Science For Everyone - Hydrocarbons

Copyright © Karl Dahlke, 2019

A hydrocarbon is a compound that contains only hydrogen and carbon. That seems straightforward, but there are also carbohydrates, e.g. this can of Coke contains 28 grams of carbohydrates, and one should not get them mixed up. A carbohydrate contains carbon, hydrogen, and oxygen. You can remember this because the suffix "ate" always implies oxygen. Sulfate = sulfur and oxygen, nitrate = nitrogen and oxygen, phosphate = phosphorus and oxygen, carbonate = carbon and oxygen, chlorate = chlorine and oxygen, and so on. Thus sodium phosphate contains sodium, phosphorous, and oxygen. We'll get to that later. Let's start with hydrocarbons, a class of molecules containing just C and H.

Any hydrocarbon can be described by its carbon framework; all the remaining bonds are satisfied by hydrogens. The simplest example is methane, with one carbon atom. Since carbon has 4 bonds, it is surrounded by 4 hydrogen atoms. The formula is CH4, and a picture might look like this.

H
HCH
H

This picture is a bit misleading, because methane is a 3 dimensional molecule, and I have flattened it onto 2 dimensions for the convenience of the printed page. In 3 space it is a tripod with a carbon in the middle, three hydrogens for feet, and a hydrogen on top, forming a tetrahedron. That's how carbon bonds try to arrange themselves. All the pictures in this chapter, and most of the pictures in this book, are flat projections of the real thing. Most of the time it doesn't matter, but when the difference is important I'll describe the 3 dimensional structure, and explain how it affects the property of the compound.

Methane probably comes pouring into your home every day, as fuel for your hot water heater, your clothes dryer, your furnace, and/or your stove. (Some of these appliances could be electric, but I'll bet at least two of them are gas.) Methane flows into your house through a sealed metal pipe, since a gas leak is a serious fire hazard. Valves open when the appliance is burning methane to produce heat, and close when the appliance is off. Like every hydrocarbon, methane produces water and carbon dioxide when it burns. These are harmless in small amounts, thus a gas stove simply vents these exhaust products into your kitchen. You can see the flames on the burners; methane is burning right in front of your eyes. 🔥 Let's take a closer look at the reaction. Two hydrogen atoms join an oxygen atom to make water, and another two hydrogen atoms join an oxygen atom to make water, and that takes care of the 4 hydrogen atoms, consuming one O2 molecule in the process. The carbon joins another O2 molecule to make CO2, which is carbon dioxide. Let's make sure the bonds are satisfied. Carbon has 4 bonds and oxygen has two. Put an oxygen on either side of the carbon, using double bonds, and all bonds are closed. Instead of CO2, you might write this as OCO, or even O=C=O, where = denotes a double bond. Here is the chemical reaction, written as an equation. The arrow means it always goes to the right, releasing energy in the process, in this case heat. It can't go backwards, because you would have to put energy into the system.

CH4 + 2O2 → CO2 + 2H2O

If you spend 4 hours cooking a turkey in a gas oven, you might notice some humidity in the kitchen - maybe even a little moisture on the walls. This is the steam that is produced by burning all that methane. Your furnace vents the CO2 and water out of the house, because, on a cold January day, it wouldn't be safe to breathe that much CO2, and the steam would make your house feel like a rain forest. The challenge, then, is to retain all the heat, while venting the byproducts of combustion into the air. That is an engineering challenge that has largely been solved, making furnaces more efficient.

The next hydrocarbon has two carbons, and is called ethane. There is a little bit of ethane in the methane that pipes into your house, about 10%. It burns just as well as methane, and causes no trouble. This picture is, once again, a flat representation of the real thing. The real molecule looks more like a dumbbell, with a triangle of hydrogens on either end, as the bonds of each carbon form a tetrahedron.

HH
HCCH
HH

At this point you can see the pattern. A chain of n carbons has n hydrogens on top, n hydrogens below, and a hydrogen on either end. These molecules are called alkanes, and they have the formula CnH2n+2. The first 8 have names, although 5 through 8 simply defer to the latin words for 5 6 7 8, just like pentagon hexagon heptagon octagon. You may recognize the word octane as having something to do with a car's engine. Indeed, octane is the gasoline that you put into your car's gas tank.

methaneCH4
ethaneC2H6
propaneC3H8
butaneC4H10
pentaneC5H12
hexaneC6H14
heptaneC7H16
octaneC8H18

All of these burn in the presence of oxygen, and produce carbon dioxide, water, and heat. Balance this propane equation so that there are the same number of atoms of each type on the left and on the right. As democritus taught us, atoms can neither be created nor destroyed; they simply rearrange themselves. This is the reaction you see when you cook on a camping stove.

