Tools of the trade: understanding chemical structures
There are so many organic compounds because carbon likes to form bonds to lots of elements. It has four outer electrons, which it likes to share.
For example, in methane (which has the molecular formula CH4), a carbon atom shares its four electrons with four hydrogen atoms. (For those of you interested in finding out how we can determine the molecular formula of a compound, take a look at the combustion analysis pdf in the downloads section below. It gives you the opportunity to do some number crunching, which the more mathematically-minded learners may enjoy.)
In this dot and cross diagram of methane each of the shared pairs of electrons is called a covalent bond. A covalent bond is typically represented by a straight line as shown by the full structural formula for methane.
Carbon also likes to share electrons with other carbon atoms and, in ethane, we see two carbon atoms share electrons to form a carbon‐carbon bond.
In ethanol we see that carbon can form a covalent bond to oxygen.
In propanone, or acetone, we see that carbon can also form two bonds to the same oxygen atom. This carbon‐oxygen double bond is called a carbonyl group.
For larger organic compounds, drawing all of the bonds, and all of the atoms can be unwieldy – in condensed structural formulae all of the atoms are shown but many, if not all, of the bonds are not included, as shown here for butanal.
The simplest way to represent organic compounds is to use skeletal structures. Each line has a carbon atom at each end, unless a different atom such as oxygen is shown, as for butanal. The end of the line stands for a carbon atom bonded to three hydrogen atoms, CH3; this is called a methyl group, sometimes abbreviated as Me. Using skeletal structures helps us to focus in on the areas that are most important. For example, the skeletal structure of butanal is much less cluttered than other representations and it emphasises the –CHO group (typically, reactions of butanal occur directly at, or adjacent to, the –CHO group).
You will notice that the carbon chain in butanal is drawn as a zig‐zag, this is because it shows the approximate shape – the shape of organic compounds is important because, as we will see, it affects their properties.
For a copy of the chemistry images (as pdfs) on this page (and also an image of a molecular model of butanal, which shows the zig-zag shape), see the downloads section below - this is a general feature of the course content and so you will be able to download the images as you work through the sections.
Isomers are molecules that have the same molecular formula, but have a different arrangement of atoms in space. More specifically, structural isomers differ in the way that some or all of the constituent atoms are joined together. For example, propan-1-ol (CH3CH2CH2OH), propan-2-ol (CH3CH(OH)CH3) and methoxyethane (CH3OCH2CH3) are structural isomers with the formula C3H8O.
Let’s consider structural isomers that have the formula C5H10O2. There are around 900 compounds with this formula, so how many can we identify? First, take a look at our open Padlet to see what compounds learners have posted. (To get the ball rolling we added ethyl propanoate, CH3CH2CO2CH2CH3.) Then pick a compound with the formula C5H10O2, ideally one which has not been posted on Padlet. Draw a skeletal structure of your isomer (making sure you show the approximate shape of the compound) and take a photo of it. Finally, post your photo together with the compound name, any interesting facts about the compound, and your name, on Padlet. You are welcome to post more than one entry.
If you need any guidance on using the Padlet, then further information is available - we would very much like for everyone to feel part of the learning community, so look forward to seeing your contributions.
Have you noticed how many skeletal structures crop up in company logos? Take for example Mercedes-Benz who use a 2-methylpropane in the middle of a circle, or the cyclopropane used by le coq sportif, the cyclohexane used by both Hexagon Composites and Corex Honeycomb or the cyclooctane used by the MG Car Company. Also, the Volkswagen logo has a similarity to 3,3-dimethylpentane, CH3CH2C(CH3)2CH2CH3 (try drawing the skeletal structure, showing the zig-zag shape). Do you know any better examples? In our everyday lives, to chemists, perhaps those zig-zag lines on furnishings, clothes (anyone remember Charlie Brown’s teeshirt?) or road markings are reminiscent of long chains of carbon atoms bonded together in alkanes. (Star Wars fans may also spot the relationship between bicyclo[1.1.1]pentane and an Imperial Shuttle!)
If you would like to learn more about drawing organic structures you may find this YouTube clip of use.
This is an additional video, hosted on YouTube.
© University of York