1.2 Rules for Naming Simple Organic Compounds-Hydrocarbons

We are now familiar with how systematic nomenclature reflects the various structural features of a molecule and vice versa. Let’s proceed to learning the rules that help us systematically construct the name of organic molecules. The systematic nomenclature of all classes of organic molecules involves an extensive set of rules that extend beyond the scope of an Organic I textbook; only the most essential rules that help you assign a name simple organic structures containing common functional groups are described.

We will begin with the hydrocarbon family, namely the alkanes and cycloalkanes, and gradually expand the complexity of the name rules to include molecules with a single functional group and eventually progress to those with more than one functionality. Along the way, you will learn how to identify the parent chain, recognize and name substituents, and apply the proper numbering system to indicate their positions. As functional groups are introduced, you will see how they appear as prefixes or suffixes, and how to determine their relative priorities. But all in good time. Let’s begin by learning the basic rules for identifying parent chains in alkane and cycloalkanes.

a.   Selecting the Parent Chain.

Organic molecules can be linear, branched, or cyclic. When beginning the nomenclature of an organic molecule, the first step is to identify the parent chain. In an acyclic hydrocarbon molecule, the parent chain is made of the longest continuous sequence of connected carbon atoms. As we progress toward molecules with rings, multiple bonds, and functional groups, there will be additional rules that will modify this basic definition.

Acyclic Hydrocarbons

When selecting the longest chain in an acyclic hydrocarbon molecule –

1. Pay attention as the longest carbon chain is not necessarily organized horizontally. Sometimes a branched path provides a longer sequence of carbons. The thick line in the two structures below shows the longest carbon chain.

1. If two or more parent chains are possible because they have the same number of carbon atoms, choose the chain with the greatest number of substituents to be the parent chain.

• If two or more parent chains are possible because they have the same number of carbon atoms and the same number of substituents, choose the chain the provides the lowest set of locant (lowest position number).

Structures with Both Cyclic and Acyclic Components

When the hydrocarbon structures contain a ring and a chain, the ring always takes precedence over the chain as the parent structure. This is true even if the chain contains more carbon atoms.

Exception: When we begin working with functional groups, a major exception to the ‘ring over chain’ rule may arise –

1. If there is a senior functional group on the chain but no functional group on the ring, the presence of the senior group imparts priority to the chain to become the parent structure.

1. If multiple functional groups are present, the senior-most functional group decides the parent selection. If it is present on the chain, then the chain is selected as the parent structure.

*The concept of senior and senior-most functional groups will be introduced later in this module.

Structures with Multiple Bonds

When multiple bonds, a carbon-carbon double bond or a carbon-carbon triple bond, are present in the molecule the general rule is to include the multiple bonds as part of the parent structure. However, there are some exceptions that are discussed later. (need to iron-out the exceptions)

1. If two or more possible chains contain multiple bonds, choose the one with the greatest number of multiple bonds.

1. When considering between a double bond and a triple bond, the chain containing the double bond takes priority.

b.   Naming the Parent Chain and Identifying the Substituents.

Above we practiced identifying the parent chain under various scenarios and now it is time to assign a name to the parent chain. The parent or root name is made of two parts – the first part, base name, is derived from the number of carbon atoms in the selected parent structure. Further, a suffix is attached to the base name which represents the family of the molecule. We will practice this with simple hydrocarbon families and slowly progress to adding families derived from functional groups.

We will separately address the nomenclature of arenes. The table below shows the construction of parent names for molecules belonging to the three hydrocarbon families – alkanes, alkenes, and alkynes. For example, a five-carbon parent chain from the alkane family is called ‘pentane’. Where ‘pent’ is the base name indicating five carbons and the suffix ‘ane’ suggest it is an alkane. Similarly, the name propene suggests that it isa molecule with three carbons and has a double bond as it belongs to the alkene family. It is worth noting that at this point we have not indicated the locant for the double bond, but we will do that in time as we progress through the rules.

