CHAPTER III

ORGANIC NOMENCLATURE

 

 

Until recently, the environmental engineer did not have to be very knowledgeable regarding organic chemistry, and the naming of organic compounds (nomenclature). However, the intensifying concern over anthropogenic organic pollutants has changed all of this. Now the environmental professional must be familiar with the basic terminology of organic chemistry. He/she must also have a certain knowledge of the chemical behavior of organic compounds as it relates to environmental processes. This chapter is intended to provide a working knowledge of organic nomenclature.

 

 

A. THE CARBON SKELETON

 

Carbon can form a nearly limitless diversity of compounds. One reason for this is carbon's ability to bind covalently with itself in long chains:

 

 

In the above structure, each carbon atom (C) is surrounded by four single bonds. This is a consequence of carbon's tendency to form four covalent bonds each. These extra bonds not used to join the carbon chain may be linked to hydrogen atoms or other structures. The particular structure shown above is an aliphatic chain. The carbons are linked in a linear fashion, without forming rings or cycles.

 

1. Unbranched Alkanes

An homologous series of simple aliphatic organic compounds is then the following:

 

Methane Ethane Propane Butane Pentane

 

The series continues: hexane (6C), heptane (7C), octane (8C), nonane (9C), decane (10C), etc. This is the simplest homologous series of organic compounds, called the normal alkanes or sometimes the normal paraffins. Alkanes are saturated hydrocarbons (containing only hydrogen and carbon) with an unbranched aliphatic structure. To emphasize their simple linear structure they are often given the prefix, "n-", meaning, normal. All alkanes have the general empirical formula, CnH2n+2.

 

 

2. Branched Alkanes & IUPAC Nomenclature

Branched alkanes are also commonly encountered. From a geometric standpoint, the smallest branched aliphatic has 4 carbons. It is called Isobutane because it is an isomer of butane (often referred to n-butane to distinguish it from isobutane). An isomer is a compound with an empirical formula identical to a second compound, but with a different structure (i.e., geometric arrangement of the atoms) is different. For this reason, branched alkanes are sometimes called isoalkanes or isoparaffins.

 

Full Structure Shorthand

Isobutane

 

In order to avoid the tedium and unnecessary detail of drawing in all of the hydrogen atoms, chemists have adopted a shorthand version of representing organic structures. The rules of this shorthand are as follows. No carbon atoms belonging to the carbon skeleton are drawn. Instead, only the bonds connecting these carbon atoms are shown. It is understood that a carbon atom is present at the junction of any two or more line segments. In addition, the lines, representing bonds, are generally drawn at a slight angle to each other (often ~120). This is often close to the actual bond angles, and it makes the separation of one carbon-carbon bond from the next more obvious. When single bonds connect the carbon atoms, a single line is used, and when double or triple bonds exist, double or triple lines are used. Hydrogen atoms connected directly to the carbon skeleton and the bonds that connect them are never shown. Instead their presence can always be determined by keeping in mind that all carbon atoms must have a total of four covalent bonds. It is understood that bonds not shown (those required to reach a total of 4 per carbon) are actually carbon-hydrogen linkages, and that a hydrogen atom is at the end of each of these bonds.

Returning to the branched alkanes, there are three possible isomers of pentane (saturated 5C hydrocarbons). Add one more carbon and the number increases to five. At ten carbons, there exists 75 possible isomers. Since we can't refer to all of the branched isomers as simply isoheptane or isodecane (the unbranched alkanes are always n-heptane, n-decane, etc.) a well defined system of naming these compounds is needed. The International Union of Pure and Applied Chemistry (IUPAC) has developed just such a system. For example, consider the following isomers of heptane.

