Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Carbon Functional Groups and Organic Nomenclature
2.3 Saturated Hydrocarbons

Throughout our course of organic chemistry, we will encounter numerous compounds that contain only carbon and hydrogen atoms, these compounds are called hydrocarbons, and a study of these compounds will be the emphasis of this chapter. First, we will concentrate on a category of hydrocarbons, called saturated hydrocarbons or alkanes, sometimes referred to as aliphatic hydrocarbons, which are compounds that contain only carbon and hydrogen atoms, and the carbon atoms are all sp3 hybridized and hence have four single bonds to each carbon atom. The term aliphatic comes from the Greek word aleiphar, which pertains to fats or oils. Saturated hydrocarbons can be acyclic or cyclic. Acyclic saturated hydrocarbons have the chemical formula CnH2n+2, where n represents an integer, such as 1, 2, 3, and so on. On the other hand, saturated hydrocarbons that are in the form of a ring, or have a ring as part of the molecule, all have the general formula CnH2n, where n represents an integer. Figure 2.3 gives examples of different representations of acyclic hydrocarbons. Note that there are other possible compounds with the same molecular formula, but just one example of each is given.

Figure 2.4 gives examples of cyclic hydrocarbons. Note that the general formula that represents cycloalkanes is CnH2n only if there is one ring in the compound, but if there are additional rings, such as the case of the last compound in Figure 2.4, then the formula is slightly different. If there are two rings, the formula becomes CnH2n−2. Therefore, it is possible to distinguish between an acyclic alkane and a cyclic and further a bicyclic alkane just based on the formula.

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Figure 2.3 Examples of acyclic saturated hydrocarbon compounds, also known as alkanes.

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Figure 2.4 Examples of cyclic saturated hydrocarbon compounds (cycloalkanes).

Problem 2.2

Determine which of the following saturated hydrocarbons are cyclic or acyclic. For hydrocarbons that are identified as cyclic, determine the number of rings present.

a) C12H24 b) C12H22 c) C12H20 d) C10H22 e) C10H20 f) C10H16

Alkanes are nonpolar molecules, so they are not miscible in water, but they are miscible in nonpolar, or weakly polar organic solvents. Alkanes are less dense than water; most alkanes have densities of approximately 0.7 g ml−1, compared to water, which has a density of 1.0 g ml−1. Thus, a mixture of water and alkanes, such as water and gasoline or oil, readily separates into two phases, with the alkane being the top layer. Alkanes that have four or less carbons (CH4, C2H6, C3H8, and C4H10) are gases at room temperature. However, under low enough temperatures and pressures, these gases can be liquefied and can be stored and transported. These alkanes burn very cleanly and hence are used as fuels for heating. Owing to the possibility of leakage, these gaseous hydrocarbons are not used widely as fuels in vehicles. Alkanes that have five to eight carbons (C5H12, C6H14, C7H16, C8H18) are liquid at room temperature and are the major components of gasoline. These liquids are very volatile and hence make very effective motor fuel. Other alkanes (C9 to about C16) are higher boiling liquids and are used mostly as kerosene and diesel fuel. Table 2.2 shows the boiling point ranges of the different categories of alkanes, along with typical uses.

Alkanes are obtained mostly from petroleum, which was first obtained from oil wells around the middle 1800s. The majority of alkanes that are used in industry are derived from petroleum and petroleum by-products. Petroleum, also called crude oil, comes from the remains of prehistoric plants, and it is pumped from wells. The principal components of crude oil are alkanes and another type of compounds called aromatics (which will be discussed in Chapter 17). Crude oil also contains other compounds, such as sulfur and nitrogen. After the crude oil is pumped from the ground (as shown in Figure 2.5), a process called refining takes place, in which the alkanes are separated into fractions based on their boiling points, also called fractional distillation. The distillation of petroleum gives different fractions based on the number of carbon atoms. Another process called cracking is used to convert some of the larger alkanes to smaller more useful alkanes, such as those of gasoline. Cracking involves heating the alkanes in the presence of a catalyst, such as SiO2 or Al2O3. Natural gas, which is about 70% CH4 (methane) and 15% C2H6 (ethane) depending on the source of the petroleum, is collected and stored from the petroleum production process. Natural gas is typically used to heat buildings and generate electricity.

Table 2.2 Various uses and properties of different categories of alkanes.

Number of carbons

Uses

Boiling point range

C1—C4

Typically called natural gas and used as fuel for cooking

Below 20 °C (volatile gases)

C5—C7

Industrial and research lab solvents

20—100 °C (volatile liquids)

C7—C12

Gasoline for vehicles and other combustion engines

20—200 °C (volatile liquids)

C12—C18

Kerosene

200—300 °C

C12 and larger

Diesel oil and heating homes

200—400 °C

C20 and larger

Grease and other lubricants

> 400 °C (nonvolatile liquids)

C20 and larger

Asphalt and tar

> 400 °C (nonvolatile solids)

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Figure 2.5 The process of pumping petroleum, also called crude oil, from the ground.

Hydrocarbons are not only used as fuels but they are also found in nature. The simplest alkanes (C1—C4) are considered to be asphyxiants (in that they will not support respiration with insufficient oxygen.). Inhalation of higher molecular weight alkanes may cause central nervous system depression, dizziness, and loss of coordination. Also, higher molecular weight alkanes will dissolve fatty substances around the nerve. As a result, extreme care must be exercised in the industrial workplace to minimize the exposure to workers. For some insects, hydrocarbons provide a waterproof layer to prevent desiccation. Some alkanes also serve as pheromones for some insects and in some cases act as recognition cues to protect insects from parasites and other undesired invasions.

DID YOU KNOW?

The saturated hydrocarbon shown below is a sex pheromone used by female tiger moths.

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2.3.1 Classification of the Carbons of Saturated Hydrocarbons

Owing to the numerous different possible saturated hydrocarbons, a system of classification has been devised. This classification is based on the number of alkyl groups that are bonded to the carbons. For example, if a carbon in a hydrocarbon is bonded to one alkyl group, that carbon is classified as a primary carbon, or sometimes the symbol 1° is used. Since carbons of saturated hydrocarbon have four single bonds, the classifications are primary (1°), secondary (2°), and tertiary (3°), as illustrated below.

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This method of classification will be used throughout our course to describe different organic compounds that contain these types of carbons.

Problem 2.3

Label the starred carbon atoms in the following compounds as primary (1°), secondary (2°), or tertiary (3°).

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