Friday, December 9, 2011

Chemistry Friday: Introduction to organic chemistry

Nope, I haven't gone all granola on you! Organic chemistry is 
a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of carbon-based compounds, hydrocarbons, and their derivatives. These compounds may contain any number of other elements, including hydrogen, nitrogen, oxygen, the halogens as well as phosphorus, silicon, and sulfur. 

In other words, organic chemistry is the study of thingies that contain carbon. It wouldn't be the study of metals, metalloids, or even salts - if it doesn't contain carbon, it isn't organic chemistry. So we'd study fats, oils, butters, surfactants, and extracts, for instance, but we wouldn't be studying Epsom salts, MSM, silicones, or zinc because those are inorganic chemistry. 

This is an alkane. Alkanes "are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds (i.e., they are saturated compounds) without any cycles (or loops; i.e., cyclic structure)." To put it in easier terms, it's a chain of carbons that contain single bonds, no double or triple bonds. 

Carbon has four electrons in its outer shell, meaning it has space for up to four electrons to join in the fun by covalent bonding. Hydrogen has an extra electron that it wants to share, meaning that each hydrogen can offer one electron. Carbon has four slots - hydrogen has one electron - so we can get four hydrogens around each carbon. The smallest alkane is called methane (meth- meaning one and -ane indicating it's an alkane). 

Alkanes are named after the number of carbon atoms. This picture is ethane - eth- meaning 2 and -ane indicating it has only single bonds. 

But why aren't there four hydrogens around each of these carbons? Take a look at one of the carbon atoms. It can make up to four bonds - it has three bonds with hydrogen, and the fourth is bonded to another carbon!

As an aside...meth- is 1, eth- is 2, prop- is 3, but- is 4, and pent- is 5. So if you see a propane molecule, you know it has 3 carbon atoms! Click here to learn more about organic chemistry nomenclature! The other thing you might see is called the trivial name, and we'll get to that soon! 

An alkene "is an unsaturated chemical compound containing at least one carbon-to-carbon double bond." (The word unsaturated means that there is at least one double bond, so the definition is a little circular.) "The simplest acyclic alkenes, with only one double bond and no other functional groups, form an homologous series of hydrocarbons with the general formula CnH2n." (If you see the word "olefine" or "olefin", this also refers to an alkene, like C14-16 olefin sulfonate, one of my favourite surfactants!)

So this molecule is ethene (eth- as in 2, -ene as in alkene, meaning there's a double bond in there). The carbon is bonded doubly to another carbon, which means there's only space for four hydrogens in this molecule as opposed to six above. That's because carbon has covalently bonded twice to the other carbon, so there's only room for two hydrogens per atom!

This is an unsaturated molecule of oleic acid, and you can see that there's a double bond along that chain that could be broken, allowing some atom to slip in and join the fun! If that the bond is broken and oxygen sneaks in there - you've got oxidation and your oil will go rancid soon!

The more double bonds we see in an oil, the shorter its shelf life will be because those double bonds can be broken and oxygen can cause rancidity. This is why our butters have longer shelf lives than oils. Something like shea butter has relatively few double bonds, while something like grapeseed oil has double and triple bonds! (For more on the chemistry of our oils, click here and start reading!) We'll be getting to what that O and OH at the end mean in the next few days! 

That was a lot to take in one day, so let's summarize:
1. Organic chemistry is the study of stuff that contains carbon.
2. Alkanes are chains of carbons with hydrogens on them and they have only single bonds, meaning they are saturated. We name alkanes by using the prefix for the number of carbons (like eth-) and the ending -ane.
3. Alkenes are chains of carbons with hydrogens on them and they contain double bonds, so they are unsaturated. We name alkenes by using the prefix for the number of carbons (like eth-) and the ending -ene.

Let's take a look at cis and trans fats tomorrow! On Sunday, we'll look at functional groups, and get back to essential oils on Monday! 


chowsr said...

It's not only oxygen that causes rancidity. Light exposure can as well. With enough light energy the electron in the double and triple bonds can be given enough energy to escape, thus breaking the bond.

Susan Barclay-Nichols said...

Hi chowsr! I've written posts on the mechanisms of rancidity - nice way to summarize the effect of light on our oils and butters!

Rancidity: A primer
Mechanisms of rancidity