Chapter 5.5: Naming Organic Molecules

	Howard University General Chemistry: An Atoms First Approach
Unit 1: Atomic Theory      Unit 2: Molecular Structure        Unit 3: Stoichiometry          Unit 4: Thermochem & Gases Unit 5:  States of Matter         Unit 6: Kinetics & Equilibria        Unit 7: Electro & Thermo Chemistry       Unit 8: Materials

Learning Objective

• To name covalent compounds that contain up to three elements.

As with ionic compounds, the system that chemists have devised for naming covalent compounds enables us to write the molecular formula from the name and vice versa. In this and the following section, we describe the rules for naming simple organic compounds.

Hydrocarbons

Approximately one-third of the compounds produced industrially are organic compounds. All living organisms are composed of organic compounds, as is most of the food you consume, the medicines you take, the fibers in the clothes you wear, and the plastics in the materials you use. Among the few organic compounds we have discussed are methane (CH₄) and methanol (CH₃OH). These and other organic compounds appear frequently in discussions and examples throughout this text.

The simplest class of organic compounds is the hydrocarbonsThe simplest class of organic molecules, consisting of only carbon and hydrogen., which consist entirely of carbon and hydrogen. Petroleum and natural gas are complex, naturally occurring mixtures of many different hydrocarbons that furnish raw materials for the chemical industry. The four major classes of hydrocarbons are the alkanesA saturated hydrocarbon with only carbon–hydrogen and carbon–carbon single bonds., which contain only carbon–hydrogen and carbon–carbon single bonds; the alkenesAn unsaturated hydrocarbon with at least one carbon–carbon double bond., which contain at least one carbon–carbon double bond; the alkynesAn unsaturated hydrocarbon with at least one carbon–carbon triple bond., which contain at least one carbon–carbon triple bond; and the aromatic hydrocarbonsAn unsaturated hydrocarbon consisting of a ring of six carbon atoms with alternating single and double bonds., which usually contain rings of six carbon atoms that can be drawn with alternating single and double bonds. Alkanes are also called saturated hydrocarbons, whereas hydrocarbons that contain multiple bonds (alkenes, alkynes, and aromatics) are unsaturated.

Alkanes

The simplest alkane is methane (CH₄), a colorless, odorless gas that is the major component of natural gas. In larger alkanes whose carbon atoms are joined in an unbranched chain (straight-chain alkanes), each carbon atom is bonded to at most two other carbon atoms. The structures of two simple alkanes are shown in Figure 6.4.3 , and the names and condensed structural formulas for the first 10 straight-chain alkanes are in Table 6.4.2. The names of all alkanes end in -ane, and their boiling points increase as the number of carbon atoms increases. In all of these compounds the hybridization of the atomic orbitals of carbon is sp³ and the molecular geometry around each carbon atom is tetrahedral

Figure 6.4.3 Straight-Chain Alkanes with Two and Three Carbon Atoms

Table 6.4.2 The First 10 Straight-Chain Alkanes

Name	Number of Carbon Atoms	Molecular Formula	Condensed Structural Formula	Boiling Point (°C)	Uses
methane	1	CH4	CH4	−162	natural gas constituent
ethane	2	C2H6	CH3CH3	−89	natural gas constituent
propane	3	C3H8	CH3CH2CH3	−42	bottled gas
butane	4	C4H10	CH3CH2CH2CH3 or CH3(CH2)2CH3	0	lighters, bottled gas
pentane	5	C5H12	CH3(CH2)3CH3	36	solvent, gasoline
hexane	6	C6H14	CH3(CH2)4CH3	69	solvent, gasoline
heptane	7	C7H16	CH3(CH2)5CH3	98	solvent, gasoline
octane	8	C8H18	CH3(CH2)6CH3	126	gasoline
nonane	9	C9H20	CH3(CH2)7CH3	151	gasoline
decane	10	C10H22	CH3(CH2)8CH3	174	kerosene

Alkanes with four or more carbon atoms can have more than one arrangement of atoms. The carbon atoms can form a single unbranched chain, or the primary chain of carbon atoms can have one or more shorter chains that form branches. For example, butane (C₄H₁₀) has two possible structures. Normal butane (usually called n-butane) is CH₃CH₂CH₂CH₃, in which the carbon atoms form a single unbranched chain. In contrast, the condensed structural formula for isobutane is (CH₃)₂CHCH₃, in which the primary chain of three carbon atoms has a one-carbon chain branching at the central carbon. Three-dimensional representations of both structures are as follows:

The systematic names for branched hydrocarbons use the lowest possible number to indicate the position of the branch along the longest straight carbon chain in the structure. Thus the systematic name for isobutane is 2-methylpropane, which indicates that a methyl group (a branch consisting of –CH₃) is attached to the second carbon of a propane molecule. Similarly, you will learn in Section 6.6 that one of the major components of gasoline is commonly called isooctane; its structure is as follows:

As you can see, the longest chain in this compound has five carbon atoms, so it is a derivative of pentane. There are two methyl group branches at one carbon atom and one methyl group at another. Using the lowest possible numbers for the branches gives 2,2,4-trimethylpentane for the systematic name of this compound.

