Alkanes is the simplest kind of organic molecules. They consist of only carbon and hydrogen atoms and have a general molecular formula CnH2n+2. Alkanes only contain single carbon-carbon or carbon-hydrogen bonds.
Organic chemists use the following basic names of alkanes to build the molecule’s name. You might want to memorize those as they come in handy every time you need to come up with an appropriate IUPAC name for a molecule.
All alkanes have the suffix –ane at the end of the molecule.
|Carbons||Molecular Formula||Condensed Structural Formula||Name|
When the molecule is not a simple straight chain and has branching, we use the following additional rules to give it a name:
Rule 1: Find the longest continuous chain of carbons. If two or more chains compete (have same number of carbons), then choose the one with more branches coming off it.
Rule 2: Identify and name all substituents (branches) coming off the parent (primary) chain. List them in alphabetical order.
To name the branches (substituents) we are going to count the number of carbons in the branch (do not count the carbons that are the part of the parent chain!). Then, we shall call them using the appropriate alkane name changing the ending suffix –ane to –yl. The suffix –yl tells us that this is a branch and not a parent compound. Simple hydrocarbon substituents, thus, are called in analogy with their parents—alkyls.
Rule 3: Number the primary chain starting from the end that has the most substituents (branches). That is, start from end that is the closest to the nearest branch.
If the numbering is ambiguous, then the substituent earlier in the alphabetical order should get the smallest number.
Note in the example above—numbering can start from either right or left end as the first substituent is on the third atom from either end. In this case, the alphabetical order of the substituents defines the numbering.
Rule 4: Combine the substituents in an alphabetical order with the parent (primary) chain.
For multiples of the same type of the substituent, use the multiplication prefixes:
Note: the number of locations in a molecule must correspond to the multiplication prefix! Also, these prefixes are not counted for the alphabetical order.
Proper “grammar” and punctuation matter! We separate numbers and letters with a dash, while we separate numbers between themselves with commas. Do not put any extra punctuation marks or breaks into the name if it is not required by the rules.
Common Branched Alkyl Group Names
We’ll talk more about the complex substituents and how to name them a bit later in this topic. However, there’s a short list of common complex substituents that we see often enough so that they have their own traditional names.
The “R” in the picture above is the “rest” of the molecule.
A couple of important notes, tough. First, the prefix iso- is counted for the alphabetical order of the molecule. So, when you’re alphabetizing your substituents to put them in the order into the name, remember that. Second, notice the way the names are spelled. The prefixes sec- and tert- are often abbreviated to s- and t- and are italicized.
Notice how the isopropyl group is positioned before the methyls in the name due to iso- being counted for the alphabetical order.
Complex Substituent Nomenclature Rules
Occasionally, the substituent is too complex to use a simple name for it. In this case, we use the complex substituent name using the following rules:
Rule 1: Numbering starts at the place of attachment of the complex substituent to the parent chain. If such numbering is not possible (special cases), then the numbering is done as if the substituent was a separate molecule, but instead of the –ane suffix we’ll add the –yl suffix with the position of the attachment.
Rule 2: Complex substituents are separated from the rest of the molecule’s name by the parentheses.
Notice how the complex substituent has its own numbering system and is separated by the parentheses.
Nomenclature Rules for Cycloalkanes
Cycloalkanes are the cyclic versions of the alkanes and have a ring structure in the molecule. They have two less hydrogens in their molecular formula, and thus have a general molecular formula CnH2n. The names of cycloalkanes are derived from the corresponding manes of alkanes by adding a prefix cyclo– to the name of an alkane.
Since the molecules of cycloalkanes are cyclic, they do not have an “end” from which we start numbering. This means, that the atom with a single substituent always gets the number 1. If multiple substituents are present, then the following rules apply:
Rule 1: the numbering always follows the shortest path between the substituents.
Rule 2: The carbon with the most substituents gets number 1. If no carbon has more substituents, then the carbon with the substituent that is earlier in the alphabetical order gets the number 1.
The rest of the nomenclature rules for cycloalkanes is exactly the same as for normal alkanes. Note, that we do not give the priority for cycloalkanes simply because it is cyclic! The longer continuous chain will still be the parent molecule.
Since in the case of cyclic compounds the only substituent will always get the number 1, it is considered redundant to include it for a cyclic parent. This is, however, not wrong according to the IUPAC.