Winter Chemistry or: Everything You Ever Wanted to Know About Ice, and Then Some

Foreword:

Hello all!  It’s been a busy holiday season full of travel, friends and family, interviews, and surprisingly little chemistry.  As such, this blog got pushed to the backburner so to speak.  But I’m back!

I’ve heard it’s been quite cold in the Northeast this last week.  While I have been rocking a t-shirt in Southern California, temperatures in the Boston area have inched into the sub-zero range, with parts of the Midwest reaching record-breaking lows.  Heavy snowfall has impeded holiday travel in much of the northern parts of the country, and iced-over roads have made driving difficult.

Bummer.
Photo credit: Yoon S. Byun, Boston Globe

 

It’s that ice part we’re going to talk about this time.  We skate on it, put it in our drinks, compress it into balls and throw it at each other, and curse Old Man Winter while we chip it off the windshields of our cars.  It is a ubiquitous formation on our planet, so let’s take a few minutes to learn about its structure and properties.

Ice, Ice, and More Ice

Did you know there 17 different documented forms of ice?  For you Vonnegut fans out there, Ice-Nine actually does exist; however, it (thankfully) won’t destroy all life on earth.  That being said, the vast majority of the ice you encounter is Ice Ih (pronounced “ice one-h”).  In fact, unless you’ve been to one of the poles where temperatures can reach -100⁰ C (-148⁰ F), that’s most likely the only ice you’ve ever seen.  The subscript “h” stands for “hexagonal” and denotes how individual molecules of water bond together in a crystal lattice.  It turns out that ice formation is very sensitive to temperature and pressure.

You may have seen a similar diagram in high-school chemistry. There are a few more phases of water than your teacher told you about.

 

In addition to the typical hexagonal lattice structure, ice can also form cubic, rhombohedral, and tetragonal geometric configurations; ice can even freeze in a completely disordered manner, forming amorphous ice.

 

Hexagonal ice lattice structure.

Hexagonal ice lattice structure.

Why is Ice Slippery?

At face value, this seems like a question a toddler might ask, just after inquiring about “why the sky is blue.”  But if the answer was as simple as “because it is,” I wouldn’t be talking about it here.  Believe it or not, this topic has been the subject of scientific debate since the mid-nineteenth century.  It has long been known that solid water (ice) is less dense than its liquid counterpart.  Indeed, water is one of only half a dozen or so substances known to exhibit this property.  It therefore follows that compressing ice will lower its freezing point, causing it to melt.

The Kelvin brothers (for whom the temperature scale is named) theoretically outlined and experimentally verified that water’s freezing point is dependent on external pressure.  This offers one explanation as to why ice-skating works.  The blade of an ice-skate is very thin, and exerts force on only a small area, resulting in very high pressure.  In 1886, an engineer by the name John Joly worked out that the pressure an ice-skater would exert on the ice would be close to 500 times atmospheric pressure, enough to lower the freezing point of ice from 0⁰ C to -3.5⁰ C.  This would allow regional melting of ice directly under the ice skater’s blade.

You may notice a problem here: how is it possible for people to ice skate in temperatures below -3.5⁰ C?  And never mind ice skating, how is ice slippery when you’re walking on it?  Your shoes certainly don’t generate enough pressure to melt ice.  As it turns out, very, very cold ice isn’t slippery on its own.  If you were to run your hand over it, the texture would be similar to sandpaper.  In fact, there is a fair amount of friction between the bottom of your shoe or the tire on your car, and ice.  This friction generates heat, just like when you rub your hands together for warmth, and the heat can be sufficient to melt a thin layer of ice.

We now have two explanatory models for why ice is slippery, but we remain with one problem.  Ever tried standing on a patch of black ice?  Even though you’re not moving, it’s still slippery.  Since there’s no movement, there isn’t any heat generated by friction.  So there must by a third model to explain this phenomena.  In 2002, scientists at the Lawrence Berkeley National Lab demonstrated that a thin layer of liquid water exists on top of ice.

