CH105: Consumer Chemistry

7.1 Recognition of Organic Structures

Scientists of the 18th and early 19th centuries studied compounds obtained from plants and animals and labeled them organic because they were isolated from “organized” (living) systems. Compounds isolated from nonliving systems, such as rocks and ores, the atmosphere, and the oceans, were labeled inorganic. For many years, scientists thought organic compounds could be made by only living organisms because they possessed a vital force found only in living systems. The vital force theory began to decline in 1828, when the German chemist Friedrich Wöhler synthesized urea from inorganic starting materials. He reacted silver cyanate (AgOCN) and ammonium chloride (NH₄Cl), expecting to get ammonium cyanate (NH₄OCN). What he expected is described by the following equation.

AgOCN + NH₄Cl → AgCl + NH₄OCN

Instead, he found the product to be urea (NH₂CONH₂), a well-known organic material readily isolated from urine. This result led to a series of experiments in which a wide variety of organic compounds were made from inorganic starting materials. The vital force theory gradually went away as chemists learned that they could make many organic compounds in the laboratory.

Today organic chemistry has been reclassified as the study of compounds that contain carbon, and inorganic chemistry is the study of the chemistry of all other elements. It may seem strange that we divide chemistry into two branches—one that considers compounds of only one element and one that covers the 100-plus remaining elements. However, this division seems more reasonable when we consider that of tens of millions of compounds that have been characterized, the overwhelming majority are carbon compounds.

Organic compounds, like inorganic compounds, obey all the natural laws. Often there is no clear distinction in the chemical or physical properties among organic and inorganic molecules. Nevertheless, it is useful to compare typical members of each class, as in Table 7.1 (Keep in mind, however, that there are exceptions to every category in this table.) To further illustrate typical differences among organic and inorganic compounds, Table 7.1 also lists properties of the inorganic compound sodium chloride (common table salt, NaCl) and the organic compound hexane (C₆H₁₄), a solvent that is used to extract soybean oil from soybeans (among other uses). Many compounds can be classified as organic or inorganic by the presence or absence of certain typical properties, as illustrated in Table 7.1.

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7.2 Introduction to Alkanes

Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes have the general formula C_nH_2n+2 and can be subdivided into the following three groups: the linear straight-chain alkanes, branched alkanes, and cycloalkanes (Fig. 7.2). Alkanes are also saturated hydrocarbons, that is all of the carbon atoms are ‘saturated’ with hydrogen atoms and do not contain any carbon-carbon double bonds or triple bonds. Alkanes are the simplest and least reactive hydrocarbon species containing only carbons and hydrogens. They are commercially very important, being the principal constituent of gasoline and lubricating oils and are extensively employed in organic chemistry; though the role of pure alkanes (such as hexanes) is delegated mostly to solvents. The distinguishing feature of an alkane, making it distinct from other compounds that also exclusively contain carbon and hydrogen, is its lack of unsaturation. That is to say, it contains no double or triple bonds, which are highly reactive in organic chemistry. Though not totally devoid of reactivity, their lack of reactivity under most laboratory conditions makes them a relatively uninteresting, though very important component of organic chemistry. As you will learn about later, the energy confined within the carbon-carbon bond and the carbon-hydrogen bond is quite high and their rapid oxidation produces a large amount of heat, typically in the form of fire.

Figure 7.2. Examples of Alkanes

Figure 7.3 The Three Simplest Alkanes

From propane (C₃H₈₎ onward, you will notice that the only difference between longer chain hydrocarbons involves the addition of   CH₂ units as you move up the series (Fig. 7.4). Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH₂ group) is called a homologous series, and can be defined mathematically. The members of such a series, called homologs. In organic chemistry, homologs have properties that vary in a regular and predictable manner. Thus, the principle of homology gives organization to organic chemistry in much the same way that the periodic table gives organization to inorganic chemistry. Instead of a bewildering array of individual carbon compounds, we can study a few members of a homologous series and from them deduce some of the properties of other compounds in the series.

 

Figure 7.4 Members of a Homologous Series

In Figure 7.4, note that each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula. The principle of homology allows us to write a general formula for alkanes: C_nH_{2n+ 2}. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C₈H_{(2 × 8) + 2} = C₈H₁₈.

Concept Review Exercises

1. In the homologous series of alkanes, what is the molecular formula for the member just above C₈H₁₈?

2. Use the general formula for alkanes to write the molecular formula of the alkane with 12 carbon atoms.

Answers

1. C₉H₂₀

2. C₁₂H₂₆

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Branched Chain Alkanes

We can write the structure of butane (C₄H₁₀) by stringing four carbon atoms in a row,

–C–C–C–C–

and then adding enough hydrogen atoms to give each carbon atom four bonds:

The compound butane has this structure, but there is another way to put 4 carbon atoms and 10 hydrogen atoms together. Place 3 of the carbon atoms in a row and then branch the fourth one off the middle carbon atom:

Now we add enough hydrogen atoms to give each carbon four bonds.

