Inorganic and Organic Chemistry

Observations in Science

2.6 Inorganic and Organic Chemistry

gives us some useful fl exibility and also emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table.

All of the polymer molecules we have mentioned are carbon based. Their skele- tons consist entirely of carbon atoms. Because elements in the same group of the pe- riodic table have similar chemical behavior, you might be wondering whether similar polymers could be produced based on silicon, which appears right below carbon in Group 14. Silicon-based polymers, known as silicones, do exist, but they differ from carbon polymers in important ways. Covalent bonds can be formed between silicon atoms, but they are not as strong as those between carbon atoms. So chains of silicon atoms become unstable beyond a length of around ten atoms, and silicon analogs of polymers such as polyethylene cannot be produced. Instead of pure silicon, the back- bone chains in silicone polymers consist of alternating silicon and oxygen atoms. The Si!O bond is strong enough to allow these chains to grow quite long. Additional atoms or groups of atoms bound to the silicon atoms infl uence the properties of the polymer. The range of polymers that can be produced is not nearly as diverse as for carbon, but silicone polymers are widely used in applications including greases, caulk- ing materials, water repellents, and surfactants.

bizarre distinction because carbon is just one element in a periodic table that contains more than 100 others. But the chemistry of carbon is so rich, diverse, and important that organic chemists actually outnumber inorganic chemists. Because this text is in- tended as a brief overview of important chemical principles, we will focus most of our attention on the behavior of molecules in general rather than delving too deeply into the details of either subfi eld. In this section, we will describe briefl y some important similarities and differences between organic and inorganic chemistry and introduce terminology and notation that we will need as we progress through the text.

Inorganic Chemistry—Main Groups and Transition Metals

Many inorganic compounds exist as relatively small molecules whose atoms are joined together through covalent bonds. One such compound is silicon tetrachloride, SiCl₄, which has important uses in the production of semiconductors. Figure 2.15 shows some visual representations of SiCl₄ that we might use to illustrate how its atoms are actually arranged into molecules. Four chlorine atoms surround a central silicon atom, and each chlorine shares one pair of electrons with the silicon.

Silicon and chlorine are both main group elements, found in Groups 14 and 17 of the periodic table, respectively. As mentioned in the previous section, elements from the same group tend to display similar chemical properties. Thus once we know that SiCl₄ exists, we might expect that other pairs of elements from the same groups might form similar compounds. And this prediction is correct: compounds such as SnCl₄ and CF₄ do exist and have structures and bonds analogous to those in Figure 2.15 for SiCl₄.

Other compounds of the main group elements form extended ionic structures, such as that of NaCl in Figure 2.9. But despite the difference in the types of chemical bonds employed, we can still readily predict that similar compounds should exist for other pairs of elements from the same groups. From the periodic table, we see that sodium is in Group 1 and chlorine is in Group 17. So we can expect that other pairs of elements from these columns of the table will form ionic solids, too. Again, our pre- diction is accurate; compounds such as LiCl, NaF, and KBr have structures analogous to that of NaCl. The reason for the existence of these similar compounds is simple.

All of the metals in Group 1 form cations with a 1+ charge, and all of the elements in Group 17 form anions with a 1− charge. Any of these cations can combine with any of the anions in a 1:1 ratio to form neutral compounds.

The chemistry of transition metals is somewhat more complicated than that of the main group elements, though, because most transition metals can form multiple cations with different charges. Iron commonly forms two different monatomic cations:

Fe²⁺ and Fe³⁺. As a result of this, iron can form a more diverse set of compounds than Group 1 metals. It can combine with chlorine to form either FeCl₂ or FeCl₃, and these two compounds have signifi cantly different physical properties (Figure 2.16). Largely because they can form multiple cations, the chemistry of transition metals does not We will study chemical bonding and

molecular shapes in detail in Chapter 7.

We will study chemical bonding and molecular shapes in detail in Chapter 7.

The solid and dashed triangles in the structure on the left indicate that one of the chlorine atoms would be in front of the plane of the page and one would be behind that plane.

Cl Cl Cl

Cl Si

Figure 2.15 ❚ This fi gure presents three depictions of SiCl4. In the drawing at the left, each atom is represented by its symbol and the lines between the symbols depict chemical bonds. In the center panel is a “ball and stick” model, where each atom is a ball, and the bonds are shown as sticks connecting the balls. In the right-hand panel is a “space fi lling” model, where atoms are shown as balls that overlap one another strongly. Each type of model is commonly used, and each has its strengths and weaknesses.

vary as sharply from group to group. Regardless of their positions in the periodic table, for example, most transition metals can form cations with a 2+ charge. Thus predic- tions based simply on group number are not as reliable here as they are for the repre- sentative elements. When considering transition metals and their compounds, we must rely more heavily on knowledge of the specifi c chemistry of each element.