 

 

n-Heptane 2-Methylhexane 3-Methylhexane 3-Methylhexane 2-Methylhexane

 

The first structure is a simple alkane with 7 carbons (i.e., n-Heptane). The next four have seven carbons, but they are all branched. According to the IUPAC rules, aliphatic compounds are named after the longest continuous chain of carbon atoms that can be found in the structure. For all four of these, the longest component chain is 6 carbon, thus they are all hexane derivatives. Also in each case, the branch is composed of a single carbon atom. Thus, they are all termed, methylhexanes. The name methyl comes from meth-, signifying one carbon atom (e.g., methane). Instead of adding the -ane suffix indicating an alkane, -yl is added, which indicates an alkyl chain (i.e., alkane minus a hydrogen). The precise names for the methylhexanes are unambiguously assigned once the geometric location of the methyl branch is determined. To do this, the carbon atoms on the alkane backbone are numbered consecutively from one end to the other. The number of the carbon atom to which the methyl branch is bound identifies the particular methylhexane isomer. Note that the last two are not called 4-methylhexane and 5-methylhexane. This is because, these structures are identical to 3-methylhexane and 2-methylhexane respectively. One can see this by just flipping them over.

 

3. Alkenes

If one were to remove two hydrogens from each of the alkanes, leaving a carbon-carbon double bond in their place, one would have the series known as alkenes or olefins. Organic compounds such as these having double or triple bonds are often referred to as unsaturated, because they have less than the maximum possible number of hydrogens. The IUPAC names for these compounds have the same basic roots as the alkanes, however, the suffix -ene is used in place of -ane. Common usage also permits the suffix "-ylene" for alkenes, especially for ethene (i.e., ethylene). Unbranched alkenes of 4 carbons or greater can have isomers, depending on the location of the double bond. The system of numbering the carbons on the carbon backbone, and assigning the position of the double bond to the lowest number of the two carbons involved. When more than one double bond is present the suffix becomes -diene, -triene, etc. All alkenes with only one double bond have the general empirical formula, CnH2n.

 

 

Ethene Propene 1-Butene 2-Butene 1-Pentene 2-Pentene

 

 

 

3-Ethyl-5-Octene 1,3-Pentadiene

 

4. Alkynes

Removal of 4 hydrogens from two adjoining carbons in an alkane results in the formation of a carbon-carbon triple bond. The homologous series of these compounds is termed the alkynes (suffix -yne). Despite the IUPAC nomenclature, the first member of this series is most commonly known as acetylene. All alkynes with only one triple bond have the general empirical formula, CnH2n-2.

 

 

 

Ethyne Propyne 1-Butyne 2-Butyne 1-Pentyne 2-Pentyne

 

 

5. Alicyclic Hydrocarbons

 

When hydrocarbon chains are joined to make a ring, they are said to be cyclic, or more properly, alicyclic. These differ from the previously discussed aliphatic hydrocarbons which may be branched, but not in the form of rings. A compound need only have one ring to be considered alicyclic, regardless of whether there are aliphatic chains attached to it. They may also have double and triple bonds (e.g., cycloalkenes, cycloalkynes, in addition to cycloalkanes). However, if a compound contains a 6-membered ring with alternating double and single bonds, it is a member of a special class called "the aromatics" (see below).

Alicyclic compounds are often given the prefix, "cyclo". Alicyclic alkanes are sometimes called naphthenes. These compounds have the general empirical formula, CnH2n. Note that this is the same formula for the aliphatic alkenes. Thus, addition of a double-bond or closure to form a ring have the same effect on a compound's empirical formula.

 

Cyclopentane Cyclohexane

 

6. Aromatics

 

Six-membered rings tend to be quite stable. However, when a six-membered ring contains three alternating double and single bonds, it has a property known as resonance. This imparts a special stability to the molecule, and for this reason we give such molecules a special name, aromatic. The simplest aromatic compound is benzene.

 

or

Benzene

 

There are a wide range of benzene derivatives of great commercial and environmental importance. These may include the simple aromatic hydrocarbons, the hydroxy-benzenes or phenols, the biphenyls, and the fused aromatics otherwise known as polynuclear aromatics.

The simple aromatic hydrocarbons (below) can help illustrate several important points. First, notice that when a single carbon group (a methyl group) is attached to benzene, it is called toluene. The IUPAC name should be methylbenzene, but toluene has been used for so long that it is now well accepted. Similarly, the xylenes are actually dimethylbenzene isomers. These three isomers are distinguished by the relative positions of the methyl groups. When groups attached to the aromatic ring (known as substituents) are on adjacent carbons, they are said to be in the ortho position (represented by the prefix, "o-"). When they are two carbons away they are in the meta position ("m-"). And when they are on opposite sides of the ring, they are in the para position ("p-"). This is not only true for xylenes, but for any aromatic compound with multiple substituents.