Later, as we introduce functional groups in our examples, you will notice that the family of the molecule will be defined by the senior functional group, and the suffix will change accordingly

Cyclic hydrocarbons

When a cyclic hydrocarbon is selected as the parent chain, the prefix cyclo- is added to the name derived based on the number of carbon atoms present within the cyclic structure. As indicated earlier, any additional linear chains will be treated as substituents unless they carry a senior functional group.

The above table shows a few examples of cycloalkanes, and corresponding alkenes with a single double bond or alkynes with a single triple bond can be similarly named as cycloalkenes and cycloalkynes respectively.

c. Naming Branches or Alkyl Substituents

Alkyl substituents are the shorter carbon-based branches that replace a hydrogen on any carbon of the parent chain. The name of a branch is derived by taking the name of the corresponding hydrocarbon and replacing the suffix ‘-ane’ with an ‘-yl’.

If the branch is a cyclic structure, it retains the ‘cyclo-’ prefix and gets the suffix ‘-ane’ changed to ‘-yl’ from its root name similarly.

When a Branch Has One or More Branches

So far, we have seen simple straight-chain substituents such as methyl, ethyl, propyl, etc., or simple cyclic substituents such as cyclopropyl, cyclobutyl, cyclopentyl, etc. But what if there were branches coming off the branch itself? How do we name such branched substituents? First, let’s treat the simple cases where there is a single branch coming off the substituent. We will address multiple branches on a substituent after we have learned the numbering system used in systematic naming.

Many short, branched substituents had common names before the introduction of the IUPAC nomenclature system. To avoid very long, confusing names, IUPAC allows a few traditional common names for such small, branched substituents. You will encounter these names frequently in textbook, laboratories, and professional literature, so it’s important to recognize and use them. In older literature you may find more common names that were in use when those books or articles were written, however the use of some of these names has been discontinued. Below is the most current list of permitted common names for substituents. You will be introduced to the systematic naming of more complex branched substituents once we have covered rules for numbering substituents (Section d).

d. Numbering the Parent Chain and Assigning Locants

Once the parent chain is identified, each carbon atom in the parent chain is assigned a number, beginning at one end of the chain and counting sequentially to the other. These numbers are called locants and they tell us where each substituent is attached. However, in both linear and cyclic parent structures, the direction (left-to-right or right-to-left and clockwise or anticlockwise) for numbering needs to be decided.

Acyclic Structures with One Substituent.

Let’s first consider linear structures and understand numbering with the help of the example below.

The longest chain in the above molecule is five carbons long, and thus the root name is pentane. It has one alkyl substituent which is one carbon long, thus, it has a methyl substituent. There are two possible ways to number the pentane chain. First is left to right, which places the methyl group on carbon-2, and second from right to left, which places the methyl group on carbon-3. The number assigned to the carbon is the locant for the methyl group, but each numbering scheme assigns a different number to the methyl group, and we need to choose one. The rule is to choose the number that assigns the lowest locant to the substituent. Thus, the correct name for the above molecule is 2-methylpentane. In linear chains with one substituent, this is easily accomplished by looking for the end that is closest to the substituent.

It is important to note that the convention to write the locant in the systematic nomenclature is to separate the location of the substituent from its name by a hyphen.

Acyclic Structures with Multiple Substituents.

Moving on to multiple substituents, you should note that the rule for choosing the direction of number is still the same – assign the lowest set of locants. Consider the molecule below –

The root name for the molecule based on the longest chain is nonane and it has two substituents – ethyl and methyl. The right-to-left numbering scheme assigns the locants 4-methyl and 7-ethyl, while the left-to-right numbering scheme assigns the locants 3-ethyl and 6-methyl. 3 and 6 make the lowest set of locants as 3 is smaller than 4 and 6 is smaller than 7. Thus, the systematic name for this molecule is 3-ethyl-6-methylnonane.

It is important to take notice of few additional conventions that got introduced with multiple substituents.