There is a hydrocarbon that corresponds to this structure, which means that two different compounds have the same molecular formula (C₄H₁₀), but a different arrangement of the atoms in space.  Recall that compounds having the same molecular formula but a different arrangement in space are called structural isomers. Structural isomers have different chemical and physical properties. Figure 7.5 shows the ball and stick model for the straight chain butane and the branched isomer, isobutane.

Figure 7.5 Butane and Isobutane. The ball-and-stick models of these two compounds show them to be isomers; both have the molecular formula C₄H₁₀.

Notice that C₄H₁₀ is depicted with a bent chain in Figure 7.5. The four-carbon chain may be bent in various ways because the groups can rotate freely about the C–C bonds. However, this rotation does not change the identity of the compound. It is important to realize that bending a chain does not change the identity of the compound; all of the following represent the same compound:

The formula of isobutane shows a continuous chain of three carbon atoms only, with the fourth attached as a branch off the middle carbon atom of the continuous chain.

Unlike C₄H₁₀, the compounds methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds.

Note

A continuous (unbranched) chain of carbon atoms is often called a straight chain even though the tetrahedral arrangement about each carbon gives it a zigzag shape. Straight-chain alkanes are sometimes called normal alkanes, and their names are given the prefix n-. For example, butane is called n-butane.

Concept Review Exercises

1. In alkanes, can there be a two-carbon branch off the second carbon atom of a four-carbon chain? Explain.

2. A student is asked to write structural formulas for two different hydrocarbons having the molecular formula C₅H₁₂. She writes one formula with all five carbon atoms in a horizontal line and the other with four carbon atoms in a line, with a CH₃ group extending down from the first attached to the third carbon atom. Do these structural formulas represent different molecular formulas? Explain why or why not.

Answers

1. No; the branch would make the longest continuous chain of five carbon atoms.

2. No; both are five-carbon continuous chains.

Key Takeaway

• Alkanes with four or more carbon atoms can exist in isomeric forms.

Exercises

1. Briefly identify the important distinctions between a straight-chain alkane and a branched-chain alkane.

2. Draw the structural isomers for the following alkanes.

   1. 
   2. 

3. Indicate whether the structures in each set represent the same compound or isomers.

   1. CH₃CH₂CH₂CH₃ and

      

   2. CH₃CH₂CH₂CH₂CH₃ and

      

Answers

1. Straight-chain alkanes and branched-chain alkanes have different properties as well as different structures.

3. 1. no
   2. yes

Additional Practice

1. Write the condensed structural formula for each structural formula.

   1. 
   2. 
   3. 

2. A condensed structural formula for isohexane can be written as (CH₃)₂CHCH₂CH₂CH₃. Draw the line-angle formula for isohexane.

3. Draw a line-angle formula for the compound CH₃CH₂CH(CH₃)CH₂CH₂CH₃.

4. Give the structural formula for the compound represented by this line-angle formula:

   

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Cycloalkanes

Cycloalkanes are very important in components of food, pharmaceutical drugs, and much more. However, to use cycloalkanes in such applications, we must know the effects, functions, properties, and structures of cycloalkanes. Cycloalkanes are alkanes that are in the form of a ring; hence, the prefix cyclo- is used to name these alkanes. Stable cycloalkanes cannot be formed with carbon chains of just any length. Recall that in alkanes, carbon adopts the tetrahedral geometry in which the angles between bonds are 109.5°.

Source: Wikipedia

For some cycloalkanes to form, the angle between bonds must deviate from this ideal angle, an effect known as angle strain. Additionally, some hydrogen atoms may come into closer proximity with each other than is desirable (become eclipsed), an effect called torsional strain. These destabilizing effects, angle strain and torsional strain are known together as ring strain. The smaller cycloalkanes, cyclopropane and cyclobutane, have particularly high ring strains because their bond angles deviate substantially from 109.5° and their hydrogens eclipse each other. Thus, both of these ring conformations are highly unfavorable and unstable. Cyclopentane is a more stable molecule with a small amount of ring strain, while cyclohexane is able to adopt the perfect geometry of a cycloalkane in which all angles are the ideal 109.5° and no hydrogens are eclipsed; it has no ring strain at all. Cycloalkanes larger than cyclohexane have ring strain and are not as commonly encountered in organic chemistry. Figure 7.6 provides examples of cycloalkane structures.

Classification of Carbon Bonds

Carbon atoms participating in chemical bonds within a molecule can be classified based on the number of carbon-carbon bonds that are formed. 

• primary carbon atom: one carbon neighbor
• secondary carbon atom: two carbon neighbors
• tertiary carbon atom: three carbon neighbors
• quaternary carbon atom: four carbon neighbors

Within any given molecule, each carbon atom can be classified (Fig. 7.8).

Figure 7.8.  Classification of Carbon Atoms Within a Molecule

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7.3 Properties of Alkanes

Alkanes are the simplest family of hydrocarbons – compounds containing carbon and hydrogen only with only carbon-hydrogen bonds and carbon-carbon single bonds. Alkanes are not very reactive and have little biological activity; all alkanes are colorless and odorless. Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families. Let’s consider their physical properties first.