Organic Chemistry

All organic compounds feature carbon skeletons, similar to those we have already seen in our introduction to polymers. Other elements frequently found in organic compounds include hydrogen, oxygen, and nitrogen. Despite this rather short list of elements, more than 18 million organic compounds exist. This vast number of com- pounds arises from some unusual aspects of the chemistry of carbon itself. Most im- portantly, carbon atoms readily attach to one another to form chains, and these chains can grow quite long. Many of the polymer molecules we have been discussing in this chapter contain thousands of carbon atoms. Furthermore, some of these long chains are straight, whereas others are branched at one or more places. And fi nally, any pair of carbon atoms can bond to one another in three different ways, by sharing either one, two, or three pairs of electrons. When taken all together, these factors allow car- bon to form a vast array of compounds.

The diversity of organic compounds presents some challenges. It is not uncom- mon for several different organic compounds to have the same molecular formula, for example, but to display different properties depending on exactly how the atoms are arranged into molecules. (Different compounds with the same molecular formula are called isomers.) So organic chemists frequently must depict molecules not only by their formulas, but in some way that also conveys important information about the arrange- ment of the atoms. This could be done using structural formulas of the sort we saw in Figure 2.15. But because organic chemistry often deals with very large compounds and complex structures, that option is somewhat unwieldy. A shorthand notation known as a line structure has emerged as the most common method for describing organic compounds simply and unambiguously. The line structure is a modifi ed version of the structural formula. As in any structural formula, lines are used to depict bonds between atoms. But in a line drawing, many of the elemental symbols are omitted. By defi nition, an organic compound is based on carbon atoms. So to reduce clutter in a line drawing, the ‘C’ symbols for carbon atoms are not written. Furthermore, because organic com- pounds almost always contain many hydrogen atoms, the ‘H’ symbol for any hydrogen atom that is attached directly to a carbon atom is also not written. Symbols are written for any elements other than carbon and hydrogen, as well as for any hydrogen atoms that are not directly attached to carbon. We can illustrate the relationship between a structural formula and a line drawing with an example.

E X A M P L E P RO B L E M 2 . 3

Poly(methyl methacrylate) is widely known as Plexiglas^®. The structural formula for the monomer, methyl methacrylate, is shown below. Write the corresponding line structure for methyl methacrylate.

C C H HH

O

C C

H H

H O

H C H

Strategy We convert the structure into a line drawing by removing the symbols for all carbon atoms and for hydrogen atoms attached directly to carbons. Bonds to or between

© Cengage Learning/Charles D. Winters

Iron(II) chloride, FeCl₂ Greenish-yellow color

Density 3.16 g cm^-3 Melts at 670°C Iron(III) chloride, FeCl₃ (Here forming as the solid at

the bottom of the test tube) Orange-brown color

Density 2.90 g cm^-3 Melts at 306°C

Figure 2.16 ❚ Transition metals typically form more than one type of cation, giving them a very diverse chemistry. Iron, for example, forms cations with both 2+ and 3+ charges, and this allows it to form two different ionic compounds with chlorine. FeCl2 and FeCl3

have different appearances and properties.

© Cengage Learning/ Charles D. Winters

carbon atoms remain, so carbon atom positions become either intersections between lines or ends of lines. Bonds between carbon and hydrogen atoms are omitted.

Solution First, we will remove the symbols and bonds for all of the hydrogen atoms because they are all bound directly to carbon.

C C

O C C O

C

Next we remove the symbols for the carbon atoms, leaving intact the lines that depict the remaining bonds. This gives us the fi nal line structure.

O O

Discussion The line structure is much more compact than the original structural formula. An experienced chemist quickly recognizes where the atoms whose symbols are not shown need to be.

Check Your Understanding The structural formula for styrene, which is the monomer for the common plastic polystyrene, is shown below. Convert this to a line drawing.

C

C C

H H

H

C

C C

H H H C

H

C H

We will use these line structures throughout the rest of this textbook, and you may also encounter them in other places, such as the information sheets that accom- pany prescription drugs. In many instances, it will be necessary to interpret the line drawing to determine the molecular formula, so we should develop a way to do that systematically. In addition to the rules we used before to transform a structural dia- gram into a line structure, we will need to introduce two important generalizations about chemical bonding.