Alkenes

The simplest alkenes are ethylene, C₂H₄ or CH₂=CH₂, and propylene, C₃H₆ or CH₃CH=CH₂ (part (a) in Figure 6.4.4 ). The names of alkenes that have more than three carbon atoms use the same stems as the names of the alkanes (Table 6.4.2) but end in -ene instead of -ane. The hybridization around any of the carbon attached to a double bond is sp² . The molecular geometry around that carbon atom is trigonal planar.

Once again, more than one structure is possible for alkenes with four or more carbon atoms. For example, an alkene with four carbon atoms has three possible structures. One is CH₂=CHCH₂CH₃ (1-butene), which has the double bond between the first and second carbon atoms in the chain. The other two structures have the double bond between the second and third carbon atoms and are forms of CH₃CH=CHCH₃ (2-butene). All four carbon atoms in 2-butene lie in the same plane, so there are two possible structures (part (a) in Figure 6.4.4 ). If the two methyl groups are on the same side of the double bond, the compound is cis-2-butene (from the Latin cis, meaning “on the same side”). If the two methyl groups are on opposite sides of the double bond, the compound is trans-2-butene (from the Latin trans, meaning “across”). These are distinctly different molecules: cis-2-butene melts at −138.9°C, whereas trans-2-butene melts at −105.5°C.

Figure 6.4.4 Some Simple (a) Alkenes, (b) Alkynes, and (c) Cyclic Hydrocarbons The positions of the carbon atoms in the chain are indicated by C1 or C2.

Just as a number indicates the positions of branches in an alkane, the number in the name of an alkene specifies the position of the first carbon atom of the double bond. The name is based on the lowest possible number starting from either end of the carbon chain, so CH₃CH₂CH=CH₂ is called 1-butene, not 3-butene. Note that CH₂=CHCH₂CH₃ and CH₃CH₂CH=CH₂ are different ways of writing the same molecule (1-butene) in two different orientations.

The name of a compound does not depend on its orientation. As illustrated for 1-butene, both condensed structural formulas and molecular models show different orientations of the same molecule. Don’t let orientation fool you; you must be able to recognize the same structure no matter what its orientation.

Note the Pattern

The positions of groups or multiple bonds are always indicated by the lowest number possible.

Alkynes

The simplest alkyne is acetylene, C₂H₂ or HC≡CH (part (b) in Figure 6.4.4 ). Because a mixture of acetylene and oxygen burns with a flame that is hot enough (>3000°C) to cut metals such as hardened steel, acetylene is widely used in cutting and welding torches. The names of other alkynes are similar to those of the corresponding alkanes but end in -yne. For example, HC≡CCH₃ is propyne, and CH₃C≡CCH₃ is 2-butyne because the multiple bond begins on the second carbon atom. The hybridization of a carbon atom attached to the triple bond is sp, and the geometric shape is linear

Note the Pattern

The number of bonds between carbon atoms in a hydrocarbon is indicated in the suffix:

• alkane: only carbon–carbon single bonds
• alkene: at least one carbon–carbon double bond
• alkyne: at least one carbon–carbon triple bond

Cyclic Hydrocarbons

In a cyclic hydrocarbonA hydrocarbon in which the ends of the carbon chain are connected to form a ring of covalently bonded carbon atoms., the ends of a hydrocarbon chain are connected to form a ring of covalently bonded carbon atoms. Cyclic hydrocarbons are named by attaching the prefix cyclo- to the name of the alkane, the alkene, or the alkyne. The simplest cyclic alkanes are cyclopropane (C₃H₆) a flammable gas that is also a powerful anesthetic, and cyclobutane (C₄H₈) (part (c) in Figure 6.4.4). The most common way to draw the structures of cyclic alkanes is to sketch a polygon with the same number of vertices as there are carbon atoms in the ring; each vertex represents a CH₂ unit. The structures of the cycloalkanes that contain three to six carbon atoms are shown schematically in Figure 6.4.5.