This layer persists down to temperatures of about -20⁰ C (-4 ⁰ F), at which point water molecules do not have sufficient energy to break out of their crystal lattice structure.  See, water molecules are held together by electrostatic forces, similar to how a balloon will stick to your hair if you rub it against something.  In the case of water, this force is called a hydrogen bond.  When solid ice interacts with air, the molecules in air disrupt the formation of a crystal lattice, resulting in a thin film of liquid-like water.

This layer was measured to be approximately one hundred-millionth of an inch thick, 6 water molecules across.  If the sole of your shoe were scaled to be as large as the tallest building on earth (Burj Khalifa, 2,722 ft) then the water layer would still be ten-thousand times thinner than a penny on the sidewalk.  

Seriously, I did the math.

That penny is a long way down.

So there we have it: seventeen different types of ice, condensed matter physics, pressure, friction, equilibrium, and intermolecular forces.  All that to explain the chemistry of why ice is slippery.  I hope you’ve found this post interesting and informative, and I’ll do my best to keep the chemistry coming.

Cheers,

Mitchell

Sources:

https://archive.org/stream/scientificprocee5188687roya#page/452/mode/2up

http://iopscience.iop.org/0953-8984/14/8/108/pdf/0953-8984_14_8_108.pdf

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All Bad Things…

Heads up: there are some chemistry terms you may or may not be familiar with in this post.  I’ve tried my best to explain as I go, and make everything as self-explanatory as possible.  If you find yourself lost, head over to the new glossary section, where I have compiled some simple definitions.

Part Three

Up until this point, the synthetic chemistry presented in Breaking Bad has been quite factual.  Conversion of pseudoephedrine to d-methamphetamine using reagents mentioned in the show is a well-known and documented synthesis.  You probably guessed there’s a “but” following the previous statement.  I’ll get to that, but first let’s talk about the synthetic route I propose Walt most likely used.

It is revealed in season one, in the episode “A No-Rough-Stuff-Type Deal,” that their process involves phenyl-2-propanone or phenylacetone, a chemical which Walt and Jesse initially make in a tube furnace.  Phenylacetone is a prochiral compound, meaning that while it is not chiral itself, it can be made chiral after only a single chemical reaction.

Chirality visual

A carbon with four bonds generally takes the shape of a tetrahedron (left) with the carbon in the center, and the four bonded substituents at the peaks of the tetrahedron. Skeletally, this is represented in the middle image. A carbon is said to be chiral if the four substituents it is bonded to are all different (right).

And that chemical reaction involves methylamine, a difficult to acquire chemical that is central to the plot of several episodes in the series.

“We’re going to use reductive amination to yield methamphetamine.  Four Pounds.”           -Walter White

This makes the homework pretty simple.  Walt and Jesse treat phenylacetone with methylamine, a reaction which yields an intermediate called an imine.

synth of imine

Phenylacetone or “P2P” (left) is treated with methylamine (above arrow) to yield an imine intermediate, shown in square brackets.

The process also releases one molecule of water for every imine formed.  You’ll notice the intermediate compound very closely resembles methamphetamine, except for one key detail: the carbon-nitrogen double bond.  With that bond in place, the intermediate is not chiral, and it certainly isn’t methamphetamine.  Luckily, we haven’t yet done the “reductive” part of the reductive amination.  If a mild reducing agent is added to the mixture (usually either gaseous hydrogen or sodium cyanoborohydride), methamphetamine results.

reduction

Reduction of the imine intermediate with hydrogen gas (or a number of other reagents) yields racemic methamphetamine.

Those of you following along since part one may notice a problem here.  We have indeed synthesized methamphetamine; however, we have done so as a racemic mixture.  That is, we have a mixture of dextro and levorotary methamphetamine.  A fifty-fifty mixture, in fact.  Then how is it that Walt claimed to produce 99.1% enantiomerically pure methamphetamine if the reaction cannot possibly do any better than 50%?  The short answer is, we don’t know.  They leave that part out of the methods described in the show.  At one point, Walt sends Jesse to acquire “40 grams of thorium nitrate,” which has catalytic uses, but no documented use in asymmetric synthesis.

