Structural isomerism

What is structural isomerism? What are isomers? Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds. For example, both of the following are the same molecule. They are not isomers. Both are butane. There are also endless other possible ways that this molecule could twist itself. There is completely free rotation around all the carbon-carbon single bonds.
	Note:  Isomerism is 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 have a look at this set or something similar from Amazon. 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.
If you had a model of a molecule in front of you, you would have to take it to pieces and rebuild it if you wanted to make an isomer of that molecule. If you can make an apparently different molecule just by rotating single bonds, it's not different - it's still the same molecule.
	Note:  It's really important that you understand this. If you aren't sure, then you must get hold of (or make) some models.
What are structural isomers? In structural isomerism, the atoms are arranged in a completely different order. This is easier to see with specific examples. What follows looks at some of the ways that structural isomers can arise. The names of the various forms of structural isomerism probably don't matter all that much, but you must be aware of the different possibilities when you come to draw isomers. Types of structural isomerism Chain isomerism These isomers arise because of the possibility of branching in carbon chains. For example, there are two isomers of butane, C4H10. In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is branched.
	Note:  Although the chain is drawn as straight, in reality it's anything but straight. If you aren't happy about the ways of drawing organic molecules, follow this link. Use the BACK button on your browser to return to this page.
Be careful not to draw "false" isomers which are just twisted versions of the original molecule. For example, this structure is just the straight chain version of butane rotated about the central carbon-carbon bond. You could easily see this with a model. This is the example we've already used at the top of this page. Pentane, C5H12, has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below. If in doubt make some models. Position isomerism In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton. For example, there are two structural isomers with the molecular formula C3H7Br. In one of them the bromine atom is on the end of the chain, whereas in the other it's attached in the middle. If you made a model, there is no way that you could twist one molecule to turn it into the other one. You would have to break the bromine off the end and re-attach it in the middle. At the same time, you would have to move a hydrogen from the middle to the end. Another similar example occurs in alcohols such as C4H9OH These are the only two possibilities provided you keep to a four carbon chain, but there is no reason why you should do that. You can easily have a mixture of chain isomerism and position isomerism - you aren't restricted to one or the other. So two other isomers of butanol are:
	Note:  It's essential if you are asked to draw isomers in an exam not to restrict yourself to chain isomers or position isomers. You must be aware of all the possibilities.
You can also get position isomers on benzene rings. Consider the molecular formula C7H7Cl. There are four different isomers you could make depending on the position of the chlorine atom. In one case it is attached to the side-group carbon atom, and then there are three other possible positions it could have around the ring - next to the CH3 group, next-but-one to the CH3 group, or opposite the CH3 group. Functional group isomerism In this variety of structural isomerism, the isomers contain different functional groups - that is, they belong to different families of compounds (different homologous series). For example, a molecular formula C3H6O could be either propanal (an aldehyde) or propanone (a ketone). There are other possibilities as well for this same molecular formula - for example, you could have a carbon-carbon double bond (an alkene) and an -OH group (an alcohol) in the same molecule. Another common example is illustrated by the molecular formula C3H6O2. Amongst the several structural isomers of this are propanoic acid (a carboxylic acid) and methyl ethanoate (an ester).
	Note:  To repeat the warning given earlier: If you are asked to draw the structural isomers from a given molecular formula, don't forget to think about all the possibilities. Can you branch the carbon chain? Can you move a group around on that chain? Is it possible to make more than one type of compound? Be careful though! If you are asked to draw the structures of esters with the molecular formula C3H6O2, you aren't going to get a lot of credit for drawing propanoic acid, even if it is a valid isomer.

