1.5: Naming Alkanes - Isomers & An Introduction to Chemical Nomenclature

We now turn our attention to the names of the alkanes in Table 1-1, our first foray into chemical nomenclature. They are obviously systematic in some way. We will take some time to explain precisely what the system behind those names is. The goal of chemical nomenclature is to provide every compound a unique name that conveys all of the information needed to unambiguously communicate its structure. This is useful because there are cases where different compounds have exactly the same compositions so simply providing the molecular formula would not distinguish between them. For example, there are two pairs of such compounds included in Table 1-1: both hexane and 2-methylpentane have the formulas C₆H₁₄, while cyclohexane and methylcyclopentane both have the formula C₆H₁₂. By definition, compounds that have the same molecular formula but different structures are isomers.  Thus cyclohexane and methylcyclopentane are isomers of each other. The same is true of hexane and 2-methylpentane: they are isomers of each other, and they have different names to reflect their different structures.

The rules for naming alkanes are fairly straightforward, although they can get cumbersome for complex structures. To start with some simple examples, we'll stick with acyclic alkanes; we will cover cyclic alkanes when we consider additional ideas specific to structures with closed rings. First, all alkanes carry an -ane suffix. Second, the -ane suffix is attached to a prefix that conveys the number of carbon atoms in the longest chain of atoms in the molecule. The prefixes used for chains up to ten carbons in length are listed in Table 1-2. Note that the prefixes do not denote the total number of carbon atoms. Because none of the compounds in Table 1-2 have branches off the chain, the number of carbon atoms equals the length of the longest chain, so the names simply consist of the prefix and the suffix, e.g., hexane.

Table 1-2. Names of Straight Chain Alkanes Having Ten or Fewer Carbon Atoms
Number of Carbon Atoms	Molecular Formula	Prefix	Systematic Name	Line Diagram
1	CH4	meth-	methane	CH4
2	C2H6	eth-	ethane
3	C3H8	pro-	propane
4	C4H10	but-	butane
5	C5H12	pent-	pentane
6	C6H14	hex-	hexane
7	C7H16	hept-	heptane
8	C8H18	oct-	octane
9	C9H20	non-	nonane
10	C10H22	dec-	decane
*With no carbon-carbon bonds, methane does not lend itself to the line structures used for the other compounds on this list. It is usually just written as “CH4” or with a structural formula showing its component atoms.

Some of the compound names in Table 1-2 are probably familiar to you. Methane, for example, is the major component of natural gas. Propane is widely used for heating and in outdoor grills, butane is used in disposable cigarette lighters, and the quality of gasoline is often described by its “octane” rating, although octane itself is a poor fuel for most internal combustion engines. The names of unbranched alkanes consist solely of the quantifying prefix and the –ane suffix. For example, an unbranched alkane consisting of five carbon atoms is named pentane, one with eight carbons is octane, etc. Sometimes an n- prefix is used for unbranched alkanes, e.g., octane can also be called n-octane; this removes ambiguity because referring to a compound as “octane” could refer to n-octane or to any alkane that has a total of eight carbons. While this is not standard nomenclature, it is frequently used.

Before delving into the nomenclature of branched alkanes, it is useful to take note that pattern that exists in the line diagrams and molecular formulas of the unbranched compounds. Note that (except for methane) every chain has two terminal  -CH₃ units and, as you go down the column, there is an additional  -CH₂- unit in the internal part of the chain. Thus this series of alkanes have the molecular formula C_nH_2n+2, where n is the total number of carbons in the molecule. In fact, all acyclic alkanes have the formula C_nH_2n+2. This is a rule without exceptions and is a natural consequence of the fact that, by definition, alkanes consist only of carbon and hydrogen and do not contain any multiple bonds. Cyclic alkanes have fewer hydrogen atoms than acyclic alkanes because they have more carbon-carbon bonds (for a given value of n), and therefore fewer places available for hydrogen to bond to carbon. For example, cyclohexane has six carbon-carbon atoms, while hexane has only five; their formulas are C₆H₁₂ and C₆H₁₄, respectively. The same is true with methylcyclopentane, C₆H₁₂, and 2-methylpentane, C₆H₁₄; the former has 6 carbon-carbon bonds while the latter has only 5, but the latter has two additional hydrogen atoms to compensate for the lack of a sixth carbon-carbon bond..

 

Exercise 

Problem 1-3: What are the molecular formulas for acyclic alkanes having 15, 20 and 30 carbon atoms? 