 

Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene

 

When 3 or more substituents are present, the numbering system is most commonly used. One of the six carbons is assigned the number "1", and the others are numbered consecutively as you go clockwise or counterclockwise around the ring. The direction you choose to go around the ring doesn't matter, because an aromatic compound can always be flipped on its vertical axis to get the mirror image. These two forms are indistinguishable and represent the same compound.

 

 

 

Example: Automotive Gasoline:

Gasoline is a blend of light hydrocarbon fractions from petroleum. It is blended at the refinery with the very practical objective to perform well in combustion engines. Since, its not blended to any specific chemical composition, its precise makeup is hard to characterize. Of all the major petroleum products, gasoline probably has the highest alkane content, a low alicyclic content, and a low to medium aromatic content. It differs from jet fuel in that the latter is higher in alicyclics and lower in alkanes. The fuel oils have higher aromatic contents, average alicyclic contents and very low levels of alkanes.

Table 3.1 shows the composition of two contrasting automotive gasolines. The 76 product is a winter-grade (volatility class D) regular fuel. The Amoco is a summer-grade (volatility class A) premium fuel. The winter-grade fuel (76) is characterized by more volatile constituents. This is especially seen in the higher alkane content. The premium (Amoco) has a higher octane rating and a substantially higher toluene concentration.

Efficient operation of automobile engines requires that just the right mix of hydrocarbon vapor and air is obtained in the carburator. If the hydrocarbon vapor concentration is too high, there will be too little oxygen for good combustion. Conversely, if the vapor concentration is too low, ignition will not occur at all. For this reason, manufacturers blend their gasoline to achieve the right volatility for the anticipated temperture conditions (i.e., location and time of year).

High octane gasolines are blended to minimize knocking, or irregular ignition. As a hydrocarbon becomes more heavily branched, it tends to ignite more smoothly in an automobile engine. One of the most efficient compounds in this regard is 2,2,4-trimethylpentane[1], which is arbitrarily given an octane rating of 100. n-Heptane, on the other hand, is very poor, and is given a rating of zero. Aromatics have high octane ratings of 100 or more. This is why the high-aromatic content Amoco is a "premium grade".

In practice, octane ratings are determined empirically by comparing the performance of a gasoline with standard mixtures of n-heptane and 2,2,4-trimethylpentane. Two reading are generally taken, one on a cold engine (the R or research rating), the other on warm engine (the M or motor rating). The overall octane rating is then taken as the average of the two, i.e.,

In 1922 it was discovered that the octane rating of a gasoline could be increased by addition of small amounts of tetraethyl lead. This compound has a central lead atom surrounded by four ethyl groups. It resembles a highly-branched alkane and greatly inhibits knocking. For the next 50 years leaded gasoline was the norm, until congress required all new car to have catalytic converters. Since tetraethyl lead poisoned the catalyst, unleaded gasoline was needed. To help achieve the desired no-knock performance, octane-boosting additives, such as methyl tert-butyl ehter (MTBE; 115 octane rating), tert-butyl alcohol and methyl alcohol (105 octane rating), are now used.

 

 

 

Table 3.1

Composition of Two Unleaded Gasolines

(from: Sigsby et al., 1987)

 

Compound

% by Weight

 

76 Regular

Amoco Premium

n-Alkanes

 

 

n-Butane

7.75

3.52

n-Pentane

3.06

2.37

n-Hexane

1.32

0.83

n-Heptane

1.23

0.42

n-Octane

0.76

0.20

n-Nonane

0.27

0.18

Branched Alkanes

 

 

Isobutane

1.86

1.4

2,2-Dimethylbutane

0.41

0.08

2,3-Dimethylbutane

0.86

0.78

Isopentane

6.16

7.12

2-Methylpentane

2.76

2.76

3-Methylpentane

1.76

1.47

2,4-Dimethylpentane

1.15

0.86

 


