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: Notice that the way the methane is drawn bears no resemblance to the actual 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 problems if you aren't careful. 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. 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 as models. One structure is in reality a simple rotation of the other one. Consider a slightly more complicated molecule, C2H5Cl. The displayed formula could be written as either of these: But, again these are exactly the same. Look at the models. The commonest way to draw structural formulae 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 could even condense it further to CH3COOH, and would probably do this if you had to write a simple chemical equation involving ethanoic acid. You do, however, lose something by condensing the acid group in this way, because you can't immediately see how the bonding works. You still have to be careful in drawing structures in this way. Remember from above that these two structures both represent the same molecule: The next three structures all represent butane. All of these are just versions of four carbon atoms joined up in a line. The only difference is that there has been some rotation about 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 even more important when you start to have branched chains of carbon atoms. The following structures again all represent the same molecule - 2-methylbutane. The two structures on the left are fairly obviously the same - all we've done is flip the molecule over. The other one isn't so obvious until you look at the structure in detail. There are four carbons joined up in a row, with a CH3 group attached to the next-to-end one. That's exactly the same as the other two structures. If you had a model, the only difference between these three diagrams is that you have rotated some of the bonds and turned the model around a bit. 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. If you made a model of one of them, you could turn it into any other one simply by rotating one or more of the carbon-carbon bonds. 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: For example, you might want to show the 3-D arrangement of the groups around the carbon which has the -OH group in butan-2-ol. Butan-2-ol has the structural formula: Using conventional bond notation, you could draw it as, for example: The only difference between these is a slight rotation of the bond between the centre two 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-hand model, it is tucked behind the carbon atom. In the right-hand model, it is in the same plane. The change is very slight. It doesn't matter in the least which of the two arrangements you draw. You could easily invent other ones as well. Choose one of them and get into the habit of drawing 3-dimensional structures that way. My own habit (used elsewhere on this site) is to draw two bonds going back into the paper and one coming out - as in the left-hand diagram above. 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: In a skeletal diagram of this sort
Beware! Diagrams of this sort take practice to interpret correctly - and may well not be acceptable to your examiners (see below). 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. And this is cyclohexene, which is similar but contains a double bond: But the commonest of all is the benzene ring, C6H6, which has a special symbol of its own. 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. What if you still aren't sure? Draw the most detailed formula that you can fit into the space available. If in doubt, draw a fully displayed formula. You would never lose marks for giving too much detail. Apart from the most trivial cases (for example, burning hydrocarbons), never use a molecular formula. Always show the detail around the important part(s) of a molecule. For example, the important part of an ethene molecule is the carbon-carbon double bond - so write (at the very least) CH2=CH2 and not C2H4. Where a particular way of drawing a structure is important, this will always be pointed out where it arises elsewhere on this site.
THE NAMES OF ORGANIC COMPOUNDS This page explains how to write the formula for an organic compound given its name - and vice versa. It covers alkanes, cycloalkanes, alkenes, simple compounds containing halogens, alcohols, aldehydes and ketones. At the bottom of the page, you will find links to other types of compound. Background How this page is going to tackle the problem There are two skills you have to develop in this area: You need to be able to translate the name of an organic compound into its structural formula. You need to be able to name a compound from its given formula. The first of these is more important (and also easier!) than the second. In an exam, if you can't write a formula for a given compound, you aren't going to know what the examiner is talking about and could lose lots of marks. However, you might only be asked to write a name for a given formula once in a whole exam - in which case you only risk 1 mark. So, we're going to look mainly at how you decode names and turn them into formulae. In the process you will also pick up tips about how to produce names yourself. In the early stages of an organic chemistry course people frequently get confused and daunted by the names because they try to do too much at once. Don't try to read all these pages in one go. Just go as far as the compounds you are interested in at the moment and ignore the rest. Come back to them as