DRAWING ORGANIC MOLECULES
This page explains the various ways that organic molecules can be
represented on paper or on screen - including molecular formulae, and
various forms of structural formulae. Molecular formulae A molecular formula simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined together. For example, the molecular formula of butane is C4H10, and the molecular formula of ethanol is C2H6O. Molecular formulae are very rarely used in organic chemistry, because they don't give any useful information about the bonding in the molecule. About the only place where you might come across them is in equations for the combustion of simple hydrocarbons, for example: In cases like this, the bonding in the organic molecule isn't important. Structural formulae A structural formula shows how the various atoms are bonded. There are various ways of drawing this and you will need to be familiar with all of them. Displayed formulae A displayed formula shows all the bonds in the molecule as individual lines. You need to remember that each line represents a pair of shared electrons. For example, this is a model of methane together with its displayed formula: For example, consider the simple molecule with the molecular formula CH2Cl2. You might think that there were two different ways of arranging these atoms if you drew a displayed formula. | |
Note: This is all much easier to understand if you have actually got some models to play with. If your school or college hasn't given you the opportunity to play around with molecular models in the early stages of your organic chemistry course, you might consider getting hold of a cheap set. The models made by Molymod are both cheap and easy to use. An introductory organic set is more than adequate. Google molymod to find a supplier and more about them, or click on the picture or text link below to see a typical example from Amazon. (Don't click on the "Buy" button unless you really want to buy it!) Share the cost with some friends, keep it in good condition and don't lose any bits, and resell it via eBay or Amazon at the end of your course. Alternatively, get hold of some coloured Plasticene (or other children's modelling clay) and some used matches and make your own. It's cheaper, but more difficult to get the bond angles right. | |
Consider a slightly more complicated molecule, C2H5Cl. The displayed formula could be written as either of these: For anything other than the most simple molecules, drawing a fully displayed formula is a bit of a bother - especially all the carbon-hydrogen bonds. You can simplify the formula by writing, for example, CH3 or CH2 instead of showing all these bonds. So for example, ethanoic acid would be shown in a fully displayed form and a simplified form as: You still have to be careful in drawing structures in this way. Remember from above that these two structures both represent the same molecule: This is even more important when you start to have branched chains of carbon atoms. The following structures again all represent the same molecule - 2-methylbutane. To overcome this possible confusion, the convention is that you always look for the longest possible chain of carbon atoms, and then draw it horizontally. Anything else is simply hung off that chain. It doesn't matter in the least whether you draw any side groups pointing up or down. All of the following represent exactly the same molecule. How to draw structural formulae in 3-dimensions There are occasions when it is important to be able to show the precise 3-D arrangement in parts of some molecules. To do this, the bonds are shown using conventional symbols: Butan-2-ol has the structural formula: Notice that no attempt was made to show the whole molecule in 3-dimensions in the structural formula diagrams. The CH2CH3 group was left in a simple form. Keep diagrams simple - trying to show too much detail makes the whole thing amazingly difficult to understand! Skeletal formulae In a skeletal formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it. For example, we've just been talking about butan-2-ol. The normal structural formula and the skeletal formula look like this:
There are, however, some very common cases where they are frequently used. These cases involve rings of carbon atoms which are surprisingly awkward to draw tidily in a normal structural formula. Cyclohexane, C6H12, is a ring of carbon atoms each with two hydrogens attached. This is what it looks like in both a structural formula and a skeletal formula. | |
Note: Explaining exactly what this structure means needs more space than is available here. It is explained in full in two pages on the structure of benzene elsewhere in this site. It would probably be better not to follow this link unless you are actively interested in benzene chemistry at the moment - it will lead you off into quite deep water! | |
Deciding which sort of formula to use
There's no easy, all-embracing answer to this problem. It depends
more than anything else on experience - a feeling that a particular way
of writing a formula is best for the situation you are dealing with. Don't worry about this - as you do more and more organic chemistry, you will probably find it will come naturally. You'll get so used to writing formulae in reaction mechanisms, or for the structures for isomers, or in simple chemical equations, that you won't even think about it. There are, however, a few guidelines that you should follow. What does your syllabus say? Different examiners will have different preferences. Check first with your syllabus. If you've down-loaded a copy of your syllabus from your examiners' web site, it is easy to check what they say they want. Use the "find" function on your Adobe Acrobat Reader to search the organic section(s) of the syllabus for the word "formula". You should also check recent exam papers and (particulary) mark schemes to find out what sort of formula the examiners really prefer in given situations. You could also look at any support material published by your examiners. |
Thursday, 5 September 2013
DRAWING ORGANIC MOLECULES
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