USING CURLY ARROWS IN REACTION MECHANISMS

Using curly arrows to show the movement of electron pairs Curly arrows (and that's exactly what they are called!) are used in mechanisms to show the various electron pairs moving around. You mustn't use them for any other purpose. The arrow tail is where the electron pair starts from. That's always fairly obvious, but you must show the electron pair either as a bond or, if it is a lone pair, as a pair of dots. Remember that a lone pair is a pair of electrons at the bonding level which isn't currently being used to join on to anything else. The arrow head is where you want the electron pair to end up. For example, in the reaction between ethene and hydrogen bromide, one of the two bonds between the two carbon atoms breaks. That bond is simply a pair of electrons. Those electrons move to form a new bond with the hydrogen from the HBr. At the same time the pair of electrons in the hydrogen-bromine bond moves down on to the bromine atom. There's no need to draw the pairs of electrons in the bonds as two dots. Drawing the bond as a line is enough, but you could put two dots in as well if you wanted to. Notice that the arrow head points between the C and H because that's where the electron pair ends up. Notice also that the electron movement between the H and Br is shown as a curly arrow even though the electron pair moves straight down. You have to show electron pair movements as curly arrows - not as straight ones. The second stage of this reaction nicely illustrates how you use a curly arrow if a lone pair of electrons is involved. The first stage leaves you with a positive charge on the right hand carbon atom and a negative bromide ion. You can think of the electrons shown on the bromide ion as being the ones which originally made up the hydrogen-bromine bond.
	Note:  There are another three lone pairs around the outside of the bromide ion - making four in all. These aren't normally shown because they don't actually do anything new and interesting! However, it is essential that you show the lone pair you are interested in as a pair of dots. If you don't, you risk losing marks in an exam.
The lone pair on the bromide ion moves to form a new bond between the bromine and the right hand carbon atom. That movement is again shown by a curly arrow. Notice again, that the curly arrow points between the carbon and the bromine because that's where the electron pair ends up. That leaves you with the product of this reaction, bromoethane:
	Note:  You can read a full description of this mechanism together with other similar reactions of ethene and the other alkenes by following this link.
Using curly arrows to show the movement of single electrons The most common use of "curly arrows" is to show the movement of pairs of electrons. You can also use similar arrows to show the movement of single electrons - except that the heads of these arrows only have a single line rather than two lines. shows the movement of an electron pair shows the movement of a single electron The first stage of the polymerisation of ethene, for example, could be shown as: You should draw the dots showing the interesting electrons. The half arrows show where they go. This is very much a "belt-and-braces" job, and the arrows don't add much. Whether you choose to use these half arrows to show the movement of a single electron should be governed by what your syllabus says. If your syllabus encourages the use of these arrows, then it makes sense to use them. If not - if the syllabus says that they "may" be used, or just ignores them altogether - then they are as well avoided. There is some danger of confusing them with the arrows showing electron pair movements, which you will use all the time. If, by mistake, you use an ordinary full arrow to show the movement of a single electron you run the risk of losing marks.
	Note:  If you are a working to one of the UK-based syllabuses for 16 - 18 year olds and haven't got a copy, find out how to get a syllabus by following this link.

THE NAMES OF AROMATIC COMPOUNDS

	Note:  If you aren't sure about naming aliphatic compounds follow this link before you go on.
Naming aromatic compounds isn't quite so straightforward as naming chain compounds. Often, more than one name is acceptable and it's not uncommon to find the old names still in use as well. Background The benzene ring All aromatic compounds are based on benzene, C6H6, which has a ring of six carbon atoms and has the symbol: Each corner of the hexagon has a carbon atom with a hydrogen attached.
	Note:  If you don't understand this structure, it is explained in full in two pages on the structure of benzene elsewhere in this site. Following this link could well take you some time!
The phenyl group Remember that you get a methyl group, CH3, by removing a hydrogen from methane, CH4. You get a phenyl group, C6H5, by removing a hydrogen from a benzene ring, C6H6. Like a methyl or an ethyl group, a phenyl group is always attached to something else. Aromatic compounds with only one group attached to the benzene ring Cases where the name is based on benzene chlorobenzene This is a simple example of a halogen attached to the benzene ring. The name is self-obvious. The simplified formula for this is C6H5Cl. You could therefore (although you never do!) call it phenyl chloride. Whenever you draw a benzene ring with one other thing attached to it, you are in fact drawing a phenyl group. In order to attach something else, you have to remove one of the existing hydrogen atoms, and so automatically make a phenyl group. nitrobenzene The nitro group, NO2, is attached to a benzene ring. The simplified formula for this is C6H5NO2. methylbenzene Another obvious name - the benzene ring has a methyl group attached. Other alkyl side-chains would be named similarly - for example, ethylbenzene. The old name for methylbenzene is toluene, and you may still meet that. The simplified formula for this is C6H5CH3. (chloromethyl)benzene A variant on this which you may need to know about is where one of the hydrogens on the CH3 group is replaced by a chlorine atom. Notice the brackets around the (chloromethyl) in the name. This is so that you are sure that the chlorine is part of the methyl group and not somewhere else on the ring. If more than one of the hydrogens had been replaced by chlorine, the names would be (dichloromethyl)benzene or (trichloromethyl)benzene. Again, notice the importance of the brackets in showing that the chlorines are part of the side group and not directly attached to the ring. benzoic acid (benzenecarboxylic acid) Benzoic acid is the older name, but is still in common use - it's a lot easier to say and write than the modern alternative! Whatever you call it, it has a carboxylic acid group, -COOH, attached to the benzene ring. Cases where the name is based on phenyl Remember that the phenyl group is a benzene ring minus a hydrogen atom - C6H5. If you draw a benzene ring with one group attached, you have drawn a phenyl group. phenylamine Phenylamine is a primary amine and contains the -NH2 group attached to a benzene ring. The old name for phenylamine is aniline, and you could also reasonably call it aminobenzene. Phenylamine is what it is most commonly for UK-based exam purposes.