 

Exercise

Problem 1-4: Compare the structures of octane and cyclooctane. The former is included in Table 1-2, while the latter consists of a closed ring of 8 carbon atoms. How many carbon-carbon bonds do these compounds have? What are their molecular formulas. Are they isomers or not

 

Before delineating how branched alkanes are names, let's take a look at the example from Table 1-1: 2-methylpentane, shown again at right. This relatively simple example illustrates the three pieces of information that must be included when naming such compounds:

• the length of the longest chain
• the composition of any substituents
• the position of any substituents

Addressing the first bullet point above, notice that despite having six carbon atoms, the base name for this compound is not hexane, but pentane. This is because the longest chain in the structure is five carbon atoms in length, not six. The sixth carbon is in the -CH₃ side-chain, called a methyl group; this addresses the second bullet point – the substituent composition. A substituent is any structure attached to an organic frame in place of a hydrogen atom. They are usually not stable molecules on their own, but can be looked at as fragments of molecules – they must be attached to other fragments to make stable molecules. The -CH₃ group is the simplest example of an alkyl substituent. Alkyl substituents, or alkyl groups, are essentially alkanes that are missing one hydrogen (and therefore have the general formula C_nH_2n+1); the missing hydrogen is the site where the alkyl group forms a bond to something else, in this case, the five-carbon chain. Finally, the last bullet point states that we need to specify where along the chain the substituent is positioned. In 2-methylpentane, the methyl group is on the second carbon from the end of the chain, as indicated by the digit, called a locant, that precedes the name of the substituent – think of it as an address. As seen in this example, the locant is separated from the substituent name with a hyphen.

The way you number the carbon atoms when naming alkanes deserves some further comment because it can sometimes be unclear.  There are actually three ways we could have numbered the chain, two of which yield the same (correct) name and one that yields an incorrect name (Figure 1-16). By convention, the correct name for compounds is that which has the lowest possible locant value for the first substituent, that is, the correct way to number the carbon atoms in a given chain is that which gives the smallest initial locant. The first two numbering schemes in Figure 1-16 are different - one starts at the far left carbon and the other starts at the one positioned above the chain - but they are equivalent in that each has a methyl group on the second carbon of the chain. In this case it doesn't matter which numbering scheme you use because they give exactly the same result: 2-methylpentane. But the numbering scheme on the right differs: it places the first carbon on the far right, positioning the methyl group on the fourth carbon of the chain. The name that would result from this numbering system is 4-methylpentane. It contains the same information but is an incorrect name because the locant, 4, is higher than it would be with an alternative numbering scheme. [7] 

Figure 1-16. Three ways of numbering the longest continuous chain of carbon atoms in 2-methylpentane. The left and center schemes are equivalent and give the correct name, but the scheme on the right does not. The correct numbering schemes both feature a methyl group (-CH3) on the second carbon of the chain. The scheme on the right places the methyl group in the fourth position; this would result in name, 4-methylpentane, which is not correct.

 

The above process forms the basis for most organic nomenclature and we will build on this foundation going forward, adding rules to resolve situations that the three points above don’t account anticipate. To summarize how the name 2-methylpentane was determined, we:

1. based the name of the compound on the length of the longest chain of carbon atoms (the base name was pentane),

2. included the name of the alkyl substituent and,

3. indicated its position with the smallest locant possible (there is a methyl on the second carbon of the chain).

These conventions will be modified later in this chapter and in subsequent chapters as we consider more complex structures as well as compounds that are not alkanes, but the general approach is similar in all cases.

Returning to the two isomers of C₆H₁₄ shown in Table 1-1, one might wonder how many other isomers exist? The exhaustive collection is shown in Figure 1-17; as you can see there are five compounds that share the molecular formula C₆H₁₄. Note that 2-methyl- and 3-methylpentane are shown but 4-methylpentane is not. As we explained above, 2-methylpentane and 4-methylpentane correspond to the same molecule, but only the former name is correct because the locant, 2, is smaller than the alternative, 4. We leave it to the reader to explain why there is no 1-methyl- or 5-methylpentane either. 

n-hexane	2-methylpentane	3-methylpentane	2,3-dimethylbutane	2,2-dimethylbuttane
Figure 1-17. The complete set of isomers of with the formula C6H14.

