some rules for naming

alkanes

Introduction

In the section called 'Structures and Formulas', the role of the IUPAC (International Union of Pure and Applied Chemistry) was described with respect to the naming of organic compounds. Prior to the action taken by the IUPAC committee, organic chemical names were coined by chemists and these names usually had little relevance to the chemical structure they were representing. Generally names were based on the use or origin of the substances and even those which did have relevance did not adequately describe the structure of the chemical. Chemists had to commit to memory the chemical substance and its associated name, or look it up in reference volumes.

The IUPAC's aims were to make sure that each organic substance had its own unique name and that a chemist given that name could construct the substance's formula from it. The committee's book of rules is complex and is a study on its own, however simple rules on some of the more common organic chemicals can be extracted from it and will serve as a guide to these substances. Usually, the rules that apply to one group of substances will also apply to other groups. Only a selection of the possible organic substances are presented here as otherwise the subject would become too complex. The groups of substances (not in alphabetical order) covered on this CD include alkanes, alkenes, alkynes, carboxylic acids, alcohols, aldehydes, ketones, esters and amides.

The Alkanes

Alkanes are hydrocarbon compounds which in their simplest forms are straight chains of carbon atoms with hydrogen attached at every unused bond and these are sometimes referred to as acyclic alkanes because they do not have any ring structures present. The first 10 alkanes are:

Name / Molecular formula / Structural formula
methane / CH4 / CH4
ethane / C2H6 / CH3CH3
propane / C3H8 / CH3CH2CH3
butane / C4H10 / CH3CH2CH2CH3
pentane / C5H12 / CH3(CH2)3CH3
hexane / C6H14 / CH3(CH2)4CH3
heptane / C7H16 / CH3(CH2)5CH3
octane / C8H18 / CH3(CH2)6CH3
nonane / C9H18 / CH3(CH2)7CH3
decane / C10H22 / CH3(CH2)8CH3

Rules for alkanes

1. Any alkane ends with "ane".

2. The main stem of the name is derived from the longest carbon chain. eg. if there are 7 carbon atoms in the longest chain, then the stem will be hept......

3. If there are any alkyl side chains (eg. CH3 or C2H5) present, then these side chains are added as prefixes to the main name.

4. If there is more than one of the same type of alkyl substituent, a multiplying prefix for the alkyl is used of the following format:

2: di ; 3: tri ; 4: tetra ; 5: penta ; 6: hexa ; ...... etc.

for example: dimethyl, tributyl, etc.; however when placing the alkyls in order in the formula, remember that the prefixes do not count alphabetically, so that triethyl would come before dimethyl. The exception to this is when older prefixes such as 'iso' or 'cyclo' are used as these do count alphabetically. For instance, 'isopropyl' would be placed in front of 'dimethyl'.

5. The carbon atoms in the principal chain are numbered from one end to the other in order to give the lowest combination of numbers ( the lowest at the first point of difference) for positions of the side chains. The position of every side chain must be specified by a number.

Figure 1. 2,2,6-trimethylheptane. The other possible name would be generated if the name began from the right hand side of the above diagram, but if this was done the result would be 2,6,6-trimethylheptane and since the numerical combination is larger than the one given in the figure, the latter name is incorrect. The chemical name must always have the lowest combination of numbers and this means inspecting the structural formula carefully to find out which combination does give the lowest number condition.

6. Prefixes are arranged in alphabetical order if there are different side chains; for example 3-ethyl-4-methyl......

7. Punctuation is extremely important and there must always be a comma between two numbers and a hyphen between a number and a letter.

Here are some more examples:

Figure 2. Methylpropane, a very simple molecule which takes its name from the longest carbon chain (three and therefore propane) and the alkyl side chain which has only a single carbon atom and therefore is a methyl group.

Figure 3. 2,3-dimethylpentane. The longest carbon chain is five and so pentane is the stem. The methyl groups are attached at the second and third atoms of carbon, remembering always to start from the end of the chain where any additives or substitutives are nearest. Since there are two similar methyl groups, the prefix 'di' is used.

Figure 4. 3-bromo-1-chloropentane. This molecule's name shows some more of the rules in operation. The name is not 1-chloro-3-bromopentane, even though the chlorine atom is right at the end of the chain. Recall that alphabetical precedence must take place and so the bromine atom comes first. The numbers of the respective carbon atoms still show where the two substitute atoms are to be found.