Boom Advent #16 – Molecular Madness

Christmas is a festival of the senses. In our first post we covered the sight of tinsel and last week we explored that feeling of heartburn and how we can deal with it. There are loads of molecules we could pick to highlight Christmas but this week, we’ve picked a very smelly molecule. That wonderful scent of a real Christmas tree – that rich, woody aroma – is all thanks to a molecule called pinene and we’ve spent all season pining for it!

Giving Christmas a fir chance

Nowadays, many of us have fake Christmas trees. Plastic trees have the advantage of being long lasting and not dropping hundreds and hundreds of pine needles on the floor. However, there is one thing that plastic tress will never have, the wonderful smell of a real Christmas tree. If you’ve ever had or been near a real fir or spruce tree, you will know that smell is.

pine cones are full of pinene

The smell of Christmas tree is down to a molecule called pinene. However, that is only half the story. This molecule has structural isomerism. Isomers are molecules that have the same molecular formula (so the same number and type of atoms) but are connected in different ways.

Same Pinene, but different

The two isomers are known as α-pinene (alpha) and β-pinene (beta). These isomers have slightly different properties and also slightly different smells. Before we get into that, let’s see what the structure is and how the two types differ.

pinene isomers structure

It’s an awkward looking structure! Both isomers have a hexagonal ring of carbon atoms, with a bridging, arch like carbon above it. Connected to this hexagon, are three more carbon atoms.

In α-pinene, there is a double bond between two carbons within the hexagon ring. However, in β-pinene, the double bond is between two carbon atoms that are connected to the hexagon ring. This all sounds quite confusing so it’s probably easier to see in a picture. Drawing these structures is not easy though and sometimes the pictures are more confusing than the descriptions.

Seeing double

Things can get even more complicated as both α- and β-pinene can have mirror-images of themselves. In both molecules, the bridging, arch like carbon can be above the ring or below the ring. We don’t need to worry about these mirror-image forms.

all four isomers of pinene

How do alpha and beta pinene differ?

Well, both molecules have very strong odours. The smell of the oil or resin of pine trees contains both types, although that woody, pine-smell is mostly caused by β-pinene.

As pure substances, both isomers are liquids. They cannot mix with water, but will easily mix with vinegar and ethanol.

Pinene and Fire

However, both isomers are very flammable and this is why pinecones or indeed Christmas trees burn very well. Historically, Christmas trees were also decorated with candles, a practice that seems very dangerous given how flammable pinene is!

Please be careful with candles this Christmas

Trees, cones and herbs

Pinene is an example of the wonderful selection of molecules that exist. There are lots of molecules that can all have the same chemical formula, but very different properties. However, the two isomers of pinene are very similar in structure and properties. Every Christmas we experience the smell of pinene. This fantastic aroma of pinene can be found in Christmas trees, pinecones and even rosemary. If you don’t have a real Christmas tree, you can still the experience the smell of pinene by walking through a pine forest. Although, perhaps wait until the weather is a little warmer and drier…


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