	Note:  In all cases where there is some possibility of alternative names, you need to know what your examiners are likely to call a particular compound. Refer to your syllabus and recent exam papers. If you are working to a UK-based syllabus for 16 - 8 year olds, and haven't got these, follow this link to find out how to get hold of them.
phenylethene This is an ethene molecule with a phenyl group attached. Ethene is a two carbon chain with a carbon-carbon double bond. Phenylethene is therefore: The old name for phenylethene is styrene - the monomer from which polystyrene is made. phenylethanone This is a slightly awkward name - take it to pieces. It consists of a two carbon chain with no carbon-carbon double bond. The one ending shows that it is a ketone, and so has a C=O group somewhere in the middle. Attached to the carbon chain is a phenyl group. Putting that together gives: phenyl ethanoate This is an ester based on ethanoic acid. The hydrogen atom in the -COOH group has been replaced by a phenyl group.
	Note:  If you aren't happy about naming esters, follow this link before you go on.
phenol Phenol has an -OH group attached to a benzene ring and so has a formula C6H5OH. Aromatic compounds with more than one group attached to the benzene ring Numbering the ring Any group already attached to the ring is given the number 1 position. Where you draw it on the ring (at the top or in any other position) doesn't matter - that's just a question of rotating the molecule a bit. It's much easier, though, to get in the habit of drawing your main group at the top. The other ring positions are then numbered from 2 to 6. You can number them either clockwise or anti-clockwise. As with chain compounds, you number the ring so that the name you end up with has the smallest possible numbers in it. Examples will make this clear. Some simple examples Substituting chlorine atoms on the ring Look at these compounds: All of these are based on methylbenzene and so the methyl group is given the number 1 position on the ring. Why is it 2-chloromethylbenzene rather than 6-chloromethylbenzene? The ring is numbered clockwise in this case because that produces a 2- in the name rather than a 6-. 2 is smaller than 6.
	Warning!  You will find all sorts of variations on this depending on the age of the book you look it up in, and where it was published. What I have described above isn't in strict accordance with the most modern interpretation of the IUPAC recommendations for naming organic compounds. The names should actually be 1-chloro-2-methylbenzene, 1-chloro-3-methylbenzene, and so on. The substituted groups are named in alphabetical order, and the "1" position is assigned to the first of these - rather than to the more logical methyl group. This produces some silly inconsistencies. For example, if you had the exactly equivalent compounds containing nitro groups in place of the chlorines, the names would change completely, to 1-methyl-2-nitrobenzene, 1-methyl-3-nitrobenzene, etc. In this case, the normal practice of naming the hydrocarbon first, and then attaching other things to it has been completely wrecked. Do you need to worry about this? NO! It is extremely unlikely that you would ever be asked to name these in an exam, and it is always easy to write a structure from one of these names - however illogical it may be! There is a simple rule for exam purposes. Unless you are specifically asked for the name of anything remotely complicated, don't give it. As long as you have got the structure right, that's all that matters.