1. A hydrogen atom in an organic molecule can form only one covalent bond to one other atom.

2. Every carbon atom in an organic molecule always forms exactly four covalent bonds.

In combination, these two facts allow us to fill in all of the carbon and hydrogen atoms that are not explicitly written in a line structure. First, we place a carbon any- where that we see either an intersection between lines or the end of a line. Then we add hydrogen atoms as needed to bring each carbon’s number of bonds up to four.

E X A M P L E P RO B L E M 2 . 4

A temperature resistant plastic called poly(phenylene oxide) is a key component of resins such as GE’s Noryl^®, which is widely used in computer cases and automobile dashboards. The line structure below represents 2,6-dimethylphenol, which is the monomer from which poly(phenylene oxide) is made. What is the molecular formula for 2,6-dimethylphenol?

OH

Strategy First, we can pencil in carbon atoms at the appropriate positions. Then we will add hydrogen atoms as needed. Once all of the atoms have been identifi ed, it will be easy to count them to produce the needed formula.

Solution We place a carbon atom at the end of a line or the intersection between lines.

C

C C

C C

C

C C

OH

Next we count the number of bonds shown for each carbon. If that number is three or less, we add as many hydrogen atoms as needed to bring it up to four.

H₃C C CH₃

C H HC CH

C C

OH

Now all of the atoms are shown explicitly. Counting, we arrive at the molecular for- mula as C₈H₁₀O.

Discussion The double lines in the ring in this structure represent double bonds, in which two pairs of electrons are shared between two atoms. Notice that in locating the carbon atoms, we treated the double lines the same as we did the single lines: each intersection represents a carbon atom, no matter how many lines meet.

Check Your Understanding Once, poly(vinylpyrrolidone) was used in the man- ufacture of hairsprays, and it is still used in the glue that holds the layers together in plywood. The line structure for the vinylpyrrolidone monomer is shown below. Find the corresponding molecular formula.

N O

Functional Groups

Given the vast number of organic compounds, the need for some systematic way to understand their chemistry should be apparent. One of the most important con- cepts for an organic chemist is the idea that certain arrangements of atoms tend to display similar chemical properties whenever they appear together. Such an ar- rangement of atoms is called a functional group. One of the simplest functional groups, and one that is central to many polymerization reactions, is a pair of carbon atoms joined by a double bond. If the double bond is converted to a single bond, then each carbon atom can form a new bond to another atom. Thus, the charac- teristic reaction of a carbon-carbon double bond is addition, in which new atoms or groups of atoms are attached to a molecule. Line structures make it very easy to identify any C"C groups in a molecule, and thus to locate positions at which addi- tion reactions might be feasible.

The simplest organic compounds are hydrocarbons, molecules that contain only carbon and hydrogen atoms. We can imagine the formation of more complicated molecules by replacing one or more of a hydrocarbon’s hydrogen atoms with a func- tional group. Compounds in which a hydrogen atom is replaced by an !OH func- tional group, for example, are collectively referred to as alcohols. The presence of the

!OH group conveys certain properties to this class of molecules, including the abil- ity to mix with water to a much greater extent than the corresponding hydrocarbons.

Often, the notion of functional groups infl uences the way in which we choose to write chemical formulas. If the chemical formula for an alcohol is written so that the !OH group is emphasized, then it will be easier to recognize that this group is present.

So the formula for the simplest alcohol, methanol, is most often written as CH₃OH rather than as CH₄O. Similarly, ethanol is generally written as C₂H₅OH rather than C₂H₆O. Other common functional groups are listed in Table 2.3.

The role of addition reactions in producing polymers will be examined in Section 2.8.

The role of addition reactions in producing polymers will be examined in Section 2.8.

Not all organic compounds that contain !OH groups are alcohols.

Carboxylic acids contain a !COOH functional group, for example.

Not all organic compounds that contain !OH groups are alcohols.

Carboxylic acids contain a !COOH functional group, for example.

Table

❚

^2.3

Some common functional groups

Functional Group Class of Compounds Example

C C Alkenes Ethylene

!C#C! Alkynes Acetylene

!X (X = F, Cl, Br, I) Organic halides Methyl chloride

!OH Alcohols, phenols Ethanol, phenol

C!O!C Ethers Diethyl ether

N ^Amines Methylamine

C O

OH

Carboxylic acids Acetic acid

N C O

Amides Acetanilide

(continues)