• 1) each locant number is separated from its substituent’s name by a hyphen and each substituent is also separated from the next substituent by a hyphen.
• 2) Additionally, the last substituent is written continuously with root name without commas, hyphens, or spaces.
• 3) In this example it is subtle, but the two substituents are written in the alphabetical order of the first letter in their names: ethyl before m This is not in arranged by the increasing numerical value of the locant. This can be easily visualized by swapping the positions of the two substituents.

Now the name is 6-ethyl-3-methylnonane. The alphabetization of the substituent name is maintained not the order or the locant when writing the systematic name.

When choosing a numbering scheme in a multi-substituent structure, IUPAC applies the lowest set of locants rule by comparing the locants one at a time, in increasing order. The numbering decision is made at the first point of difference between the two ordered locant sets. Once the difference is found, the scheme with the lower numbers set is chosen, and all subsequent locants are ignored, even if they also differ. Let’s see this through the example below –

Here the parent chain is octane. There are four methyl substituents at different locants, and two numbering schemes are possible – 2,4,6,7 and 2,5,6,7. We begin by comparing the first locant which is identical in both schemes – 2. This creates a tie and we continue to the next locant where we encounter the first point of difference – 4 vs 5. At this point we choose the 2,4,6,7 -numbering scheme over the 2,5,6,7-numbering scheme. We ignore comparing the remaining locants even if they are different. Thus, the correct IUPAC name for the molecule is 2,4,6,7-tetramethylnonane.

A common trick taught in through the years in the classroom, various texts, and online videos involves summing up the locant numbers and proceeding to use the numbering scheme with the lowest sum. While this trick works often, it is not always correct as demonstrated with the example below.

There are three methyl substituents attached to the parent decane chain in the above molecule. The two numbering schemes possible are 2,7,8-trimethyl and 3,4,9-trimethyl. If the commonly taught trick is used, the sum of 2+7+8 = 17 and 3+4+9 – 16, which suggests 3,4,9-trimethyldecane is the IUPAC name. However, as per the IUPAC actual rule, 2<3 and is the first point of difference when comparing the two numbering schemes. Hence, the correct systematic name is 2,7,8-trimethyldecane and not 3,4,9-trimethyldecane.

In addition to the ‘first point of difference rule’, we have introduced a few more writing conventions in the name.

• 1) When identical substituents are present in the molecule –
  1. All position numbers are written in front of the substituent name separated by a hyphen.
  2. These locant numbers are each separated by commas.
• 2) Multiplicative prefixes (di, tri, tetra, penta, hexa, etc.)are used before the substituent’s name to indicate the total number of substituents of the same type. As in the above example, tetramethyl indicates that there are four methyl groups present at the indicated locants in the octane molecule.
• 3) If two or more hydrogens are substituted at the same carbon, whether by different substituent groups or the same, the locant number is repeated in the locant set. This is seen in the example below –

What if there is still a tie between two numbering schemes? Consider the example of 3-ethyl-5-methylheptane shown below. The two substituents, ethyl and methyl are located at the same distance from either end of the parent chain. In such a situation, the alphabetical order of the substituent is used to break the tie. Thus, ethyl gets the lower locant – 3.

Numbering Cyclic Substituents with Alkyl Substituents

When naming cyclic molecules, the ring is chosen as the parent structure even if the ring has fewer carbon atoms than the chain, unless a senior-priority functional group on the chain changes the priority order. If same functional group is present on both the ring and the chain, the ring still holds priority for selection of the chain. For now, we will only focus on rules for numbering rings with alkyl substituents and introduce modifications to these rules after we have introduced naming molecules with other functional group families.