2-hydroxybenzoic acid This might also be called 2-hydroxybenzenecarboxylic acid. There is a -COOH group attached to the ring and, because the name is based on benzoic acid, that group is assigned the number 1 position. Next door to it in the 2 position is a hydroxy group, -OH. benzene-1,4-dicarboxylic acid The di shows that there are two carboxylic acid groups, -COOH, one of them in the 1 position and the other opposite it in the 4 position. 2,4,6-trichlorophenol This is based on phenol - with an -OH group attached in the number 1 position on the ring. There are 3 chlorine atoms substituted onto the ring in the 2, 4 and 6 positions. methyl 3-nitrobenzoate This is a name you might come across as a part of a practical exercise in nitrating benzene rings. It's included partly for that reason, and partly because it is a relatively complicated name to finish with! The structure of the name shows that it is an ester. You can tell that from the oate ending, and the methyl group floating separately from the rest of the name at the beginning. The ester is based on the acid, 3-nitrobenzoic acid - so start with that. There will be a benzene ring with a -COOH group in the number 1 position and a nitro group, NO2, in the 3 position. The -COOH group is modified to make an ester by replacing the hydrogen of the -COOH group by a methyl group. Methyl 3-nitrobenzoate is therefore:

THE NAMES OF MORE ORGANIC COMPOUNDS

	Note:  If you haven't already looked at that page, it would be a good idea to do so before you go on. The names on this second page aren't explained in quite as much detail as those on the introductory page - it assumes that you have already understood the main principles. If in doubt, follow this link first.
More types of organic compound Carboxylic acids Carboxylic acids contain the -COOH group, which is better written out in full as: Carboxylic acids are shown by the ending oic acid. When you count the carbon chain, you have to remember to include the carbon in the -COOH group. That carbon is always thought of as number 1 in the chain. Example 1:  Write the structural formula for 3-methylbutanoic acid. This is a four carbon acid with no carbon-carbon double bonds. There is a methyl group on the third carbon (counting the -COOH carbon as number 1). Example 2:  Write the structural formula for 2-hydroxypropanoic acid. The hydroxy part of the name shows the presence of an -OH group. Normally, you would show that by the ending ol, but this time you can't because you've already got another ending. You are forced into this alternative way of describing it. The old name for 2-hydroxypropanoic acid is lactic acid. That name sounds more friendly, but is utterly useless when it comes to writing a formula for it. In the old days, you would have had to learn the formula rather than just working it out should you need it. Example 3:  Write the structural formula for 2-chlorobut-3-enoic acid. This time, not only is there a chlorine attached to the chain, but the chain also contains a carbon-carbon double bond (en) starting on the number 3 carbon (counting the -COOH carbon as number 1). Salts of carboxylic acids Example:  Write the structural formula for sodium propanoate. This is the sodium salt of propanoic acid - so start from that. Propanoic acid is a three carbon acid with no carbon-carbon double bonds. When the carboxylic acids form salts, the hydrogen in the -COOH group is replaced by a metal. Sodium propanoate is therefore: Notice that there is an ionic bond between the sodium and the propanoate group. Whatever you do, don't draw a line between the sodium and the oxygen. That would represent a covalent bond. It's wrong, and makes you look very incompetent in an exam! In a shortened version, sodium propanoate would be written CH3CH2COONa or, if you wanted to emphasise the ionic nature, as CH3CH2COO- Na+.
	Note:  The confusing thing about these salts (and even more so for the esters that are coming up next) is that they are named the wrong way round. In the formula, the sodium is at the end, but appears first in the name. Why? Salts are always named with the metal first - think of sodium chloride or potassium iodide. So for consistency you would need to reverse the formula of sodium propanoate - NaOOCCH2CH3. But if you reverse the formula, you can't see immediately that it is related to propanoic acid. So you learn to live with the inconsistency.
