Part 24: The Chem in SpaceChem - Round 7
The Chem in SpaceChem, Round 7Note: I wrote a post about the chemical structure of nitrous oxide, in case you’re interested.
14: LP Isomers
Input molecules: 1-butene
Output molecules: 2-butene
Net reaction: C4H8 --> C4H8 (isomerization)
1-butene, C4H8
Melting point: -185 °C, -302 °F
Boiling point: -6.5 °C, 20 °F
Molecular mass: 56.11u
Density: 2.37 g/L
1-butene is stable by itself but it is highly flammable. It will react with quite a lot of other stuff as well, and it will exothermically polymerize. It is produced by cracking of crude oil, and used to make many chemical products, such as solvents, plastics, resins, adhesives and so on.
Like many simple hydrocarbons found in oil, it’s a precursor for a lot of things, but there’s not that much to say about the substance itself.
2-butene, C4H8
Melting point: -139 °C, -219 °F (cis); -106 °C, -158 °F (trans)
Boiling point: 3.7 °C, 39 °F (cis); 1.1 °C, 34 °F (trans)
Molecular mass: 56.11u
Density: 2.42 g/L (cis); 2.44 g/L (trans)
There are two stereoisomers of 2-butene, with slightly different properties. The cis form has the two CH3 groups pointing in the same way (i.e. if the double bond is horizontal, they are both pointing upwards), the trans has one pointed in the opposite direction of the other.
2-butene is stable by itself but it is highly flammable. It is produced by cracking of crude oil, and used to make several chemical products, such as solvents and synthetic rubber. Its main use (probably together with 1-butene) is to produce octanes, an important component of gasoline.
Feasibility of the reaction
Feasibility: high.
This reaction will not go by itself, but adding a simple metal or metal oxide catalyst and some heating will do the trick. It will go even better if the catalyst can lend hydrogens (Well, H+). The basic reason for that is not hard to understand: if the hydrogen can first bind to the carbon that is going to lose the double bond, it’s easier for the double bond to go and ‘jump’ to the middle carbons. To get the product, the third carbon then has to lose an hydrogen atom again.
Reaction energy: Slightly exothermic, about -12 kJ/mol
There’s quite a lot of different numbers for this reaction floating around. I guess the value depends on the way it’s measured. They all agree that it’s a tiny number, though. 12 kJ/mol is really low, it basically means that there’s barely any energy change. That’s to be expected: 1-butene and 2-butene are of course very similar molecules.
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15: Natural Chemo
Input molecules: ‘Acetone Enolate’
Output molecules: Oxetane
Net reaction: C3H6O --> C3H6O (isomerization)
’Acetone enolate’, propen-2-ol, C3H6O
Melting point: -94 °C, -138 °F
Boiling point: 56 °C, 133 °F
Molecular mass: 58.08u
Density: 0.791 kg/L
All these values are for acetone.
Why did I pick the acetone values? Because our substance here is a tautomer. A tautomer is yet another kind of isomer, one that spontaneously switches forms. In any sample of acetone, there is a small amount (less than 1 in a million molecules) of the enol present. Some enols can be purified, but they typically react back to the ketone form quickly. It’s not possible to measure the physical properties of the enol itself, you’d always measure (mostly) acetone.
Note the difference between tautomers and resonance forms, which I already talked about. Resonance forms are different ways to draw a single molecule that actually exist as a hybrid. Tautomers are actually different structures, but the molecule easily switches between them.
The –ate in the name refers to an ion. It is the enol form after it loses an H+. When the tautomerization is catalyzed by a base, the enolate will exist during the reaction as a very short-lived intermediate. The molecule used in the reaction is actually the enol itself, in this case propen-2-ol.
Anyway, even though less than one in a million acetone molecules is an enol at any one time, the form is very important in chemistry, because it can react in ways a ketone can’t. For instance, it’s involved in making bromoacetone, and it’s also possible to bind something else to the O in the enol form. Equilibrium rules tell us that when the enol form reacts, more acetone will turn into propenol, keeping the reaction going.