Table 7.3 describes some of the properties of some of the first 10 straight-chain alkanes.  Nearly all alkanes have densities less than 1.0 g/mL and are therefore less dense than water (the density of H₂O is 1.00 g/mL at 20°C).

Melting Points and Boiling Points

Both the melting points and boiling points of alkanes are characteristic of the intermolecular forces found between the molecules. The electronegativity difference between carbon and hydrogen (2.1 – 1.9 = 0.2) is small; therefore, the C-H bond is nonpolar, meaning that the only attractions between one molecule and its neighbors will be London dispersion forces. These  forces will be very small for a molecule like methane but will increase as the size of the molecules increase. Therefore, the melting and boiling points of the alkanes increases with the molecular size, due to the increase in London dispersion forces. (i.e. the intermolecular forces are stronger in larger hydrocarbons, therefore, more energy is required to cause phase changes). Figure 7.9 shows the melting and boiling point trends for the first 16 hydrocarbons. Notice that the first four alkanes are gases at room temperature, and solids do not start to appear until about C₁₇H₃₆.

Figure 7.9. Melting Points and Boiling Points for Straight Chain Alkanes

Adapted from: Techstepp

Regarding isomers, the more branched the chain, the lower the boiling point tends to be. London dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules (with substantial amounts of branching) to lie close together (compact) compared with long, thin molecules. Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact.

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Solubility

Alkanes (both normal and cycloalkanes) are virtually insoluble in water but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds. When a molecular substance dissolves in water, the following must occur:

• breaking of the intermolecular forces within the substance. In the case of the alkanes, these are the London dispersion forces.
• breaking of the intermolecular forces in the water so that the substance can fit between the water molecules. In water, the primary intermolecular attractions are hydrogen bonds.

Breaking either of these attractions requires energy, although the amount of energy required to break the London dispersion forces in a compound, such as methane, is relatively negligible; this is not true of the hydrogen bonds in water. Recall that hydrogen bonds are much stronger. 

To simplify, a substance will dissolve if sufficient energy is released when the new bonds are formed between the substance and the water to make up for the energy required to break the original attractions. The only new attractions between the alkane and the water molecules are the London dispersion forces. These forces to do not release a sufficient amount of energy to compensate for the energy required to break the hydrogen bonds in water. Therefore, the alkane does not dissolve as shown in Figure 7.10.

Solubility in organic solvents

In most organic solvents, the primary forces of attraction between the solvent molecules are the London dispersion forces. Therefore, when an alkane dissolves in an organic solvent, the London dispersion forces are broken and are replaced by new London dispersion forces between the mixture. The two processes more or less cancel each other out energetically; thus, there is no barrier to solubility.

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Alkane Properties and Environmental Hazards: A Closer Look

Due to the solubility and density of alkanes, oil spills into the ocean or other bodies of water can have devastating environmental consequences.  The oil cannot dissolve or mix with the water, and because it is less dense than water, it floats on top of the surface of the water creating an oil slick, as depicted in Figure 7.11. Since the oil slick remains at the surface of the water, the organisms most affected by oil slicks are those found on the surface of the ocean or near the shorelines, including sea otters and seabirds. The chemical constituents of the oil are toxic though ingestion, inhalation, and through skin and eye irritation.

Figure 7.11 Oil Spills. Crude oil coats the water’s surface in the Gulf of Mexico after the Deepwater Horizon oil rig sank following an explosion. The leak was a mile below the surface, making it difficult to estimate the size of the spill. One liter of oil can create a slick 2.5 hectares (6.3 acres) in size. This and similar spills provide a reminder that hydrocarbons and water don’t mix.

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7.4 Chemical Reactivity of Alkanes

Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. Both of these bonds are nonpolar. Therefore, there is no portion of the molecule that carries any significant amount of positive or negative charge, which is required for other ionic and polar molecules to be attracted to it. Therefore alkanes generally do not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents. Consider butane as an example:

Neither positive ions nor negative ions are attracted to a nonpolar molecule. In fact, the alkanes undergo so few reactions that they are sometimes called paraffins, from the Latin parum affinis, meaning “little affinity.”

The result is that alkanes have very little reactivity and only undergo three major types of reactions, including the following:

• Combustion Reactions – burn them – destroying the entire molecule;
• Halogenation Reactions (substitution type) – react them with some of the halogens, breaking the carbon-hydrogen bonds;
• Cracking Reactions – use heat and/or a catalyst to crack alkanes, breaking carbon-carbon bonds.

 

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Combustion Reactions

The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history. The practical importance of this reaction cannot be denied, but the massive and uncontrolled chemical changes that take place in combustion make it difficult to deduce mechanistic paths. Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. No other common reaction involves such a profound and pervasive change, and the mechanism of combustion is so complex that chemists are just beginning to explore and understand some of its elementary features.

CH₃CH₂CH₃  +  5O₂  →  3CO₂  +  4H₂O  +  heat