 

As two of the structures in Figure 1-17 illustrate, it is possible to have multiple substituents off the main chain, either on the same carbon atom or different ones. These two compounds have only four carbons in the longest chain and are therefore named as substituted butanes. In these examples, both substituents are methyl groups (-CH₃) and the fact that there are two of them is indicated by a di- prefix that is placed in front of “methyl” in the name; the positions of the methyl groups are indicated numerically as explained above. The names are therefore 2,3-dimethylbutane and 2,2-dimethylbutane. Note that when there is more than one substituent off the main chain of the same type, the total number of these is denoted with a prefix: di-, for two substituents, as in 2,3-dimethylbutane, tri- for three, tetra- for four, and so on.

 

Exercise

Problem 1-5. Prove to yourself that 3,3-dimethylbutane is an incorrect name for 2,2-dimethylbutane.

 

The isomers of hexane shown in Figure 1-17 include structures that have zero, one, or two methyl groups attached to the longest carbon chain. It is impossible to have larger substituents when the longest chain is only four carbons in length. With longer carbon chains, however, it is possible to have larger alkyl substituents. Table 1-3 presents some of the more commonly encountered alkyl groups. Note that all have an -yl suffix and have names that are based on the corresponding alkane name. Thus the -CH₃ group, which is one hydrogen short of methane, is called methyl. Ethyl is derived from ethane, propyl is derived from propane, etc. Alkyl groups with more than two carbons present a new complication, however: they can attach to a larger chain in multiple ways. Consider, for example, two alkyl groups, both having the formula -C₃H₇. Figure 1-18 illustrates the two ways that three-carbon fragments can be attached to a longer chain. On the left 4-propyloctane and is presented with two numbering systems: one for the longest chain (in red) and one for the side chain (in blue). Note that the three carbon side-chain is connected at the first carbon (the correct numbering of the side chain always makes the locant of the attachment point as small as possible). On the right is 4-isopropyloctane. The structures differ with respect to where on the side chain it is connected to the longer chain: on the left it is the first carbon of the side chain and on the right it is the second. The alkyl substituent on the right is called the isopropyl group, where the iso- prefix differentiates it from the alkyl group that connects at the first position. 

 

Table 1-3. Common Alkyl Substituents
Number of Carbon Atoms	Name	Structure
1	methyl	—CH3
2	ethyl	—CH2-CH3
3	propyl	—CH2-CH2-CH3
3	isopropyl
4	butyl	—CH2-CH2-CH2-CH3
4	2-methylpropyl (or isobutyl*)
4	butan-2-yl (or sec-butyl*)
4	tert-butyl
* The names isobutyl and sec-butyl are both widely used but are no longer recommended by IUPAC.

 

 

4-propyloctane	4-isopropyloctane
Figure 1-18. There are two alkyl groups with the formula -C3H7, propyl and isopropyl. They differ with respect to the position at which they are connected to a larger structure: propyl attaches at the first carbon and isopropyl attaches at the second.

As you might imagine, the larger the alkyl group, the more possibilities there are for isomeric forms. For example, there are four different alkyl groups with the formula -C₄H₉. Their structures are illustrated in Figure 1-19, as are the ways they attach to the rest of the molecule, represented as “R”. [8] The names for these are worth explaining in brief. The unbranched alkyl group takes the name “butyl” as expected. “Isobutyl” is a commonly encountered name for 2-methylpropyl. The iso- prefix, which we encountered above in isopropyl, is reserved for structures that have a methyl group on the next-to-last carbon of the chain; that wasn’t apparent from isopropyl, but you can see that relationship clearly with isobutyl. The “tert” in tert-butyl is short for tertiary, meaning that the carbon that attaches to the main chain is a “tertiary carbon”, defined as one bound to three other carbons in the alkyl group [9]. The term is a useful way of describing certain features and we will make use of this idea on several occasions. Finally, the most peculiar name of the four groups in Figure 1-19 is the clumsy “butan-2-yl”. If it was limited to this example it may well be worth ignoring, but that type of naming is actually common for more complex organic molecules, so let’s unpack it a bit. The -yl suffix tells us that this is a substituent on a molecule, like alkyl groups. Importantly, the “2-” locant tells us that the substituent is connected to the rest of the molecule at the second carbon. And “butan-” tells us that this is a four-carbon chain. So, putting it together, “butan-2-yl” tells us that the substituent is a four carbon alkyl group connected at the 2 position. Examine the structure to confirm for yourself that this naming is consistent with your interpretation of the line structure in Figure 1-19. Because the attachment point of butan-2-yl is a carbon that is bound to two other carbons in the alkyl group, it is sometimes called sec-butyl.

