Organic Chemistry I For Dummies, 2nd Edition (2014)

IN THIS PART …

See the structures of hydrocarbons.

Name simple hydrocarbons.

Work with organic reactions using arrow pushing.

Synthesize alkenes and alkynes.

IN THIS CHAPTER

Seeing how straight-chain alkanes are named

Naming a branched alkane

Alkanes are compounds that contain only carbon-carbon single bonds. Because they have the maximum possible number of hydrogens, alkanes are said to be saturated hydrocarbons. Alkanes have the molecular formula of C_nH_2n+2, where n is the number of carbons in the molecule. An alkane that has eight carbons, for example, would have a molecular formula of C₈H₁₈.

Naming molecules is an important skill to have. Unlike biology, where organisms are given seemingly arbitrary names, organic molecules are named systematically. When you understand the nomenclature rules and how to apply them, you can name most molecules you come across. And thank goodness for that, as there are millions of known organic compounds. Can you imagine the amount of memorization that would be required if each one were given some arbitrary name? I don’t even want to think about it.

Furthermore, every unique molecule in chemistry must have a different name. George Foreman may be able to name each of his five sons George, but in chemistry, if two molecules are different, then they must have different names to distinguish them.

All in a Line: Straight-Chain Alkanes

The names of alkanes all end with the suffix –ane. The prefixes for alkanes with one, two, three, and four carbons come from historical roots, while alkanes with more than five carbons have prefixes that derive from Greek (see Table 7-1). These prefixes simply must be memorized. The mnemonic, “Mary eats peanut butter” may help you remember the prefixes for the first four alkanes that have historical roots — methane, ethane, propane, and butane. After that, knowing the Greek numbers (for example, knowing that hex refers to six, and oct refers to eight) makes the names more familiar.

TABLE 7-1 The Names of the Straight-Chain Alkanes

Reaching Out: Branching Alkanes

Oh, if life were only that simple! All the alkanes in Table 7-1 have carbon atoms that are joined in a line, so they’re called straight-chain alkanes. Alkanes, though, are not limited to staying in a line — they can have structures that branch. For example, the alkane with the formula C₄H₁₀can have two possible structures (see Figure 7-1) — one alkane that’s a straight chain (butane), and one alkane that’s branched (isobutane).

FIGURE 7-1: The isomers of C₄H₁₀.

Molecules that have the same molecular formula but different chemical structures are referred to as isomers, using organic-speak. Branching makes the nomenclature of these molecules a bit more difficult, but these branching molecules can still be named systematically. With a bit of practice, you’ll soon get the hang of naming all alkanes — including branched ones — and nomenclature questions on exams will become gimmes.

Several steps need to be followed in order to name a branched alkane. In the following sections, I detail how to perform each step, using an example to clarify what I mean as I go along.

Finding the longest chain

The first, and potentially the trickiest, step is to find the longest chain of carbon atoms in the molecule. This task can be tricky because, as a reader of English, you’re so used to reading from left to right. Often, however, the longest chain of carbons is not the chain that follows simply from left to right, but one that snakes around the molecule in different directions. Organic professors like to make the parent chain one that curves around the molecule and doesn’t necessarily flow from right to left, so you have to keep on your toes to make sure that you’ve spotted the longest carbon chain.

The longest carbon chain for the molecule shown in Figure 7-2 is seven carbons long, so the parent name for this alkane is heptane (refer back to Table 7-1).

FIGURE 7-2: The right and wrong ways to count the parent chain.

Numbering the chain

Number the parent chain starting with the end that reaches a substituent first. A chain can always be numbered in two ways. In the case of the molecule in Figure 7-2, the numbering could start at the top and go down, or it could start at the bottom and go up. The correct way to number the parent chain is to start with the end that reaches the first substituent sooner. A substituent is organic-speak for a fragment that comes off of the parent chain. In this example, if you number from the top down, the first substituent comes at carbon number three (see Figure 7-3); if you number from the bottom up, the first substituent comes at carbon number four. The correct numbering in this case, then, starts at the top and goes down.

FIGURE 7-3: The right and wrong ways to number the carbons of the parent chain.