Esters Esters are one of a number of compounds known collectively as acid derivatives. In these the acid group is modified in some way. In an ester, the hydrogen in the -COOH group is replaced by an alkyl group (or possibly some more complex hydrocarbon group). Example 1:  Write the structural formula for methyl propanoate. An ester name has two parts - the part that comes from the acid (propanoate) and the part that shows the alkyl group (methyl). Start by thinking about propanoic acid - a 3 carbon acid with no carbon-carbon double bonds. The hydrogen in the -COOH group is replaced by an alkyl group - in this case, a methyl group. Ester names are confusing because the name is written backwards from the way the structure is drawn. There's no way round this - you just have to get used to it! In the shortened version, this formula would be written CH3CH2COOCH3. Example 2:  Write the structural formula for ethyl ethanoate. This is probably the most commonly used example of an ester. It is based on ethanoic acid ( hence, ethanoate) - a 2 carbon acid. The hydrogen in the -COOH group is replaced by an ethyl group. Make sure that you draw the ethyl group the right way round. A fairly common mistake is to try to join the CH3 group to the oxygen. If you count the bonds if you do that, you will find that both the CH3 carbon and the CH2 carbon have the wrong number of bonds. Acyl chlorides (acid chlorides) An acyl chloride is another acid derivative. In this case, the -OH group of the acid is replaced by -Cl. All acyl chlorides contain the -COCl group: Example:  Write the structural formula for ethanoyl chloride. Acyl chlorides are shown by the ending oyl chloride. So ethanoyl chloride is based on a 2 carbon chain with no carbon-carbon double bonds and a -COCl group. The carbon in that group counts as part of the chain. In a longer chain, with side groups attached, the -COCl carbon is given the number 1 position. Acid anhydrides Another acid derivative! An acid anhydride is what you get if you dehydrate an acid - that is, remove water from it. Example:  Write the structural formula for propanoic anhydride. These are most easily worked out by writing it down on a scrap of paper in the following way: Draw two molecules of acid arranged so that the -OH groups are next to each other. Tweak out a molecule of water - and then join up what's left. In this case, because you want propanoic anhydride, you draw two molecules of propanoic acid. Amides Yet another acid derivative! Amides contain the group -CONH2 where the -OH of an acid is replaced by -NH2. Example:  Write the structural formula for propanamide. This is based on a 3 carbon chain with no carbon-carbon double bonds. At the end of the chain is a -CONH2 group. The carbon in that group counts as part of the chain. Nitriles Nitriles contain a -CN group, and used to be called cyanides. Example 1:  Write the structural formula for ethanenitrile. The name shows a 2 carbon chain with no carbon-carbon double bond. nitrile shows a -CN group at the end of the chain. As with the previous examples involving acids and acid derivatives, don't forget that the carbon in the -CN group counts as part of the chain. The old name for this would have been methyl cyanide. You might think that that's easier, but as soon as the chain gets more complicated, it doesn't work - as the next example shows. Example 2:  Write the structural formula for 2-hydroxypropanenitrile. Here we've got a 3 carbon chain, no carbon-carbon double bonds, and a -CN group on the end of the chain. The carbon in the -CN group counts as the number 1 carbon. On the number 2 carbon there is an -OH group (hydroxy). Notice that you can't use the ol ending because you've already got a nitrile ending. Primary amines A primary amine contains the group -NH2 attached to a hydrocarbon chain or ring. You can think of amines in general as being derived from ammonia, NH3. In a primary amine, one of the hydrogens has been replaced by a hydrocarbon group. Example 1:  Write the structural formula for ethylamine. In this case, an ethyl group is attached to the -NH2 group. This name (ethylamine) is fine as long as you've only got a short chain where there isn't any ambiguity about where the -NH2 group is found. But suppose you had a 3 carbon chain - in this case, the -NH2 group could be on an end carbon or on the middle carbon. How you get around that problem is illustrated in the next example. Example 2:  Write the structural formula for 2-aminopropane. The name shows a 3 carbon chain with an amino group attached to the second carbon.  amino shows the -NH2 group. Ethylamine (example 1 above) could equally well have been called aminoethane. Secondary and tertiary amines You are only likely to come across simple examples of these. In a secondary amine, two of the hydrogen atoms in an ammonia molecule have been replaced by hydrocarbon groups. In a tertiary amine, all three hydrogens have been replaced. Example 1:  Write the structural formula for dimethylamine. In this case, two of the hydrogens in ammonia have been replaced by methyl groups. Example 2:  Write the structural formula for trimethylamine. Here, all three hydrogens in ammonia have been replaced by methyl groups. Amino acids An amino acid contains both an amino group, -NH2, and a carboxylic acid group, -COOH, in the same molecule. As with all acids the carbon chain is numbered so that the carbon in the -COOH group is counted as number 1. Example:  Write the structural formula for 2-aminopropanoic acid. This has a 3 carbon chain with no carbon-carbon double bonds. On the second carbon (counting the -COOH carbon as number 1) there is an amino group, -NH2.