Table 3.1 (continued)

Composition of Two Unleaded Gasolines

(from: Sigsby et al., 1987)

 

Compound

% by Weight

 

76 Regular

Amoco Premium

Branched Alkanes (cont)

 

 

2,3,3-Trimethylpentane

2.26

1.82

3-Methylhexane

1.91

1.04

2,3,5-Trimethylhexane

0.18

0.13

2,2,5-Trimethylhexane

0.81

0.76

2-Methylheptane

0.37

0.10

3-Methylheptane

0.70

0.23

4-Methylheptane

1.20

0.25

3,4-Dimethyloctane

1.12

1.42

2-Methyldecane

1.83

1.38

Alkenes

 

 

2-Butene

0.50

0.26

1-Pentene

0.32

0.18

2-Pentene

1.40

1.14

1-Hexene

0.64

0.64

2-Hexene

0.33

0.27

3-Hexene

0.80

0.73

Branched Alkenes

 

 

2-Methyl-2-butene

1.22

1.50

2-Methyl-2-pentene

0.61

0.65

Dimethylhexene

0.28

0.25

Cyclic Compounds

 

 

Cyclopentene

0.37

0.31

Cyclopentane

0.48

0.42

Methylcyclopentane

1.17

0.77

Cyclohexene

2.73

1.31

Methylcyclohexane

1.57

0.33

Aromatics

 

 

Benzene

1.76

1.96

Toluene

5.54

20.25

Ethylbenzene

1.17

0.94

m- and p-Xylene

4.58

2.60

o-Xylene

2.46

1.61

n-Propylbenzene

0.70

0.90

1,3,5-Trimethylbenzene

2.74

3.35

1,2,4-Trimethylbenzene

3.75

4.59

1,2,3-Trimethylbenzene

1.21

1.26

1-Methyl-3-ethylbenzene

1.52

1.53

Isobutylbenzene

0.42

0.48

1-Methyl-3-n-propylbenzene

1.15

1.32

 

B. FUNCTIONAL GROUPS

 

In addition to hydrocarbon chains, one can build organic compounds by adding what we call functional groups. These are common sub-molecules or structures that contain an "O", an "N" an "S" or a variety of other elements, in addition to carbon and/or hydrogen. Some examples follow:

 

 

 

Primary

Secondary

Name

Structure

Suffix/Prefix

Term

Alcohols

-ol

hydroxy

Acids

-oic acid

carboxy

 

-ate

 

Ketones

-one

keto

Aldehydes

-al

 

Amines

-yl amine

amino

Esters

Alkyl(R) __-ate

 

Ethers

 

 

Halides

chloride

chloro-

 

bromide

bromo-

Amide

 

 

Nitriles

-nitrile

 

 

 

 

 

C. HETEROATOMS and HETEROCYCLICS

 

Heteroatoms often refer to nitrogen, oxygen, sulfur that are incorporated into ringed molecules, called heterocycles. Thre are a broad array of heterocyclic compounds one finds in nature, notably the nucleic acids base units, pyrimidines and purines. Some important ones are shown below

 

Thymine Cytosine Uracil

Pyrimidines

Adenine Guanine

Purines

 

 

Pyridine Indole

Others

 

Literature Cited

 

Sigsby, J.E., S. Tejada, W. Ray, J.M. Lang and W. Duncan, 1987. "Volatile Organic Compound Emissions from 46 In-Use Passanger Cars," Environ. Sci. Technol. 21(5)466-475.

 

 

General References

 

Sawyer, C.N., P.L. McCarty and G.F. Parkin, 1994. Chemistry for Environmental Engineering, 4th Edition, McGraw-Hill, Inc., New York. (chapter 5)

 

Hutchinson, E. 1964. Chemistry: The Elements and their Reactions, 2nd Edition, W.B. Saunders Co., Philadelphia, (chapter 29), or almost any other basic text on general chemistry or organic chemistry

 


 

 

 

 



[1]this compoud has been mistakenly called isooctane, or sometimes just octane. This is where the name, "octane rating" comes from.