C3H8 + ?O2 → ?CO2 + ?H2O

Hint - balance the carbons, then the hydrogens, then the oxygens.

Each alkane has its role in our industrialized society. Methane and ethane flow through pipes into our homes to provide heat, and these gases won't liquify, even on the coldest winter nights. Propane is also a gas, but it can be turned into a liquid by high pressure. Metal propane canisters provide fuel for camping stoves and heaters, while large propane tanks heat buildings that are too remote for pipelines. My grandfather heated is workshop with a propane stove. The tank, holding 100 gallons, was wisely placed outside the shop and away from power tools. A thin pipe carried propane through the wall and into the stove for combustion. He only used it during the cold winter months. Once a year a truck came and refilled the tank.

Butane is the fuel of choice for lighters. It is also a gas, but is easily liquified by a couple atmospheres of pressure. The container can be lighter and thinner than its propane carrying cousin, perfect for pocket or purse.

A car's gas tank isn't going to be pressurized, that's a bad idea, especially in the event of an accident, thus the fuel has to be a liquid, and remain a liquid on a hot summer day in an Arizona parking lot. Octane C8H18 is the answer.

Of course an alkane can have more than 8 carbons along its backbone. Motor oil, which lubricates your car's engine, consists of alkanes of lengths 20 to 30. This is thicker than gasoline.

Lengths of 40 to 50 form a white or colorless wax, as you might find in a candle. This is a solid, though candles can melt on a hot summer day if stored in the attic. Because the chains are so long, and harder to vaporize, a candle might not burn as cleanly as methane or octane. Some of the carbon does not combine with oxygen to make CO2, hence it rises to the ceiling in the form of soot. You may have seen black smudges on the ceiling where candles burn beneath. This is less of a concern today, thanks to Thomas Edison.

Gas Liquid Solid

With some experience, you can look at a molecule and predict whether it is a gas at room temperature. Gas molecules tend to be small, light, and symmetric. Methane is the perfect example: one carbon and 4 hydrogen atoms, which don't weigh very much, arranged in a perfect tetrahedron. It remains a gas, even at very low temperatures. Ethane is almost as light, and retains most of the symmetry. Propane and butane are still small and light, but they begin to lose some of the symmetry as they flop around like snakes in 3 space. Pentane is the first alkane that is a liquid at room temperature, but just barely. Here are the first 8 alkanes again, with their boiling points. This is the temperature where they switch from liquid to gas. Fahrenheit is given first, then celsius.

methaneCH4-258-164
ethaneC2H6-127-88
propaneC3H8-44-42
butaneC4H10341
pentaneC5H129736
hexaneC6H1415669
heptaneC7H1620998
octaneC8H18259126

Boiling point rises steadily as the chains get longer. After 20 carbons, the liquid becomes more viscous, until it finally becomes a waxy solid around length 45.

Isobutane

Butane is the first alkane that comes in different forms. All 4 carbons can connect in a chain, or, the fourth carbon can hang off the second. Here is an illustration, omitting the hydrogens.

n-butaneisobutane
CCCC
CCC
C

Isobutane has more symmetry than this flat projection would indicate. In 3 dimensions, the central carbon has 3 carbons below it, like the feet of a tripod, and each of those carbons has 3 hydrogens below it. Then a hydrogen sits atop like a cap. If you spin the molecule 120 degrees about its central axis it looks the same.

Normal butane and isobutane have the same number of atoms, of the same type. They weigh the same, and are roughly the same size. But isobutane has more symmetry, thus you would expect it to become a gas more easily. Indeed it does. Normal butane boils at 32 °F, or 0 °C, the freezing point of water, while isobutane becomes a gas at 10°F, or -12°C, significantly cooler.

Since n-butane and isobutane both have the formula C4H10, is there another way to distinguish the molecules, without drawing pictures? There is, using a notation called smiles, Simplified Molecular Input Line Entry System. Other inline notations exist, but smiles is perhaps easiest for a human to read and unravel. I find it fairly intuitive. Various software packages can convert a .smi file, containing a smiles description of a molecule, into a 2 or 3 dimensional representation.