Figure 6.4.5 The Simple Cycloalkanes

Aromatic Hydrocarbons

Alkanes, alkenes, alkynes, and cyclic hydrocarbons are generally called aliphatic hydrocarbonsAlkanes, alkenes, alkynes, and cyclic hydrocarbons (hydrocarbons that are not aromatic).. The name comes from the Greek aleiphar, meaning “oil,” because the first examples were extracted from animal fats. In contrast, the first examples of aromatic hydrocarbons, also called arenes, were obtained by the distillation and degradation of highly scented (thus aromatic) resins from tropical trees.

The simplest aromatic hydrocarbon is benzene (C₆H₆), which was first obtained from a coal distillate. The word aromatic now refers to benzene and structurally similar compounds. As shown in part (a) in Figure 6.4.6, it is possible to draw the structure of benzene in two different but equivalent ways, depending on which carbon atoms are connected by double bonds or single bonds. We learned that this is a consequence of the p orbitals on each carbon forming a bonding molecular π orbital which extends over the planar carbon chain. Toluene is similar to benzene, except that one hydrogen atom is replaced by a –CH₃ group; it has the formula C₇H₈ (part (b) in Figure 6.4.6 ). As you will soon learn, the chemical behavior of aromatic compounds differs from the behavior of aliphatic compounds. Benzene and toluene are found in gasoline, and benzene is the starting material for preparing substances as diverse as aspirin and nylon. Notice that the C atoms in aromatic molecules are sp² hybridized and the molecules are planar.

Figure 6.4.6 Two Aromatic Hydrocarbons: (a) Benzene and (b) Toluene

Figure 6.4.7 illustrates two of the molecular structures possible for hydrocarbons that have six carbon atoms. As you can see, compounds with the same molecular formula can have very different structures. The hybridization of the carbon atoms in these cases is sp³ and the molecular geometry around each C atom is tetrahedral

Figure 6.4.7 Two Hydrocarbons with the Molecular Formula C₆H₁₂

Example 10

Write the condensed structural formula for each hydrocarbon.

1. n-heptane
2. 2-pentene
3. 2-butyne
4. cyclooctene

Given: name of hydrocarbon

Asked for: condensed structural formula

Strategy:

A Use the prefix to determine the number of carbon atoms in the molecule and whether it is cyclic. From the suffix, determine whether multiple bonds are present.

B Identify the position of any multiple bonds from the number(s) in the name and then write the condensed structural formula.

Solution:

1. A The prefix hept- tells us that this hydrocarbon has seven carbon atoms, and n- indicates that the carbon atoms form a straight chain. The suffix -ane tells that it is an alkane, with no carbon–carbon double or triple bonds. B The condensed structural formula is CH₃CH₂CH₂CH₂CH₂CH₂CH₃, which can also be written as CH₃(CH₂)₅CH₃.

2. A The prefix pent- tells us that this hydrocarbon has five carbon atoms, and the suffix -ene indicates that it is an alkene, with a carbon–carbon double bond. B The 2- tells us that the double bond begins on the second carbon of the five-carbon atom chain. The condensed structural formula of the compound is therefore CH₃CH=CHCH₂CH₃.

   

3. A The prefix but- tells us that the compound has a chain of four carbon atoms, and the suffix -yne indicates that it has a carbon–carbon triple bond. B The 2- tells us that the triple bond begins on the second carbon of the four-carbon atom chain. So the condensed structural formula for the compound is CH₃C≡CCH₃.

4. A The prefix cyclo- tells us that this hydrocarbon has a ring structure, and oct- indicates that it contains eight carbon atoms, which we can draw as

   

   The suffix -ene tells us that the compound contains a carbon–carbon double bond, but where in the ring do we place the double bond? B Because all eight carbon atoms are identical, it doesn’t matter. We can draw the structure of cyclooctene as

   

Exercise

Write the condensed structural formula for each hydrocarbon.

1. n-octane
2. 2-hexene
3. 1-heptyne
4. cyclopentane

Answer:

1. CH₃(CH₂)₆CH₃
2. CH₃CH=CHCH₂CH₂CH₃
3. HC≡C(CH₂)₄CH₃

4. 

The general name for a group of atoms derived from an alkane is an alkyl group. The name of an alkyl group is derived from the name of the alkane by adding the suffix -yl. Thus the –CH₃ fragment is a methyl group, the –CH₂CH₃ fragment is an ethyl group, and so forth, where the dash represents a single bond to some other atom or group. Similarly, groups of atoms derived from aromatic hydrocarbons are aryl groups, which sometimes have unexpected names. For example, the –C₆H₅ fragment is derived from benzene, but it is called a phenyl group. In general formulas and structures, alkyl and aryl groups are often abbreviated as RThe abbreviation used for alkyl groups and aryl groups in general formulas and structures..