they arise during the natural flow of your course. Cracking the code A modern organic name is simply a code. Each part of the name gives you some useful information about the compound. For example, to understand the name 2-methylpropan-1-ol you need to take the name to pieces. The prop in the middle tells you how many carbon atoms there are in the longest chain (in this case, 3). The an which follows the "prop" tells you that there aren't any carbon-carbon double bonds. The other two parts of the name tell you about interesting things which are happening on the first and second carbon atom in the chain. Any name you are likely to come across can be broken up in this same way. Counting the carbon atoms You will need to remember the codes for the number of carbon atoms in a chain up to 6 carbons. There is no easy way around this - you have got to learn them. If you don't do this properly, you won't be able to name anything! Types of carbon-carbon bonds Whether or not the compound contains a carbon-carbon double bond is shown by the two letters immediately after the code for the chain length. For example, butane means four carbons in a chain with no double bond. Propene means three carbons in a chain with a double bond between two of the carbons. Alkyl groups Compounds like methane, CH4, and ethane, CH3CH3, are members of a family of compounds called alkanes. If you remove a hydrogen atom from one of these you get an alkyl group. For example: A methyl group is CH3. An ethyl group is CH3CH2. These groups must, of course, always be attached to something else. Types of compounds The alkanes Example 1: Write the structural formula for 2-methylpentane. Start decoding the name from the bit that counts the number of carbon atoms in the longest chain - pent counts 5 carbons. Are there any carbon-carbon double bonds? No - an tells you there aren't any. Now draw this carbon skeleton: Put a methyl group on the number 2 carbon atom: Does it matter which end you start counting from? No - if you counted from the other end, you would draw the next structure. That's exactly the same as the first one, except that it has been flipped over. Finally, all you have to do is to put in the correct number of hydrogen atoms on each carbon so that each carbon is forming four bonds. If you had to name this yourself: Count the longest chain of carbons that you can find. Don't assume that you have necessarily drawn that chain horizontally. 5 carbons means pent. Are there any carbon-carbon double bonds? No - thereforepentane. There's a methyl group on the number 2 carbon - therefore 2-methylpentane. Why the number 2 as opposed to the number 4 carbon? In other words, why do we choose to number from this particluar end? The convention is that you number from the end which produces the lowest numbers in the name - hence 2- rather than 4-. Example 2: Write the structural formula for 2,3-dimethylbutane. Start with the carbon backbone. There are 4 carbons in the longest chain (but) with no carbon-carbon double bonds (an). This time there are two methyl groups (di) on the number 2 and number 3 carbon atoms. Completing the formula by filling in the hydrogen atoms gives: Example 3: Write the structural formula for 2,2-dimethylbutane. This is exactly like the last example, except that both methyl groups are on the same carbon atom. Notice that the name shows this by using 2,2- as well as di. The structure is worked out as before: Example 4: Write the structural formula for 3-ethyl-2-methylhexane. hexan shows a 6 carbon chain with no carbon-carbon double bonds. This time there are two different alkyl groups attached - an ethyl group on the number 3 carbon atom and a methyl group on number 2. Filling in the hydrogen atoms gives: If you had to name this yourself: How do you know what order to write the different alkyl groups at the beginning of the name? The convention is that you write them in alphabetical order - hence ethyl comes before methyl which in turn comes before propyl. The cycloalkanes In a cycloalkane the carbon atoms are joined up in a ring - hencecyclo. Example: Write the structural formula for cyclohexane. hexan shows 6 carbons with no carbon-carbon double bonds.cyclo shows that they are in a ring. Drawing the ring and putting in the correct number of hydrogens to satisfy the bonding requirements of the carbons gives: The alkenes Example 1: Write the structural formula for propene. prop counts 3 carbon atoms in the longest chain. en tells you that there is a carbon-carbon double bond. That means that the carbon skeleton looks like this: Putting in the hydrogens gives you: Example 2: Write the structural formula for but-1-ene. but counts 4 carbon atoms in the longest chain and en tells you that there is a carbon-carbon double bond. The number in the name tells you where the double bond starts. No number was necessary in the propene example above because the double bond has to start on one of the end carbon atoms. In the case of butene, though, the double bond could either be at the end of the chain or in the middle - and so the name has to code for the its position. The carbon skeleton is: And the full structure is: Example 3: Write the structural formula for 3-methylhex-2-ene. The longest chain has got 6 carbon atoms (hex) with a double bond starting on the second one (-2-en). But this time there is a methyl group attached to the chain on the number 3 carbon atom, giving you the underlying structure: Adding the hydrogens gives the final structure: Be very careful to count the bonds around each carbon atom when you put the hydrogens