Thus, acetone is a common building block for more complex organic molecules. Its main use is as a solvent, though. Acetone has the advantage that compared to other organic solvents, it’s not very toxic. Outside the lab, it’s an important ingredient in nail polish remover, glue remover and paint thinner. If you accidentally use a permanent marker on a whiteboard, you can probably remove it by rubbing it with some acetone (try this at own risk, as it might slightly damage the whiteboard).
Watch out, though, because it can also dissolve a bunch of synthetic solids (such as plastics). I heard that it can dissolve contact lenses, and if you get acetone in your eye while you’re wearing them, the lens will actually fuse to your eye and you need to get it surgically removed. Sounds very painful to me. This is one of the reasons why wearing contact lenses in the lab is discouraged.
Oxetane, 1,3-propylene oxide, C3H6O
Melting point: -97 °C, -143 °F
Boiling point: 48 °C, 118 °F
Molecular mass: 58.08u
Density: 0.893 kg/L
I can’t find too much information about oxetane itself. The C-O-C group makes it an ether, which are rather stable molecules. Still, 4-membered rings are strained, increasing reactivity. It reacts with strong Lewis acids, in which case the ring opens. In another ring-opening reaction it polymerizes. Other than that, I found it has an ‘agreeable odor’ .
Oxetane is usually formed by reacting KOH with 3-chloropropylacetate. Organic reactions are usually described by moving electrons around, and the chlorine will gladly hold an extra electron. An important property of ring molecules such as oxetanes is that substituents (side-groups other than a hydrogen) make the ring form easier. For other molecules that contain an oxetane group, other routes of production are commonly used.
And those molecules are quite important, especially in medicinal chemistry. The oxetane group stabilizes the molecule, so it doesn’t get metabolized as fast in the human body, and more molecules will reach the target. Wikipedia notes that the group appears in Taxol/Paclitaxel, a large and complicated molecule that is naturally formed in yew trees (Taxus). This molecule inhibits cell division and has been used as chemotherapy treatment for years. I guess that’s what ‘Natural Chemo’ is referring to. Anyway, scientists tried to synthesize Taxol artificially, but this proved very difficult. It is produced nowadays by a Taxus cell strain growing in a lab, which is a much easier and safe method.
Feasibility of the reaction
Feasibility: low.
It’s not going to work like this. You’d need to completely debond one of the carbons and reattach it elsewhere. No way to do that in one step.
Interestingly, acetone can be used to make oxetane compounds, using the ‘Paternò–Büchi reaction’. This reaction is a photocycloaddition. You first have to use light of a certain wavelength to excite acetone and temporarily weaken the double bond. Then it can react with an alkene to form an oxetane with a bunch of methyl-groups or larger substituents.
Alternatively, you could use the enol form of a larger ketone or an alcohol with another functional group that can leave (a second –ol works, in that case water will be produced). In the right conditions, such a molecule will react with itself, forming a ring. Again, with substituents. It doesn’t seem you can make just oxetane with these reactions. You need the common production route with 3-chloropropylacetate.
Reaction energy: Endothermic, about 138 kJ/mol. Based on acetone, as the enol form can’t be measured directly. Both molecules in gas phase.
Oxetane has more stored energy than acetone. This is probably at least partially due to the strain in the ring. It takes energy to bend a 4-membered ring into shape.
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Challenge 7: The Chem in SpaceChem
Input molecules: Maleic acid
Output molecules: 1,4-dioxane-2,3-dione
Net reaction: C4H4O4 --> C4H4O4 (isomerization)
Maleic acid, cis-butenedioic acid, C4H4O4
Melting point: 135 °C, 275 °F (decomposes)
Molecular mass: 116.07u
Density: 1.59 kg/L
Maleic acid is another great example of cis/trans stereoisomerism. trans-butenedioic acid has a completely different name: fumaric acid. That’s because the two forms have very different properties. Fumaric acid doesn’t melt until somewhere above 200 °C. In maleic acid, the acid groups (O=C-O-H) are close enough to form internal hydrogen bonds, making it soluble in water. Fumaric acid won’t dissolve in water.
