All the members of a particular homologous series share similar chemical properties (as discusses in a previous post. This is because they all possess the same functional group. The functional group of an organic compound is an atom (or group of atoms) which determines the chemical properties of the compounds within a homologous series. The chart below shows each homologous series and its corresponding functional group:
So, if you have the full structural formula of a compound, you should be able to determine which homologous series it belongs to based on its functional group:
The functional group in the compound above is the alkanoic acid (carboxylic acid) functional group, and thus the compound is an alkanoic acid. (You may have already noticed the pattern with the naming of this compound, but we will discuss this in the latter half of this post).
When naming organic compounds, the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature is used. Two things are needed for you to be able to name a specific compound:
the molecular formula, which tells you the total number of carbon atoms in the molecule
the homologous series, which allows you to determine which functional group is in the molecule
Naming Straight-Chain Alkanes and Alkenes
In straight chain (unbranched) alkanes and alkenes, the name is made up of two parts, the prefix and the suffix. The prefix tells the number of carbon atoms in the chain while the suffix tells the functional group present. That is, for alkanes (functional group C---C) the suffix will be '-ane' while for alkenes (functional group C===C) the suffix will be '-ene.'
The prefixes for organic compounds (based on the number of carbon atoms in the chain) are different from those used in naming covalent compounds, especially the first four:
For example, if you were told to name an alkane of molecular formula C2H6, you would first determine the prefix. There are two carbon atoms, so the prefix is 'eth-'. The compound is an alkane, so the suffix is '-ane,' and thus, the name of the compound is ethane.
Naming Straight-Chain Alkanols and Alkanoic Acids
To name a straight-chain/unbranched alkanol or alkanoic acid, simply use the name of the corresponding alkane with the same number of carbon atoms as is in the compound. Then remove the 'e' at the end of the alkane's name and add the suffix '-ol' for alkanols and '-oic acid' for alkanoic acids instead.
So, if you are told to name an alkanol of molecular formula C4H9OH (OH is the functional group), you would first determine the name of the alkane with 4 carbon atoms, butane. Remove the 'e' and replace it with '-ol' since the compound is an alkanol, and the named compound is 'butanol.'
The procedure is essentially the same for naming an alkanoic acid, for example, C2H5COOH. There are 3 carbon atoms here, and the alkane with 3 carbon atoms is called propane. The 'e' is removes and replaced with 'oic acid' since the compound is an alkanoic acid (functional group COOH), giving us propanoic acid.
(Note: the total number of carbon atoms is counted when naming unbranched compounds)
Naming Branched-chain Alkanes and Alkenes
This is where naming gets a little more interesting. Naming branched chain compounds requires you to understand alkyl groups. An alkyl group is essentially and alkane with one hydrogen atom removed, so it has a general formula of 'CnH2n+1.' They are represented by the symbol 'R' and are always linked to either a hydrogen atom or some functional group. In alkanes, an alkyl group is always bonded to a hydrogen (R―H). An alkanoic acid would be represented as R―COOH and and alkanol would be represented as R―O―H.
For each alkyl group, the formula is determined by substituting the value of n into the general formula. Then, the name of the alkyl group is the prefix for the corresponding number of carbon atoms followed by the suffix '-yl'.
So, when n=1, the formula of the alkyl group is C(1)H2(1)+1, or CH3. The name of that alkyl group is methyl-. Similarly, the alkyl groups ethyl- (C2H5), propyl- (C3H7), butyl- (C4H9), pentyl- (C5H11), etc follow the same pattern.
Now that we have established alkyl groups, we can begin to understand how to name certain compounds. We will name the following branched chain compound:
Which homologous series does the compound belong to? The one above contains only single bonded carbon atoms and has a molecular formula of C6H14 (which matches the general formula of an alkane) and is therefore an alkane. So, the name of the compound must end in '-ane.'
Which chain is the longest (main) chain and how many carbon atoms does it contain? This will allow us to determine what the prefix should be. The longest chain above has 4 carbon atoms, and so the prefix is 'but-.'
What is the name of the alkyl group which branches off the main chain? This compound's branch has a formula of C2H5, so its name is 'ethyl-.'
Number the carbon atoms in the main chain so that the atom to which the branch is connected has the lowest possible number
The ethyl branch is at position 2 (as opposed to position 3 if we count from the right).
Combine all of this information to create the name of the compound:
Naming Organic Compounds with More Than One Functional Group
The above method works for compounds with one functional group, but you will also be required to name compounds with multiple functional groups/branches. When naming these compounds, we need to indicate the type(s) of functional groups, the number of functional groups, and the positions of the functional groups that the compound contains.
We will look at determining the name of the following compound:
Count the number of carbon atoms in the main chain to get the main part of the name. Our compound has 4 carbon atoms, and is therefore based on butane.
The functional group in this compound is that of an alkanoic acid, so the compound's name must end in '-oic acid.' Thus, the compound is a variation of butanoic acid. There are also two hydroxyl groups (OH) attached to the main chain. We must find the positions of these as well:
The hydroxyl groups are at the 2 and 3 positions (regardless of which way you count from). In the name then, we must include 2,3-dihydroxy since there are two hydroxyl groups at these two positions.
Combine all of this information into a name: