Organic Nomenclature The naming of molecules Suitable for Year 12 Chemistry Made by Richard Henty The first thing you need to memorise, is the prefix given to molecules with a certain number of carbons in a straight chain. 1 = Meth- 2 = Eth- 3 = Prop- 4 = But- 5 = Pent- 6 = Hex- 7 = Hept- 8 = Oct- 9 = Non- 10 = Dec- You will see these prefixes come up in molecules like methane, ethanol, propyl butanoate, cyclohexene. Notice that the ending (suffix) changes. Alkanes. The simplest molecules, particularly if they are straight chained. For these, count the number of linked carbons, find the prefix and put -ane (the suffix) after it. H H H H 4 carbons, but- & -ane = butane H C C C C H H H H H H 3 carbons H H C H = propane H C H 8 carbons = octane H C H H C H H H H H H H C H H H C C C C C C H Even though it is “bent”, it H H H H H H is called a straight chain. How many carbon atoms are there in the longest carbon chain? There are fourteen H carbon atoms in the longest chain. It H C H does not matter that it looks bent. H C H H H H H H H C C C C C C H H H H H H C H H H C H H H H H H H H H H C C C C C C C C C H H H H H H H H H H Alkanes that have extra carbons and hydrogens sticking off of them are called branched alkanes. The size of the branch will affect its name. H This molecule has two branches, one with three H C H carbons and one with two carbons. The other fourteen carbons make up the alkane “backbone”. H C H H H H H H H C C C C C C H H H H H H C H H H C H H H H H H H H H H C C C C C C C C C H H H H H H H H H H H H H H H H H H H C H H H C C C C C C C C H H H H H H H C H H C H C C H H H H H H H C C H H H H C C H H H H H H C C C C H H C H H H H H H C H H H H A branch that has one carbon H C C C H in it is called a methyl group. H H A branch that has two carbons H C H in it is called an ethyl group. H C H A branch that has three carbons in it is called a propyl group. H The branch name is added at the start of the alkane name, like another prefix. Numbering my be used. For example: methylpropane These contain a single branch where ethylpentane only one position is possible. propyloctane These contain one branch, but there 2-methylpentane are two possible positions, so we 3-ethylnonane must show what carbon it is on. 2,2-dimethylbutane These contain multiples of the same 2,3,4-trimethylhexane branch, so we must number them 3,3,4,4-tetramethylheptane and use another prefix as well. 2-methyl-3-ethylheptane Two different branches, with their positions numbered When numbering, you can count from either end of the the longest straight chain. But, the branch number must be the lowest possible. The longest chain in this molecule is five carbons, with a 1 carbon methyl branch. If we count from the left, the methyl group is on carbon #4. If we count from the right, the methyl group is on carbon #2. This is the preferred number. This is 2-methylpentane If there are two branches, the sum of their numbers must be the lowest. The longest chain is again five, hence pentane. Two methyl branches. From the left, on carbons #3 and #4. From the right, on carbons #2 and #3. This is the preferred numbering. This molecule is 2,3-dimethylpentane What are the names of these molecules? 2,2-dimethylbutane 2,4-dimethylpentane. The same no matter which way you count. methylbutane. No number needed (but OK if you put 2) because the only place for a methyl on butane is at carbon #2. What is the name for this big molecule? H H H H H H H H H C C C C C C C C H H H H H H C H C C H H H H H H C C H H H H H H C H H 2-methyl-6-ethyl-4-propyloctane It does not matter what order you put the branches, so long as they have the correct numbering, and dashes between them. Functional groups. Alkanes are the only molecules that do not have a functional group. A functional group is a distinct part of a molecule that is involved in a chemical reaction and gives the molecule its physical characteristics. Different functional groups result in different chemical and physical properties. You will need to be able to identify a functional group when you find it in a molecule. Most molecules have only one functional group. Some have two of the same, some have two different or more. Functional groups alter the name of the straight chain of carbons that they are found upon. Most change the suffix -ane to something else. If there is a double bond then it is now -ene (Alkene family) If there is a triple bond then it is now -yne (Alkyne family) If there is an -OH attached then it is now -anol (Alcohol family) If there is a -COOH at one end then it is now -anoic acid (Carboxylic acid family) If there is a halogen atom attached then it is still -ane, but given a prefix of chloro (if its chlorine) or bromo. Some molecules: propene, ethyne, methanol, chloroethane, butanoic acid, 2-pentene, 2-methyl-2-propanol, 2,3-dibromobutane, 3-pentyne. Alkenes (easily identified by the double bond). In the same way as if there were a branch, the alkene is numbered so that the double bond starts at the lowest numbered carbon possible. Two carbons = ethene (only one place the double bond can go, so no number) Three carbons = propene (only one place the double bond can go, so no number). Butene is the first alkene where there are two possible places for the double bond. On the left 1-butene and on the right 2-butene. Disclaimer Some text books give a different way of writing the number into the name. 2-butene may be seen as but-2-ene. The reason for this is that the number is right next to the functional group it is counting for. Either way is acceptable, because if you drew 2-butene or but-2-ene you would get the same thing. There is no confusion. The only time confusion occurs is if there are two different functional groups in the same molecule. (Rare in Year 12 or 13 Chemistry). You decide which you prefer using; here we will use the 2-butene method. Name these alkene molecules. A branch can be treated in the same way as with alkanes. 3-methyl-3-hexene 2-pentene 1-butene Alkene molecules can have the same atoms, but rearranged slightly differently about the double bond. This is because the double bond cannot rotate. These are geometric isomers, given cis - or trans- notation. Trans-2-pentene Cis-2-pentene The main branch The main branch continues on opposite continues on the same sides of the double bond. side of the double bond. What are the names of these molecules, and what are their cis or trans name if needed. 1-butene 3-methyl-trans-3-hexene cis-2-butene cis-2-pentene trans-2-pentene ethene Alcohols can have the -OH group positioned on any of the carbons within the chain. The -OH group must be numbered with the lowest number. 1-butanol 2-methyl-1-butanol 2-pentanol 3-ethyl-3-pentanol 2-methyl-2-propanol methanol Disclaimer Using the name 2-propanol is that same as using the name propan-2-ol. Both yeild the same structure. Either is appropriate. The 2-propanol method will be used here. Alcohols - Primary, secondary, tertiary strucutres. The position of the -OH with the structure classifies it as 1o, 2o, an 3o. If the C-OH is bonded to one other carbon it is primary, 1o. If the C-OH is bonded to two other carbons (one either side) then it is secondary, 2o. If the C-OH has three other carbons bonded to that carbon then it is tertiary, 3o. Carboxylic acids always have their functional group at the end of the chain on carbon one. Count the number of carbons, use the prefix and then anoic acid. Butanoic acid 2-methylbutanoic acid Methanoic acid 3-methylbutanoic acid Propanoic acid Haloalkanes - these are molecules that contain one or more halogen atom - F, Cl, Br or I. The molecule is considered to be an alkane derivative containing a halogen, so the longest carbon chain alkane is identified, and the halogen is numbered based on its position. F prefix is fluoro, Cl is chloro, Br is bromo, I is iodo 1-bromobutane 2-chlorobutane 1-chloropropane 2-methyl-2-chlorobutane Aldehydes and ketones Both of these contain a C=O (carbonyl group), but its position is significant. For an aldehyde, the C=O must be on the first or last carbon (always counted as carbon #1). Butanal For a ketone, the C=O must be within the carbon chain and not on the first or last carbon. 2-pentanone Aldehydes have the suffix -al. Ketones have the suffix -one.
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