Understanding hydrocarbons helpful in pipeline debate

Editors note: The following is a significantly-revised version of a Relativity column that first appeared in 2014.

Every now and then, I think an understanding of hydrocarbon chemistry might help with some of the debate around pipelines, oil, diluted bitumen and the environment. So here is my attempt at Hydrocarbons 101.

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The name hydrocarbon comes from the presence of both hydrogen and carbon in the molecular structure.

Carbon sits in the middle of the main group of the periodic table, balanced between anionic non-metals and metallic cations. It would have to gain four electrons to act as anionic species, which is hard to do as electrons repel each other. It would have to lose four electrons to become a cation, which is equally hard to do as it creates an overwhelming positive charge.

Instead, carbon shares electrons by forming covalent bonds with other atoms.

Maybe that should be written "co-valent" to emphasize the bond is formed by the sharing of the valence electrons.

That is, two carbon atoms will share a pair of electrons between them. The same is true for a carbon and a hydrogen atom - they share an electron pair.

Covalent bonding is very strong. For example, diamonds are a form of covalently bonding carbon resulting in the hardest natural substance.

For hydrocarbons, the simplest of all compounds is methane. This is a carbon, which has four valence electrons, surrounded by four hydrogen atoms, each with a single valence electron. The result is four covalent bonds. Everything is very stable and the bonds are very strong.

Methane, by itself, is not explosive. It doesn't really interact with other methane molecules. As a consequence and because it is such a small molecule, methane is usually found in gaseous form. It is a large component of natural gas.

Ethane consists of two carbons bonded together and surrounded by six hydrogen atoms. If you think of a single carbon having four available sites for attachment, then this arrangement makes sense as each carbon is using up one of its sites to bind to the other carbon atom while the remaining six sites covalently bond to hydrogen atoms.

Ethane is slightly larger than methane and a slightly heavier gas. It is a component of natural gas but it boils at a higher temperature than methane (-89 C versus -164 C), which means the gases can be separated using distillation. It also does not yield as much energy per carbon dioxide produced as methane.

Propane consists of three carbons and eight hydrogens with the middle carbon holding on to the carbons at the end resulting in a linear arrangement. Propane is used in stoves, barbecues and even cars. Propane can be isolated and sold separately but it is also a constituent of natural gas and generated as a by-product during petroleum refining.

With one more carbon, we have butane. It is much heavier gas with four carbons and ten hydrogen atoms and a boiling point around 0 C. This makes it useful in butane lighters.

These are the gaseous hydrocarbons. They represent the light fraction of petroleum and all are byproducts of petroleum production and refining. They also occur independently.

Pentane, with five carbons and twelve hydrogen atoms, is too heavy to be a gas at typical room temperatures as it has a boiling point of 36 C. It evaporates on human skin. This is the first of the hydrocarbon liquids and is the lightest hydrocarbon typically found in gasoline.

Hexane, heptane, and octane have six, seven, and eight carbon atoms, respectively. They are liquids under most normal circumstances. They are too heavy to boil easily but still light enough to move freely at ambient temperatures.

Indeed, the same can be said for all of the straight chain hydrocarbons until around hexadecane which has sixteen carbons in a row. Beyond sixteen carbons, the molecules are too long and too heavy to move around much so they form solid or semi-solid waxes and tars.

However, this is only strictly true if the hydrocarbon is pure. Mix in a few other compounds or create a blend of, say, octane, decane and hexadecane and the combination results in free flowing mixture. Gasoline is a blend of both simple hydrocarbons and more complex branched hydrocarbons. Diesel uses heavier hydrocarbons than gasoline. It is the mixture of hydrocarbons which allows them to flow.

In the case of bitumen, it is a mixture of really long chain hydrocarbons with a few other organic constituents thrown in. Even though it is a mixture in its natural state, the size of the hydrocarbons means it is essentially a solid or a viscous tar.

Only by blending bitumen with some of the lighter hydrocarbons do we get a free flowing form. The resulting diluted bitumen is what is called dilbit - a mixture of heavy and light hydrocarbons from which we can make gasoline and other petroleum products.

More next week.

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