Maleic acid is commercially prepared from maleic anhydride C4H2O3, which is formed by partial oxidation of benzene or butane. It doesn’t have much use by itself, and most of it is converted into the fumaric acid isomer, which happens readily with the right catalyst. Fumaric acid is used as a medicine and as a food additive (it has a fruit-like taste). It also occurs naturally in many plants. It is an intermediate in the citric acid cycle and urea cycle in humans, and it's formed in the human skin when exposed to sunlight.
1,4-dioxane-2,3-dione, ethylene oxalate, C4H4O4
Melting point: 13 °C, 55 °F (predicted and probably wrong, see below)
Boiling point: 287 °C, 549 °F (predicted)
Melting point: Above 50 °C, 122 °F (possibly)
Boiling point: None, it polymerizes instead. (possibly)
Molecular mass: 116.07u
Density: 1.42 kg/L (predicted)
It’s really hard to find information about this molecule. It exists, that I know. A site [1] that’s often useful for melting and boiling points only lists the temperature of the triple point, which is a single point at a certain temperature and pressure at which solid, liquid, and gas can exist at the same time. The temperature given is 142 °C, 287 °F, but that doesn’t tell us anything if we don’t know the corresponding pressure, which I don’t.
Instead I used a website that lists theoretically predicted data. This is less accurate than actual measurements, so I’m not sure if those values are correct.
I did find that ethylene oxalate is used to make a biodegradable polyester (a polymer containing ester groups, which makes sense, as the molecule is already a di-ester). The article [2] implies that when ethylene oxalate forms, it immediately forms oligomers, which are like polymers but with only 2-4 molecule units (polymers have at least hundreds of units). The oligomers can be broken down to single molecules by heating them in a polar organic solvent. Interestingly, they say that the molecule was still solid at at least 50 °C. They simply heated ethylene oxalate in an inert atmosphere to 170 °C to get the polymer. It first melted, then the polymerization started.
If this data is right, the melting point should be somewhere between 50 °C and 170 °C, and there is no boiling point, because the compound polymerizes first. I don’t know what’s true, but I do know that you should never blindly trust predicted data.
Feasibility of the reaction
Feasibility: low.
Once again, it’s not going to happen. You need to shift so many bonds around. What we need is not maleic acid, but oxalic acid, (ethanedioic acid), which has only two C’s between the double bonded O’s, like in the product. According to [2], a normal esterification would work: react a carbonic acid with an alcohol, get the ester and water. In this case, you would need 1,2-ethanediol (ethylene glycol).
However, there’s nothing keeping one of the alcohol groups from reacting with one oxalic acid molecule, and the other one reacting with another. That way you get oligomer side products... which is what the article said, so that makes sense. To depolymerize those into ethylene oxide monomers, an extra step is needed.
Reaction energy: Endothermic, about 92 kJ/mol .
Yes, for some reason the heat of formation for ethylene oxide is listed. It’s hard to say why this one is endothermic. Esterification is exothermic, but we are breaking a bunch of C-C bonds too (replacing them with C-O bonds), which probably costs energy. The point is, we’re going from one molecule to a completely different one, with completely different bond properties. It’s difficult to try and compare them at all.
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In case you hadn’t noticed: In each of the puzzles, there’s not just an isomerization reaction going on, but one of the molecules exhibits isomerism as well. Cis-trans in LP Isomers and The Chem of SpaceChem, and tautomerism in Natural Chemo.
[1] http://webbook.nist.gov/cgi/cbook.c...=4#Thermo-Phase
[2] http://www.freepatentsonline.com/5688586.pdf