Figure 1-19. Isomeric forms of alkyl groups with the formula -C4H9. Note that the unspecified R group in this figure represents the rest of the alkane group. * see text and note in Table 1-3 for more on these alternative names

 

One final note: when naming a compound with substituent like butan-2-yl, the name of the group goes in parentheses for clarity. For example, if this group was on the fourth carbon of a nine-carbon chain, the name would be 4-(butan-2-yl)nonane. Locant “4” corresponds to the position on the main chain, whereas locant “2” refers to where on alkyl group the connection to the main chain is located. If the parentheses were omitted, it would be unclear which groups the locants refer to. A good rule of thumb is: when a substituent group has a locant as part of its name, that name is put in parentheses. Thus 5-butylnonane does not require parentheses but 5-(2-methylpropyl)nonane does.

 

 

Nomenclature: Alkanes

As we introduce various functional groups in the coming sections and chapters, we’ll be providing overviews of the IUPAC system of nomenclature. IUPAC is the organization that is recognized as the arbiter of chemical nomenclature. The goal is always to provide all the necessary information about the structure of a compound in its name. Many compounds are known by other names, so don't think that the IUPAC system is the only way of naming compounds. It is, however, widely used and many journals require its usage to prevent ambiguity. We’ll present important common names for specific compounds when helpful, especially in the context of their chemistry.

To recap, and to provide a systemic approach to naming alkanes, the steps below provide a systematic approach. We’ll use these steps to name the structure at right. To name any alkane:

1. Identify the longest continuous chain of carbon atoms; the length will determine the base name of the compound. In the structure at left, the longest carbon chain has seven carbons and therefore will be named as a substituted heptane. Make sure that you can see the seven-membered chain.

2. Identify the substituents off the main chain and count how many of each type there are. In this example, there are three methyl groups and an ethyl group. The correct name of the compound must therefore include trimethyl and ethyl.

3. Arrange the substituent names alphabetically, meaning ethyl comes before methyl and use locants to indicate their positions on the main chain; in this case the name would be 4-ethyl-2,2,3-trimethylheptane. Make sure that the number scheme gives the smallest numbers possible; the name 4-ethyl-5,6,6-trimethylheptane is incorrect.

 

 

Example 

Problem 1-6. Name the alkane at right. It introduces a few additional ambiguities that we’ll use to expand the rules above.

 

Solution

1. Identify the longest chain. The longest carbon chain this structure has is seven carbons long, so will be named as a substituted heptane. But there are three distinctly different ways of defining the longest chain: 

   1. You could go from left to right, with a butan-2-yl group at carbon 4;

   2. You could start at the left and go upward at carbon four; this has an propyl group on carbon 4, or;

   3. You could start at the right and go upward at carbon 4; this has a 2-methylpropyl group at carbon 4. 

How do you decide how to define and number the main chain? See the next step.

2. Identify the substituents on the main chain. To define the longest chain, the path that has the greatest number of substituents is preferred. Examining the above three possibilities:

a. The path described in point 1a has two substituents: a methyl and a butan-2-yl at carbons 2 and 4, respectively;

b. The path described in point 1b has three substituents: two methyl groups (at carbons 2 and 5), and a propyl group at carbon 4;

c. The path described in point 1c has two substituents: a methyl at carbon 3 and an 2-methylpropyl at carbon 4 (where the top carbon is the first in the chain).

Because option b, shown at right, is the path with the most substituents and is the correct numbering scheme for the main chain.

3. Arranging the substituents alphabetically, we arrive at the following: 2,5-dimethyl-4-propylheptane.  

 

 

 

As the length of the carbon chain increases, it can become increasingly to readily identify the longest chain in a molecule by quick inspection, as well as to distinguish genuine isomers from structures that are equivalent but merely drawn differently. Consider the structures in Figure 1-20, for example. While they may appear to be different, these two line diagrams actually represent the same compound. Structure (a) could look like a disubstituted hexane, specifically 3-ethyl-4-propylhexane, while (b) looks like 3,4-diethylheptane. Upon closer inspection however, the longest chain of structure (a) is not the horizontal one, but rather the chain that runs more or less vertically. There are two ethyl substituents off this vertical chain: ethyl substituents on the third and fourth carbons. Neither of the two line structures, (a) or (b), is more “correct” than the other – they are just different. There is no convention, other than attempts at clarity, for how such alkanes should be drawn. Don’t assume that the “obvious” chain, usually the one oriented horizontally, is the longest in the molecule. Bottom line: if you are careful about naming structures and find that two structures have the same name, they must be the same compound even if they appear to be quite different. If you have not made a mistake in deriving the name, you should be able to reconcile the structures.
 

