How To Draw Structural Formulas For Organic Compounds

Cartoon ORGANIC MOLECULES This page explains the various ways that organic molecules tin 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 cantlet present in the molecule, just tells you lot nothing most the fashion they are joined together. For case, 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 whatever useful information about the bonding in the molecule. Nigh the only place where you might come beyond them is in equations for the combustion of simple hydrocarbons, for instance: 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. In that location are various ways of cartoon this and y'all volition demand to be familiar with all of them. *Displayed formulae* A displayed formula shows all the bonds in the molecule as private lines. You demand to call up that each line represents a pair of shared electrons. For case, this is a model of methane together with its displayed formula: Notice that the way the methyl hydride is fatigued bears no resemblance to the bodily shape of the molecule. Methane isn't flat with 90° bond angles. This mismatch between what you draw and what the molecule actually looks like can lead to bug if you aren't conscientious. For instance, consider the simple molecule with the molecular formula CH₂Cl₂. Y'all might think that in that location were two different ways of arranging these atoms if you drew a displayed formula. The chlorines could be contrary each other or at right angles to each other. But these two structures are actually exactly the same. Await at how they announced as models. One structure is in reality a uncomplicated rotation of the other one.
	Note:This is all much easier to empathise if you have really 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 on stages of your organic chemistry course, you might consider getting agree of a cheap set. The models made by *Molymod* are both cheap and like shooting fish in a barrel to apply. An introductory organic set is more than adequate. Google *molymod* to find a supplier and more about them, or search for *molymod* on Amazon. Share the cost with some friends, keep it in skillful condition and don't lose any $.25, and resell it via eBay or Amazon at the terminate of your form. Alternatively, get concur of some coloured Plasticene (or other children'due south modelling clay) and some used matches and make your ain. Information technology'south cheaper, simply more than difficult to get the bond angles right.
Consider a slightly more than complicated molecule, C₂H₅Cl. The displayed formula could be written equally either of these: Just, over again these are exactly the same. Look at the models. *The commonest way to draw structural formulae* For anything other than the nigh simple molecules, drawing a fully displayed formula is a chip of a bother - especially all the carbon-hydrogen bonds. Yous can simplify the formula by writing, for example, CH₃ or CH₂ instead of showing all these bonds. So for case, ethanoic acid would be shown in a fully displayed form and a simplified form as: You could even condense it farther to CH_iiiCOOH, and would probably do this if y'all had to write a uncomplicated chemical equation involving ethanoic acrid. You do, however, lose something by condensing the acid grouping in this way, because you can't immediately see how the bonding works. You all the same accept to be careful in drawing structures in this way. Call up from above that these ii structures both correspond the same molecule: The next three structures all correspond butane. All of these are just versions of four carbon atoms joined up in a line. The merely divergence is that there has been some rotation about some of the carbon-carbon bonds. You lot can run into this in a couple of models. Not one of the structural formulae accurately represents the shape of butane. The convention is that nosotros draw it with all the carbon atoms in a directly line - as in the kickoff of the structures above. This is even more important when you start to have branched chains of carbon atoms. The following structures once more all represent the same molecule - 2-methylbutane. The two structures on the left are adequately patently the same - all we've washed is flip the molecule over. The other 1 isn't so obvious until you look at the construction in item. There are four carbons joined upwards in a row, with a CH₃ group attached to the next-to-end one. That'southward exactly the same as the other two structures. If you had a model, the only difference betwixt these three diagrams is that y'all accept rotated some of the bonds and turned the model around a bit. To overcome this possible confusion, the convention is that you always wait for the longest possible concatenation of carbon atoms, and then draw it horizontally. Annihilation else is simply hung off that concatenation. It doesn't matter in the least whether you lot depict any side groups pointing up or down. All of the following represent exactly the same molecule. If you lot made a model of one of them, you lot could plow it into whatever other one simply by rotating one or more than of the carbon-carbon bonds. *How to depict structural formulae in three-dimensions* There are occasions when it is important to be able to show the precise 3-D arrangement in parts of some molecules. To practice this, the bonds are shown using conventional symbols: For example, y'all might desire to prove the iii-D arrangement of the groups around the carbon which has the -OH group in butan-2-ol. Butan-two-ol has the structural formula: Using conventional bond notation, y'all could draw it every bit, for example: The only difference betwixt these is a slight rotation of the bail betwixt the eye ii carbon atoms. This is shown in the two models below. Look carefully at them - particularly at what has happened to the lone hydrogen atom. In the left-paw model, information technology is tucked behind the carbon atom. In the right-hand model, it is in the aforementioned airplane. The alter is very slight. It doesn't thing in the least which of the two arrangements y'all draw. You lot could easily invent other ones also. Choose i of them and get into the habit of drawing iii-dimensional structures that mode. My own habit (used elsewhere on this site) is to describe ii bonds going back into the paper and one coming out - as in the left-hand diagram above. Notice that no attempt was made to prove the whole molecule in three-dimensions in the structural formula diagrams. The CH₂CH₃ group was left in a simple form. Keep diagrams elementary - trying to show also much item makes the whole matter amazingly difficult to sympathise! *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 information technology. For example, nosotros've just been talking about butan-2-ol. The normal structural formula and the skeletal formula wait like this: In a skeletal diagram of this sort there is a carbon cantlet at each junction between bonds in a chain and at the end of each bond (unless there is something else at that place already - like the -OH group in the instance); there are enough hydrogen atoms fastened to each carbon to make the full number of bonds on that carbon upwardly to 4. Beware! Diagrams of this sort take practice to interpret correctly - and may well non be acceptable to your examiners (come across beneath). There are, yet, some very mutual 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 ii hydrogens fastened. This is what it looks similar in both a structural formula and a skeletal formula. And this is cyclohexene, which is similar but contains a double bond: Just the commonest of all is the benzene ring, C₆H₆, which has a special symbol of its own.
	Notation:Explaining exactly what this structure ways needs more infinite than is available hither. 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 utilize There's no like shooting fish in a barrel, all-embracing respond 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 nearly this - as yous practice more and more organic chemistry, you will probably discover it volition come naturally. You'll become then used to writing formulae in reaction mechanisms, or for the structures for isomers, or in unproblematic chemical equations, that you won't even think about it. There are, notwithstanding, a few guidelines that yous should follow. *What does your syllabus say?* Dissimilar examiners will have unlike preferences. Check first with your syllabus. If you lot'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 "detect" function on your Adobe Acrobat Reader to search the organic section(s) of the syllabus for the word "formula". Yous should also check recent examination papers and (particulary) mark schemes to find out what sort of formula the examiners really prefer in given situations. Y'all could also await at whatever support material published past your examiners.
	Note:If you lot are working to a UK-based syllabus and haven't got a copy of that syllabus and recent exam papers, follow this link to find out how to get them.
*What if you even so aren't sure?* Describe the most detailed formula that y'all can fit into the space available. If in doubt, depict a fully displayed formula. You would never lose marks for giving besides much detail. Autonomously from the most trivial cases (for example, burning hydrocarbons), never use a molecular formula. Always show the detail around the important part(due south) of a molecule. For example, the important office of an ethene molecule is the carbon-carbon double bond - then write (at the very to the lowest degree) CH_two=CH_two and not C_twoH₄. Where a item way of drawing a construction is important, this volition e'er exist pointed out where information technology arises elsewhere on this site. Where would you like to go now? To the organic conventions carte du jour. . . To menu of bones organic chemistry. . . To Main Card . . . © Jim Clark 2000 (modified November 2022)