Structures of alkyl and aryl groups. The methyl group is an example of an alkyl group, and the phenyl group is an example of an aryl group.

Alcohols

Replacing one or more hydrogen atoms of a hydrocarbon with an –OH group gives an alcoholA class of organic compounds obtained by replacing one or more of the hydrogen atoms of a hydrocarbon with an −OH group., represented as ROH. The simplest alcohol (CH₃OH) is called either methanol (its systematic name) or methyl alcohol (its common name) (see Figure 6.4.4 ). Methanol is the antifreeze in automobile windshield washer fluids, and it is also used as an efficient fuel for racing cars, most notably in the Indianapolis 500. Ethanol (or ethyl alcohol, CH₃CH₂OH) is familiar as the alcohol in fermented or distilled beverages, such as beer, wine, and whiskey; it is also used as a gasoline additive. The simplest alcohol derived from an aromatic hydrocarbon is C₆H₅OH, phenol (shortened from phenyl alcohol), a potent disinfectant used in some sore throat medications and mouthwashes.

Ethanol, which is easy to obtain from fermentation processes, has successfully been used as an alternative fuel for several decades. Although it is a “green” fuel when derived from plants, it is an imperfect substitute for fossil fuels because it is less efficient than gasoline. Moreover, because ethanol absorbs water from the atmosphere, it can corrode an engine’s seals. Thus other types of processes are being developed that use bacteria to create more complex alcohols, such as octanol, that are more energy efficient and that have a lower tendency to absorb water. As scientists attempt to reduce mankind’s dependence on fossil fuels, the development of these so-called biofuels is a particularly active area of research.

Summary

Covalent inorganic compounds are named by a procedure similar to that used for ionic compounds, using prefixes to indicate the numbers of atoms in the molecular formula. The simplest organic compounds are the hydrocarbons, which contain only carbon and hydrogen. Alkanes contain only carbon–hydrogen and carbon–carbon single bonds, alkenes contain at least one carbon–carbon double bond, and alkynes contain one or more carbon–carbon triple bonds. Hydrocarbons can also be cyclic, with the ends of the chain connected to form a ring. Collectively, alkanes, alkenes, and alkynes are called aliphatic hydrocarbons. Aromatic hydrocarbons, or arenes, are another important class of hydrocarbons that contain rings of carbon atoms related to the structure of benzene (C₆H₆). A derivative of an alkane or an arene from which one hydrogen atom has been removed is called an alkyl group or an aryl group, respectively. Alcohols are another common class of organic compound, which contain an –OH group covalently bonded to either an alkyl group or an aryl group (often abbreviated R).

Key Takeaway

• Covalent inorganic compounds are named using a procedure similar to that used for ionic compounds, whereas hydrocarbons use a system based on the number of bonds between carbon atoms.

Conceptual Problems

1. Benzene (C₆H₆) is an organic compound, and KCl is an ionic compound. The sum of the masses of the atoms in each empirical formula is approximately the same. How would you expect the two to compare with regard to each of the following? What species are present in benzene vapor?

   1. melting point
   2. type of bonding
   3. rate of evaporation
   4. structure

2. Can an inorganic compound be classified as a hydrocarbon? Why or why not?

3. Is the compound NaHCO₃ a hydrocarbon? Why or why not?

5. 1. 

   2. 

6. For each structural formula, write the condensed formula and the name of the compound.​

   1. 

7. Would you expect PCl₃ to be an ionic compound or a covalent compound? Explain your reasoning.

8. What distinguishes an aromatic hydrocarbon from an aliphatic hydrocarbon?

9. The following general formulas represent specific classes of hydrocarbons. Refer to Table 6.4.2 and Table 6.8 and Figure 6.4.4 and identify the classes.

   1. C_nH_{2n + 2}
   2. C_nH_2n
   3. C_nH_{2n − 2}

10. Using R to represent an alkyl or aryl group, show the general structure of an

    1. alcohol.
    2. phenol.

Answer

11. 1. ROH (where R is an alkyl group)
    2. ROH (where R is an aryl group)

Numerical Problems​

1. Draw the structure of each compound.

   1. propyne
   2. ethanol
   3. n-hexane
   4. cyclopropane
   5. benzene

2. Draw the structure of each compound.

   1. 1-butene
   2. 2-pentyne
   3. cycloheptane
   4. toluene
   5. phenol

Answers

1. 1. 

   2. 

   3. 

   4. 

   5. 

Contributors

• Anonymous

Modified by Joshua Halpern