Figure 1-20. Not isomers. These structures both correspond to 3,4-diethylheptane, C11H24.

 

To finish this section, we return to the concept of isomers, specifically how many of them exist for a given molecular formula. We saw in Figure 1-17 that there are five isomers of alkanes that share the formula C₆H₁₄. What about other chemical formulas though? Is there a pattern concerning how many isomers exist? There is not a simple answer that question, although it is answerable. [10] Suffice it to say, the number of isomers increases geometrically with the number of carbon atoms. To illustrate, it should be easy enough to prove to yourself that neither methane, nor ethane, nor propane have any isomers: there is only one way to arrange the component atoms of those molecules that satisfies carbon’s requirement to make four bonds (and hydrogen’s requirement to make only one). There are two isomers of butane (draw them!). There are three isomers of pentane (draw those too!) and, as shown previously, five of hexane. The numbers rapidly increase from that point. There are 18 isomers of octane, 75 of decane, and 355 of C₁₂H₂₆, the straight chain form of which is named dodecane. In our earlier discussion of linoleic acid, we stated that there were thousands of possible ways to combine the component 18 carbon, 32 hydrogen and 2 oxygen atoms. While this may have initially seemed to stretch credulity, it should be a bit easier to accept given that there are 60,523 isomers of octadecane, C₁₈H₃₈. Just for fun, we’ll continue a bit more: there are 366,319 isomers of eicosane, C₂₀H₄₂, and a mind-boggling 4,111,846,763 isomers of triacontane, C₃₀H₆₂. Recall, the list of known compounds numbers only in the tens of millions, so the lion’s share of the four billion plus possible isomers of triacontane have never been isolated. But we can, given enough time and patience, draw the structures of all of them and predict their properties – they will be insoluble waxy solids that float in water – illustrating the usefulness of using functional groups, or the lack thereof in this case, as an organizing principle for chemicals.

 

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Additional Problems.

 

Problem 1-7. Name the compounds represented by the following line structures.
a)	c)	e)
b)	d)	f)

 

Problem 1-8. Draw line diagrams for each of the following alkanes:

a) 3-methylhexane

b) 3,3,4,4-tetramethylhexane

c) 3,3-diethylpentane

d) 4,6-diethyl-2,3,3,5-tetramethyldecane

e) 4-isopropyl-3,5-dimethylheptane

f) 4,5-dipropyloctane

g) 4-(butan-2-yl)-2,8-dimethylnonane

h) 4,6-dimethyl-5-(2-methylpropyl)decane

 

Problem 1-9. In this chapter the following idea was asserted: It is impossible to have substituents larger then methyl groups when the longest chain is only four carbons in length. Explain why this is true.

Problem 1-10. When attempting to name an alkane, a student comes up with the name 3-butylpentane. Explain why there is no way this name can be correct. What is the correct name for the structure that would correspond to 3-butylpentane?

 

 

 

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Footnotes and References  

[7] Incorrect according to whom? The International Union of Pure and Applied Chemistry (IUPAC). This international body makes recommendations concerning chemical nomenclature, units of measurement, element names, labeling schemes for the Periodic Table, among other things. Its recommendations are widely followed by journals and researchers. We will often refer to IUPAC recommendations in this text.

[8] It is common practice to indicate a fragment of a molecule with the abbreviation "R"; this is often done to alleviate the need of drawing larger structures when only a particular portion of it is of interest in a particular context.

[9] Carbon atoms in molecules are often designated as primary, secondary, or tertiary. Those that are primary are bound to only one other carbon atom; those that are secondary are bound to two carbon atoms, and tertiary carbons are bound to three. Primary, secondary and tertiary are often abbreviated as 1°, 2° and 3°, respectively.

[10]. For a list of the number of isomers sharing the same formula C_nH_2n+2 for values of n up to 100, see http://www.mathe2.uni-bayreuth.de/sascha/oeis/alkane.html. For information on how these values are calculated, see https://oeis.org/A000602