in. It would be very easy this time to make the mistake of writing an H after the third carbon - but that would give that carbon a total of 5 bonds. Compounds containing halogens Example 1: Write the structural formula for 1,1,1-trichloroethane. This is a two carbon chain (eth) with no double bonds (an). There are three chlorine atoms all on the first carbon atom. Example 2: Write the structural formula for 2-bromo-2-methylpropane. First sort out the carbon skeleton. It's a three carbon chain with no double bonds and a methyl group on the second carbon atom. Draw the bromine atom which is also on the second carbon. And finally put the hydrogen atoms in. If you had to name this yourself: Notice that the whole of the hydrocarbon part of the name is written together - as methylpropane - before you start adding anything else on to the name. Example 2: Write the structural formula for 1-iodo-3-methylpent-2-ene. This time the longest chain has 5 carbons (pent), but has a double bond starting on the number 2 carbon. There is also a methyl group on the number 3 carbon. Now draw the iodine on the number 1 carbon. Giving a final structure: Alcohols All alcohols contain an -OH group. This is shown in a name by the ending ol. Example 1: Write the structural formula for methanol. This is a one carbon chain with no carbon-carbon double bond (obviously!). The ol ending shows it's an alcohol and so contains an -OH group. Example 2: Write the structural formula for 2-methylpropan-1-ol. The carbon skeleton is a 3 carbon chain with no carbon-carbon double bonds, but a methyl group on the number 2 carbon. The -OH group is attached to the number 1 carbon. The structure is therefore: Example 3: Write the structural formula for ethane-1,2-diol. This is a two carbon chain with no double bond. The diol shows 2 -OH groups, one on each carbon atom. Aldehydes All aldehydes contain the group: If you are going to write this in a condensed form, you write it as -CHO - never as -COH, because that looks like an alcohol. The names of aldehydes end in al. Example 1: Write the structural formula for propanal. This is a 3 carbon chain with no carbon-carbon double bonds. Theal ending shows the presence of the -CHO group. The carbon in that group counts as one of the chain. Example 2: Write the structural formula for 2-methylpentanal. This time there are 5 carbons in the longest chain, including the one in the -CHO group. There aren't any carbon-carbon double bonds. A methyl group is attached to the number 2 carbon. Notice that in aldehydes, the carbon in the -CHO group is always counted as the number 1 carbon. Ketones Ketones contain a carbon-oxygen double bond just like aldehydes, but this time it's in the middle of a carbon chain. There isn't a hydrogen atom attached to the group as there is in aldehydes. Ketones are shown by the ending one. Example 1: Write the structural formula for propanone. This is a 3 carbon chain with no carbon-carbon double bond. The carbon-oxygen double bond has to be in the middle of the chain and so must be on the number 2 carbon. Ketones are often written in this way to emphasise the carbon-oxygen double bond. Example 2: Write the structural formula for pentan-3-one. This time the position of the carbon-oxygen double bond has to be stated because there is more than one possibility. It's on the third carbon of a 5 carbon chain with no carbon-carbon double bonds. If it was on the second carbon, it would be pentan-2-one. This could equally well be written: THE NAMES OF MORE ORGANIC COMPOUNDS This page continues looking at the names of organic compounds containing chains of carbon atoms. It assumes that you have already looked at the introductory page covering compounds from alkanes to ketones. 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+. Esters Esters are one of a number of compounds known collectively asacid 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 CH3carbon 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 -CONH2where 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 olending 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. THE NAMES OF AROMATIC COMPOUNDS This page looks at the names of some simple aromatic compounds. An aromatic compound is one which contains a benzene ring. It assumes that you are reasonably confident about naming compounds containing chains of carbon atoms (aliphatic compounds). 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. 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. 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 oneending 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. 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. 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: USING CURLY ARROWS IN REACTION MECHANISMS This page explains the use of curly arrows to show the movement both of electron pairs and of single electrons during organic reaction mechanisms. You can jump straight to the movement of single electrons further down this page if that is all you are interested in for the moment (for example, if you are currently working on free radical reactions). 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. 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: 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. 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. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Aug 28, 2010 | By Ajesh
Organic chemistry conventions
shows the movement of an electron pair
shows the movement of a single electron
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