From here on out, this discussion is purely speculative.  There is literature available on enantioselective reductive amination processes.  Instead of using either hydrogen gas or sodium cyanoborohydride, as mentioned above, a chiral hydride source could be employed.  However, the highest enantiomeric excess (ee.) found in literature for these products is only slightly better than 70%.  And if Walt could do better than that, so can we.  Certain chiral metal complexes, such as those of rhodium and titanium, have been demonstrated as incredibly expensive ways to achieve 90%+ selectivity.  But those catalysts would cost more than the meth would sell for.

There are some Lewis base catalysts that might do the job: they are relatively inexpensive, but the best yields are only in the 80% range.  After searching exhaustively, I found one procedure that might do the trick, but it’s going to cost you.  Using a catalytic mixture of (get ready for this) 1,1’-Bis{(S)-4,5-dihydro-3H-binaphthol[1,2-c:2’,1’-e]phosphino}ferrocene and Bis(1,5-cyclooctadiene)diiridium(I) dichloride you might be able to break into that ever elusive 99%+ range of purity.

catalysts

If you want to make Walt’s meth with his purity, you’ll need these catalysts, or similar ones. They aren’t cheap.

Therefore, the only possible conclusion is that Walter White is in fact a wizard.

I hope you’ve enjoyed reading about Breaking Bad chemistry; I’ve certainly enjoyed writing about it.  Hopefully, I’ll bring you all some new content early next week.  Thanks for reading!  And if you have any questions, concerns, or suggestions on what I should tackle next, check the about page for my contact info.

Cheers,

Mitchell

Sources:

Angew. Chem. Int. Ed. 2001, 40, 3425

Organometallics 1998, 17, 3308

Angew. Chem. Int. Ed. 1990, 29, 558.3

My Baby Blue

Prologue

When I started writing this, I planned on it being a single post covering some background about organic chemistry methods discussed in Breaking Bad.  But more importantly, how accurate those methods are.  However, I soon realized covering everything I wanted to would turn this post into an essay.  So I decided the best course of action would be to break it into parts.  Part one will cover the structure and properties of the drug, while parts two and three will discuss its synthesis in regards to the show.

Part One

Welcome back, faithful viewers.  As promised, this time around I will tackle (with no spoilers!) some of the chemistry from the series Breaking Bad.  For those of you living under a rock for the last five years, Breaking Bad chronicles protagonist Walter White’s descent from beloved high school chemistry teacher to methamphetamine-producing drug kingpin “Heisenberg.”  You also may be quite tired of your friends’ raving reviews about the show, and probably wish they’d shut up about it already.  But I digress.

Let’s briefly discuss the structure and properties of meth.  This is methamphetamine:

meth full formula

Or more succinctly (in what chemists call “skeletal formula”):

meth skeletal

You’ll notice the wavy line representing a carbon-carbon bond.  A wavy line exists because there are two possible enantiomers of methamphetamine.  The dash (left) represents a bond going into the screen, while the wedge (right) represents a bond sticking out at you. Remember, organic molecules are three-dimensional, not flat.

enantiomers

We differentiate these two enantiomers with the Latin prefixes dextro and levo, or simply d and l for short.  The structure on the left is levo-methamphetamine.  It’s rather innocuous and there’s a good chance you’ve unknowingly used it before; it goes by the brand name Vick’s VapoInhaler.  The one on the right is dextro-methamphetamine, and that’s the one favored by recreational methamphetamine users.  A seemingly minor difference in structure turns a highly addictive and very destructive drug into a nasal decongestant.

Methamphetamine is a psychostimulant, belonging to the broader class of psychoactive drugs.  It acts on the central nervous system by increasing the amount of dopamine in the body, causing a variety of physical and psychological effects; hyperactivity, alertness, headaches, increased libido and self-confidence, heart palpitations, psychosis, stroke, and paranoia are just a few of the many effects induced by methamphetamine usage.

Recreationally, the drug is introduced to the body virtually any way you could imagine, and in at least one way you probably can’t.  Meth tablets were given to pilots during WWII to help them stay awake on long bombing runs, and it’s currently prescribed to treat attention deficit hyperactivity disorder (ADHD) and as a last-ditch treatment for severe obesity.

Look out for my next post, where I’ll be talking about Walt and Jesse’s first cook in that run-down Winnebago.