For substituted rings, numbering begins at a carbon bearing a substituent. If only one substituent is present, the substituent is assumed to be at carbon-1, and often the locant is omitted when writing the name, e.g., 1-ethylcyclohexane or just ethylcyclohexane. When two or more substituents are attached to the ring, the numbering begins at a substituent and proceeds around the ring in direction that gives the lowest set of locants. Instead of selecting a “preferred” substituent,  each substituent is treated as a possible starting point, and the ring is numbered in both direction from that position. From each possible numbering, the resulting locant are written in increasing numerical order and compared one locant at a time, starting with the lowest. The numbering decision is made at the first point of difference between the ordered sets. Once, the first point of difference is identified, all the later locants are ignored.

Let’s put this to practice with an example.

In this molecule, we can begin the numbering either on the methyl, ethyl, or isopropyl group.

• Starting on the carbon to which the methyl is attached, we get 1,2,5 in the clockwise direction and 1,3,6 in the anticlockwise direction as the set of locants. The first point of difference occurs when comparing locants 2 and 3. Hence, we select 1,2,5 from this position as 2 is smaller than 3.
• Next, we perform the same operation assigning position-1 to carbon bearing the ethyl group. We get the locant-set 1,3,4 in the clockwise direction and 1,4,5 in the anticlockwise direction. The first point of difference is upon comparing locants 3 and 4 leading to the selection of 1,3,4 from this position.
• Finally, if we begin numbering from the isopropyl group and get the sets 1,4,6 and 1,2,4 from the clockwise and anticlockwise directions respectively. 1,2,4 will be the chosen locant set following the rule for selecting the lower number at the first point of difference.

d. Numbering the Parent Chain and Assigning Locants

Once the parent chain is identified, each carbon atom in the parent chain is assigned a number, beginning at one end of the chain and counting sequentially to the other. These numbers are called locants and they tell us where each substituent is attached. However, in both linear and cyclic parent structures, the direction (left-to-right or right-to-left and clockwise or anticlockwise) for numbering needs to be decided.

Assembling the IUPAC Name of Simple Hydrocarbons

Once the parent chain has been identified and named, substituents have been identified, and the parent structure has been correctly numbered, the final step is to assemble the complete IUPAC name. This step involves combining locants, substituent names, and the parent name using standardized formatting rules. These conventions ensure that names are written consistently and interpreted unambiguously. We have previously introduced many of the conventions in the related sections; however, they are reviewed here in one place.

In simple hydrocarbons, the assembled name consists of:

• 1) One or more prefixes that originate from substituent names and provide their locations. As we proceed through the chapter, you will notice that we will put lower priority functional groups as prefixes before the parent name.
• 2) The prefixes are followed by the parent chain name. The parent name will contain a suffix indicating the priority functional group. For simple hydrocarbons, like alkanes, alkenes, and alkynes, the parent name ends in -ane, -ene, and -yne, respectively.

Later in this chapter, you will encounter different functional groups of that take priority; their respective suffix forms will replace the -ane suffix. Alkenes and alkynes are suffix-only functions, and when present in the parent structure with a priority functional group, will be added as an additional suffix with locants.

Order of Components in the Name

As we had seen in the anatomy of the IUPAC name, the general structure of the systematic name is –

locant–substituent-locant-substituent-…-locant–substituentparent name.

Observe that:

• a) The substituent and its locant are separated by a hyphen.
  • 3-methylpentane

• b) Each locant-substituent set is also separated by a hyphen.
  • 3-ethyl-6-methylnonane

• • 3-ethyl-2-methylpentane

• c) As seen ( ref. Section), if the same substituent is present multiple times on the parent structure,
  1. their locants are written separated by commas.
  2. If the repeating substituent is attached to the same carbon, the locant is repeated separated by commas to indicate this occurrence.
     1. Multiplicative prefixes di-, tri-, tetra-, … are added before the name of the substituent to indicate the number of times it repeats in the structure.
     2. 2,4-dimethylhexane
        
