drawing double bonds in 3d
ii.2.2. Drawing 3-Dimensional Molecules
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This page explains the various ways that organic molecules can exist 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 yous nothing nigh the way they are joined together. For example, the molecular formula of butane is \(C_4H_{ten}\), and the molecular formula of ethanol is \(C_2H_6O\).
Molecular formulae are very rarely used in organic chemistry, because they do not give useful information about the bonding in the molecule. Most the merely place where you might come across them is in equations for the combustion of simple hydrocarbons, for example:
\[ C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O\]
In cases like this, the bonding in the organic molecule isn't important.
Structural formulae
A structural formula shows how the diverse atoms are bonded. There are various ways of cartoon this and you will need to be familiar with all of them.
Displayed formulae
A displayed formula shows all the bonds in the molecule every bit individual lines. Y'all need to remember that each line represents a pair of shared electrons. For instance, this is a model of marsh gas together with its displayed formula:
Notice that the way the methyl hydride is drawn bears no resemblance to the actual shape of the molecule. Methane isn't flat with ninety° bond angles. This mismatch between what you draw and what the molecule actually looks like tin lead to problems if yous aren't careful. For example, consider the unproblematic molecule with the molecular formula CHiiCl2. You might think that in that location were two unlike ways of arranging these atoms if you drew a displayed formula.
The chlorines could be opposite each other or at right angles to each other. But these two structures are actually exactly the same. Look at how they appear every bit models.
One structure is in reality a simple rotation of the other one. Consider a slightly more complicated molecule, CiiHvCl. The displayed formula could be written as either of these:
But, over again these are exactly the same. Expect at the models.
The commonest way to draw structural formulae
For annihilation other than the near simple molecules, drawing a fully displayed formula is a bit of a bother - especially all the carbon-hydrogen bonds. You tin simplify the formula by writing, for instance, CH3 or CH2 instead of showing all these bonds. For example, ethanoic acid would be shown in a fully displayed class and a simplified class as:
You could even condense information technology farther to CH3COOH, and would probably do this if you lot had to write a simple chemic equation involving ethanoic acid. You do, however, lose something by condensing the acid group in this fashion, because y'all tin't immediately see how the bonding works. You still take to exist careful in drawing structures in this fashion. Recollect from higher up that these two structures both represent the same molecule:
The next 3 structures all stand for butane.
All of these are just versions of 4 carbon atoms joined up in a line. The only difference is that at that place has been some rotation virtually some of the carbon-carbon bonds. You can see this in a couple of models.
Not one of the structural formulae accurately represents the shape of butane. The convention is that we draw it with all the carbon atoms in a straight line - as in the first of the structures above. This is fifty-fifty more important when y'all start to take branched chains of carbon atoms. The following structures over again all represent the same molecule - two-methylbutane.
The 2 structures on the left are adequately plainly the same - all we've washed is flip the molecule over. The other one isn't and so obvious until you look at the structure in detail. There are 4 carbons joined up in a row, with a CH3 group attached to the next-to-cease one. That's exactly the aforementioned every bit the other 2 structures. If you had a model, the only divergence between these three diagrams is that you lot have rotated some of the bonds and turned the model around a chip.
To overcome this possible confusion, the convention is that you lot e'er look for the longest possible chain of carbon atoms, and then draw information technology horizontally. Anything else is merely hung off that chain. It does non thing in the least whether you draw any side groups pointing up or down. All of the post-obit represent exactly the aforementioned molecule.
If you made a model of 1 of them, you could plough it into any other one simply past rotating one or more of the carbon-carbon bonds.
How to draw structural formulae in 3-dimensions
There are occasions when it is of import to be able to show the precise 3-D arrangement in parts of some molecules. To practise this, the bonds are shown using conventional symbols:
For example, you might want to show the 3-D arrangement of the groups effectually the carbon which has the -OH group in butan-2-ol.
| Example one: butan-2-ol |
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| Butan-2-ol has the structural formula: Using conventional bond notation, you could draw it every bit, for example: The merely divergence between these is a slight rotation of the bond between the centre two carbon atoms. This is shown in the 2 models beneath. Look advisedly at them - particularly at what has happened to the lone hydrogen atom. In the left-paw model, it is tucked behind the carbon cantlet. In the right-hand model, it is in the same plane. The change is very slight. Information technology doesn't affair in the to the lowest degree which of the two arrangements you draw. You could hands invent other ones too. Choose one of them and get into the habit of drawing iii-dimensional structures that way. My own addiction (used elsewhere on this site) is to depict two bonds going dorsum into the paper and ane coming out - as in the left-hand diagram above. Notice that no endeavour was made to show the whole molecule in iii-dimensions in the structural formula diagrams. The CH2CHiii group was left in a elementary form. Continue diagrams simple - trying to show too much item makes the whole thing amazingly difficult to sympathize! |
Skeletal formulae
In a skeletal formula, all the hydrogen atoms are removed from carbon bondage, leaving just a carbon skeleton with functional groups fastened to it. For case, we've just been talking about butan-2-ol. The normal structural formula and the skeletal formula await like this:
In a skeletal diagram of this sort
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there is a carbon atom at each junction betwixt bonds in a concatenation and at the terminate of each bond (unless there is something else there already - like the -OH group in the example);
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in that location are enough hydrogen atoms fastened to each carbon to make the total number of bonds on that carbon upwards to 4.
Beware! Diagrams of this sort take do to interpret correctly - and may well not be acceptable to your examiners (run into below).
In that location are, however, some very common cases where they are frequently used. These cases involve rings of carbon atoms which are surprisingly bad-mannered to draw tidily in a normal structural formula. Cyclohexane, C6H12, is a band of carbon atoms each with two hydrogens attached. This is what it looks similar in both a structural formula and a skeletal formula.
And this is cyclohexene, which is like but contains a double bond:
Just the commonest of all is the benzene ring, CsixHhalf dozen, which has a special symbol of its own.
Deciding which sort of formula to apply
In that location's no easy, all-embracing respond to this problem. It depends more 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 - every bit yous exercise more and more organic chemistry, you lot will probably discover it will come naturally. You'll get and so used to writing formulae in reaction mechanisms, or for the structures for isomers, or in simple chemical equations, that yous won't even think nigh it.
Source: https://chem.libretexts.org/Courses/Purdue/Purdue_Chem_26100:_Organic_Chemistry_I_%28Wenthold%29/Chapter_02._Structures_and_Properties_of_Organic_Molecules/2.2_Molecular_Shapes_and_Hybridization/2.2.2._Drawing_3-Dimensional_Molecules
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