Seeing the substituents

After numbering the parent chain, the next step is to determine the names of all the substituents that stick off of the parent chain. On this molecule, two substituents come off the parent chain — one substituent at carbon number three, and one substituent at carbon number four (see Figure 7-4). Substituents are named in the same manner as the parent chains, except that instead of ending with the suffix –ane they end with the suffix –yl, which indicates that the group is a substituent off the main chain. For example, the one-carbon substituent at carbon number three is amethyl substituent (not a methane substituent). A two-carbon substituent would be ethyl, a three-carbon substituent would be propyl, a four-carbon substituent would be butyl, and so on (refer to Table 7-1).

FIGURE 7-4: The locations of substituents on a parent chain.

Some complex substituents have common names rather than systematic ones. These simply must be memorized. The most important common substituents are the isopropyl group (a three-carbon group that looks like a snake’s tongue), the tert-butyl (or t-butyl) group and the sec-butyl group, shown in Figure 7-5.

FIGURE 7-5: The common names of some substituents.

In the example shown in Figure 7-4, the substituent at carbon number four is, in fact, an isopropyl group. Now you have the name of the parent chain and the names of all the substituents. The one-carbon substituent at carbon three is a methyl group, and the three-carbon substituent at carbon four is an isopropyl group. Now you just have to put it all together!

Ordering the substituents

The next step is to order the substituents alphabetically in front of the parent name, using numbers to indicate the location of the substituents. Because i comes before m in the alphabet, the isopropyl group is placed in front of the methyl group in the name of the molecule: 4-isopropyl-3-methylheptane. Note that dashes are used to separate the numbers from the substituents, and that there is no space between the last substituent and the name of the parent chain.

Of course, there’s always a stick to throw into the spokes. One quirk involving the common names of tert-butyl and sec-butyl substituents comes when placing them in alphabetical order, as the tert and sec portions of the name are ignored. In other words, tert-butyl would be ordered as if it started with the letter b, the same as with sec-butyl. Isopropyl, however, is alphabetized normally, under the letter i. (There always has to be weird exceptions like this, doesn’t there?)

More than one of a kind

What if the molecule contains more than one of the same substituent? For example, what if the compound has two methyl-group substituents on the molecule? In such a scenario, you don’t name each of the methyl groups individually. Instead, you put a prefix in front of a single substituent name to indicate the number of these substituents that the molecule contains. (See Table 7-2 for a list of prefixes.)

TABLE 7-2 Prefixes for Identical Groups

For example, if the molecule has two separate methyl groups, the prefix di– goes in front of the name methyl, making the substituent name dimethyl. In addition, two numbers are placed in front of the substituent name to show the locations of the two methyl groups on the parent chain. Students often forget that for every substituent you must explicitly list a number, even if the substituents are attached to the same carbon. If the molecule has three methyl groups coming off the parent chain, the substituent name is trimethyl; if it has four methyls, the name is tetramethyl; if it has five methyls, the name is pentamethyl; and so on. These prefixes, like tert and sec in the previous examples, are ignored when placing the substituents in alphabetical order (dimethyl would be alphabetized under m, for example). For two examples of molecules that have identical substituents, see Figure 7-6. Note the use of commas to separate the numbers preceding the substituent name.

FIGURE 7-6: Examples of multiple identical substituents.

Naming complex substituents

Sometimes, substituents will branch and will have no common name, like the substituent shown in Figure 7-7. In such a case, you name the substituent just as if it were a separate alkane, but with the parent name of the substituent ending with the suffix –yl instead of –ane, indicating that this portion of the molecule is a substituent off the main chain.

FIGURE 7-7: An alkane with a complex substituent.

The main catch in naming complex substituents, however, is that the carbon that attaches the substituent to the parent chain must be the number-one carbon; thus, when you number the carbons in the substituent chain, you force the carbon that attaches to the main chain to be carbon number one. (See Figure 7-8 for an example of how this works.)

FIGURE 7-8: The right and wrong ways to number a complex substituent.

Because the complex substituent shown in Figure 7-8 is three carbons long and has methyl groups at carbons one and three, the complex substituent is named 1,2-dimethylpropyl.

Because you’re naming a substituent off the parent chain, the name must end with the suffix –yl, not –ane.

Now you can put all the parts of the name together. By convention, when a complex substituent is included in a name, the name of the complex substituent is set off with parentheses. So the name of the molecule in Figure 7-7 is 5-(1,2-dimethylpropyl)-nonane.

Getting lots of practice with naming alkanes is essential in order to master nomenclature of organic molecules.