair of compounds from “down there”. Thus the whole conflict between the (as yet unknown) “truth” and “expected answer” could be easily avoided.

Since the electronegativities decrease down the group for both M and L, the most polar M—L bond must be Bi—F bond and BiF₃ should have the largest dipole moment. Well I couldn’t find its value. But it is known that bismuth trifluoride has ionic structure, and has the highest melting (649 °C) and boiling (900 °C) points of all binary pnictogen halides. On the other side of the spectrum, we have extremely sensitive nitrogen triiodide. A feather tickle, a loud noise and, I suppose, any attempt to measure its dipole moment will set off an explosive decomposition (see the video below):

2 NI₃ → N₂ + 3 I₂

References

1. Earnshaw, A. and Greenwood, N. (1997) Chemistry of the Elements, 2^nd Edition. Butterworth-Heinemann, Oxford.
2. Nelson, R.D., Jr., Lide, D.R., Jr. and Maryott, A.A. (1967) Selected values of electric dipole moments for molecules in the gas phase. National Standard Reference Data Series — National Bureau of Standards 10, Washington, DC.
3. Cotton, S. (2001) Nitrogen triiodide. Molecule of the Month collection, University of Bristol.
4. WebElements
5. atomistry.com