        3,3-diethylpentane – already 
        
     3. When naming amines, substituents attached to a nitrogen are indicated with “N-” instead of a locant number and when multiple nitrogen atoms are part of the parent with substituents attached to them, the substituent locants are indicated by differentiating the nitrogen atoms as N, N’, N”…
• d) All prefix substituents are alphabetically arranged. Alphabetization is based on the base name of the substituent and ignores –
  1. Multiplicative prefixes
  2. The prefixes sec– and tert–
     6-ethyl-3-methylnonane
     
     4-ethyl-2,2-dimethylheptane
     
     3-sec-butyl-5-methylheptane
     
     3-isopropyl-5-methylheptane
     
• e) The final substituent is written continuously with the parent name.
  • 3-methylpentane
    
  • 6-ethyl-3-methylnonane
    

There are a few additional conventions, such as, the use of parenthesis in separating substituents, use of hyphens and locants in connecting the suffix group to the parent name, etc. We will introduce these as we proceed through the chapter and when the need arises while naming specific structures.

Systemic Naming & Numbering of Loner Branches

Thus far, we have familiarized ourselves with the rules of identifying the parent chain and substituents. We have practiced methods for naming and numbering the parent and its substituents in simple hydrocarbons in cyclic and acyclic, linear, and simple branched structures. We were also introduced to the common names of branched substituents (ref. section/table) that have been adopted into the IUPAC naming system. These substituents, although branched, are still simple. When naming more complex branches, we will treat the substituent as a miniature organic molecule attached to the parent structure and name it systematically.

To assign a systematic IUPAC name to a branched substituent that is directly bonded to the parent structure:

1. Identify the point of attachment. The carbon atom of the substituent that is directly bonded to the parent structure is designated as carbon-1 of the substituent.
2. Identify the longest continuous carbon chain within the substituent, starting from the point of attachment. This chain is treated as the parent chain of the substituent.
3. Number the substituent chain starting from the point of attachment and identify any branches/ substituents on this chain using the same rules applied to parent chains.
4. Assemble the substituent name. List the locants and alphabetized names of any branches on the substituent chain, followed by the parent chain name of the substituent with the suffix-yl to indicate that the group is attached to a larger parent structure.

g. Incorporating Branched Substituents into the Complete IUPAC Name

In section e we learned how to assemble the IUPAC name of simple hydrocarbons by combining locant, substituent names, and the parent chain using standardized formatting rules. In those examples, all substituents were simple alkyl groups such as methyl, ethyl, and propyl. We also learned how to systematically name branched substituents by treating them as miniature organic molecules with their won parent chain, numbering, and substituents. In this section, we combine these two ideas and learn how to incorporate such branched substituents into the complete IUPAC name.

Use of Parentheses

When a substituent contains its own locants and substituents, its name is enclosed in parentheses when written as part of the full IUPAC name. This helps distinguish the internal structure of the substituent from the numbering of the parent chain.

The general format is – locant –(branched substituent)parent name

Example:

3-(1-methylethyl)heptane

• The parent chain is heptane
• The branched substituent is 1-methylethyl
• This substituent is attached at carbon-3

Remember that when branched substituents are present, two sets of locants appear in the name. The locant(s) outside the parentheses indicates the position of the substituent on the parent chain, while the locant(s) inside the parentheses describe the structure of the substituent itself.

Alphabetical Ordering with Branched Substituents

Branched substituents are included in alphabetical ordering using the name of the substituent itself. We ignore the locants inside the parentheses, multiplicative prefixes (di-, tri-, etc.) and the prefixes “sec-“ and “tert-“.

Example:

3-(1-methylethyl)-6-methylnonane

The alphabetization is based on, methylethyl vs methyl, and thus, the substituents are arranged accordingly.

Multiple Branched Substituents

If more than one branched substituent is present, each is written in parentheses and treated as a single substituent unit.

Example:

5-(1,1-dimethylethyl)-3-(2-methylpropyl)octane

Notice that each substituent has its own internal numbering and is enclosed in parentheses. Finally, is positioned using its locant on the parent chain outside the parentheses. The conventions for separating locants using commas, and the use of hyphens to separate the locants and substituent name is the same as we learned before.