The idea is simple, march along the molecule, like a chain, and list each atom, and travel down each branch in parentheses. Unsatisfied bonds are assumed to be filled with hydrogens. Thus water is represented as O, methane is C, an oxygen molecule is O=O (the = sign indicating a double bond), and carbon dioxide is O=C=O. Turning to linear hydrocarbons, butane is CCCC, and octane is CCCCCCCC. If the molecule is not a straight chain, use parentheses to indicate each side branch. Thus isobutane is written CC(C)C. Move from the first carbon to the second, then take a side trip to a third carbon, then return to the second and move on to the fourth. That effectively puts the second carbon in the middle of the other three. Any tree structure can be described in this way. As an exercise, go to wikipedia and look up any compound mentioned on this page: butane, octane, acetic acid, etc, and wikipedia will give you the high level formula and the smiles descriptor for that compound. The entry for isobutane lists C4H10, and CC(C)C. In other words, smiles is fairly standard. With practice, you can read a smiles string and deduce the structure of the molecule.

Saturated and Unsaturated

The alkanes are all saturated, which means they hold as many hydrogen atoms as possible. A hydrocarbon can hold fewer hydrogen atoms, if some of the carbons are joined together by double bonds. These are called unsaturated. Yes, this relates to saturated and unsaturated fats in your diet, and we'll see why in a later chapter. The simplest example of an unsaturated hydrocarbon is ethene, with smiles C=C. Can you determine the structure and the formula for this molecule?

Two carbons are joined by a double bond, indicated by the = sign. Thus each carbon has two bonds that are still open. These are satisfied by hydrogen. Therefore there are 4 hydrogen atoms, and the formula is C2H4. If you were drawing a picture, you would start with 2 H's on the left, then c, then a double bond, then C, then 2 H's on the right.

The smiles string for acetylene is C#C, where # indicates a triple bond. Now each carbon has one open bond, and the formula is C2H2. When the triple bond breaks, it releases a lot of energy, thus acetylene burns hot. An oxyacetylene torch can reach temperatures of 6300 °F, 3480 °C, and has many industrial uses, including welding and cutting through metal.

Let me leave you with one more compound, before we move to another planet. Acetic acid, the active agent in vinegar, has the structure CC(O)=O. The vinegar you buy from the store is dilute, about 5%; you can buy concentrated acetic acid from a chemical supplier, but you wouldn't want to drink that, or even put it on your tongue. Deduce the structure and formula.

The first carbon has 3 hydrogens, the second has an OH and a double bond O. The second half is sometimes written COOH, because it is so important to organic chemistry. Every acid that is in any way connected to life, citric acid in oranges, malic acid in apples, tannic acid in tea, ascorbic acid in limes, etc, ends in COOH, that is, carbon with a double bond oxygen and an OH group. This structure allows the hydrogen ion to break away in the presence of water and form the acid.

Titan

Titan is the largest moon of Saturn. At 40% the diameter of Earth, it is even larger than Mercury. It could be seen through the early Galilean telescopes, and was discovered by Christiaan Huygens in 1655. He named it Titan after a mythical race of giants. Unlike every other moon in the solar system, Titan retains a thick, orange-ish atmosphere. Atmospheric pressure at the surface is 45% greater than on Earth. You need a full body suit on Titan, because it's cold, but you don't need a pressure suit. A human can easily adjust to 1.45 atmospheres, and live at that pressure for years.

There are two reasons you can't run around Titan in shirt sleeves. First, it's cold, unimaginably cold, -290 °F, -179 °C. An unprotected human would die in minutes - his first breath freezing his lungs into useless caves of ice. Second, there is no oxygen. The air is 95% nitrogen and 5% methane. Methane is a liquid at these temperatures, and Titan has a methane cycle, just as Earth has a water cycle. Methane evaporates into the air, condenses out as rain, and flows into rivers, which replenish methane lakes and seas. If it's raining methane, you better get indoors in a hurry! It's hard to see how any suit could protect you from a cryogenic liquid raining down upon you. Cold air is one thing, cold liquid (like ice water) is quite another. Speaking of water, all the water on Titan is locked up in ice, as hard as granite. Molten ice on Titan is like molten rock on Earth - only seen as part of a volcanic eruption here and there.

In terms of industrialization, Titan is the reverse of Earth. The atmosphere contains hydrocarbons, methane and ethane, and there is no oxygen in the air. If you want heat, or work done, the reaction is the same: hydrocarbons + oxygen yields water, CO2, and heat; but you must supply the oxygen, the fuel is already in the air. Drive up to a gas station and fill your tank with oxygen, then drive away. If you look up the boiling point of oxygen, it is a tad lower than the temperature on Titan, which suggests oxygen remains a gas, however, Titan has a denser atmosphere, and the additional pressure allows oxygen to exist as a tenuous liquid. You could indeed pour oxygen from a nozzle into your car's gas tank, but screw the cap on tight, so the O2 doesn't evaporate away. The car needs a different engine, that brings in liquid oxygen and burns the methane vapors in the air. The real challenge is starting the engine at these unthinkably cold temperatures. You thought you had trouble starting your car in the winter.