Chemistry 105
The Chemistry of Color

Course Lecture Topic Information

Chem 105 Web Site
Dr. Kimberly Lawler-Sagarin
Elmhurst College

Discussion 6:
What makes blue jeans blue?

This discussion will take place April 24 - April 30

Assignments for the discussion board

  • Q1: Based on your student number, answer the following question:

    • students 1,6,11: Describe the geometrical structure (shape) of methane (CH4) and consider what this means in terms of the shapes and structures of other alkanes with more carbons.

    • students 2,7,12: The text describes the geometrical structure of formaldehyde, which contains a carbon-oxygen double bond. Describe the structure or discuss how you think these structural concepts would apply to molecules containing carbon-carbon double bonds, like ethene (C2H4).

    • students 3,8: Describe how the properties of alkanes change as the number of carbons increases (see text, section 16.6).

    • students 4,9,13: Answer one of the following questions about resonance: What is resonance? What kinds of molecules exhibit resonance? What are the sulfur-oxygen bonds in SO2 like (see page 186).

    • students 5,10: describe the concept of geometrical isomers and why isomers are important.

    • students 14,15,16,17: choose an aromatic hydrocarbon from those given on page 248 in the text. Find out what it is used for, how it is formed (if natural sources exist), and what properties the compound has (chemical or physical properties, health effects).

    Answers to Q1 should be about one paragraph in length.

  • Q2: Search the web and/or print sources for information on indigo or related dyes. Share some of this information with the class (about one to two paragraphs).
    • Students 1,6,11,16: indigo in history
    • Students 2,7,12,17: the synthesis of indigo
    • Students 3,8,13: indigo from the woad plant
    • Students 4,9,14: current and historical uses of indigo
    • Students 5,10,15 tyrian purple: a cousin of indigo

  • Q3: Question/Comment/Answer. For this question, you must comment on the discussion for Question 1 or 2 (Q1,Q2). Once again, there are several ways to address this. You may choose to ask a student about one of their postings. You can answer a question from another student. Or, you may comment on another student's posting by adding additional information. The question, comment or answer should be thoughtful and well researched (about one paragraph).


Text Readings
  • Chapter 6: Section 6.3: Multiple Covalent Bonds and Resonance
  • Chapter 16: Section 16.1, 16.2, 16.4, 16.6: Introduction to Organic Chemistry - alkanes, alkenes, alkynes and aromatic compounds. (You may skip the rules for naming such compounds - focus on the chemical structures).

Topic Background

Indigo

Indigo is the name of the dye used to make traditional blue jeans blue.

 

 

     The Structure of Indigo

Indigo is an organic chemical. Though the term organic is used to refer to agricultural products grown under certain conditions, it means something different to chemists. In the field of chemistry, an organic chemical is simply a chemical based on carbon. Originally, it was believed that such compounds could only be obtained from living things, hence the term organic. Today, we know that is not the case, but the name organic is still used to describe this diverse class of compounds.

Our discussion of indigo brings us into the study of organic chemistry. Many of the common dyes used today and in antiquity are organic compounds. Over the next few weeks, we will be exploring the field of organic chemistry in more depth.

So, why is indigo blue? The reason lies partially in the double carbon-carbon bonds in the indigo structure. Many dyes and other brightly colored organic molecules contain such double bonds in their structures. To understand why, we need to explore the nature of these double bonds in organic molecules in more detail. This week, we will start with some of the basics of organic chemistry.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with carbon-based compounds.

Carbon is an element with great versatility. With its four valence electrons, a carbon atom wants to make four bonds with other atoms to form an octet. It also has the capacity to form stable rings and long chains with itself. These two factors combined lead to a vast array of molecules based on a carbon backbone.

The simplest class of organic chemicals is the alkanes. Alkanes are a type of hydrocarbon (molecules containing only carbon and hydrogen). Alkanes are molecules which have only single carbon-carbon bonds. Some examples appear in the table below.

The First Six Straight Chain Alkanes
Formula Name Structure
CH4 methane
    H
    |
H - C - H
    |
    H
C2H6 ethane
    H   H
    |   |
H - C - C - H
    |   |
    H   H
C3H8 propane
    H   H   H
    |   |   |
H - C - C - C - H
    |   |   |
    H   H   H
C4H10 butane
    H   H   H   H
    |   |   |   |
H - C - C - C - C - H
    |   |   |   |
    H   H   H   H
C5H12 pentane
    H   H   H   H   H
    |   |   |   |   |
H - C - C - C - C - C - H
    |   |   |   |   |
    H   H   H   H   H
C6H14 hexane
    H   H   H   H   H   H
    |   |   |   |   |   |
H - C - C - C - C - C - C - H
    |   |   |   |   |   |
    H   H   H   H   H   H

The table above contains only a few of the straight-chain alkanes. We could continue adding carbons to the chain to form more alkanes. Such as heptane, with 7 carbons, octane with 8, and nonane, with 9. Some polymers will have chains hundreds of thousands carbons long.

Note that in the molecules above, all the carbons are connected in one straight chain. However, alkanes can adopt many other structures as well. For example, carbon chains can branch. In this case, one or more carbons may be bonded to 3 or even 4 other carbon atoms.

Isomers refer to molecules with the same chemical formula, but different structures. For example, C4H10 has two possible isomers, one straight-chain, one branched:


 
    H   H   H   H             H   H   H 
    |   |   |   |             |   |   | 
H - C - C - C - C - H     H - C - C - C - H
    |   |   |   |             |   |   | 
    H   H   H   H             H   C   H 
                                 /|\
                                H H H 

      n-butane                iso-butane

These are considered different compounds, because they have different chemical and physical properties. It is also important to note that it is possible for the molecule to rotate, and to twist around single carbon-carbon bonds. So, the structure:


        H H H 
         \|/ 
      H   C   H 
      |   |   | 
  H - C - C - C - H
      |   |   | 
      H   H   H 

      iso-butane

Is equivalent to the iso-butane structure above, rather than being a new molecule.

Larger molecules have even more possible isomers. Hexane, C6H14, for example, has five possible isomers:


 
    H   H   H   H   H   H             H   H   H   H   H 
    |   |   |   |   |   |             |   |   |   |   | 
H - C - C - C - C - C - C - H     H - C - C - C - C - C - H
    |   |   |   |   |   |             |   |   |   |   |  
    H   H   H   H   H   H             H   C   H   H   H 
                                         /|\
                                        H H H 
    
 
                                        H H H               H H H            
                                         \|/                 \|/             
    H   H   H   H   H             H   H   C   H           H   C   H   H      
    |   |   |   |   |             |   |   |   |           |   |   |   |      
H - C - C - C - C - C - H     H - C - C - C - C - H   H - C - C - C - C - H  
    |   |   |   |   |             |   |   |   |           |   |   |   |      
    H   H   C   H   H             H   C   H   H           H   C   H   H      
           /|\                       /|\                     /|\             
          H H H                     H H H                   H H H            

As you can see, a vast number of different compounds are possible, even when we limit ourselves to hydrocarbons with only carbon-carbon single bonds. With the addition of elements besides carbon and hydrogen, and other types of bonding, hundreds of thousands, even millions, of different compounds can be formed.

Alkenes are a class of organic molecules that contain carbon carbon double bonds. Some simple alkenes are found in the table below:

Some Simple Alkenes
Formula Name Structure
C2H4 ethene
(also called ethylene)
  H       H
   \     /
    C = C 
   /     \
  H       H
C3H6 propene
  H       H
   \     /
    C = C
   /     \
  H       C - H
         / \
        H   H
C4H6 butadiene
  H       H
   \     /
    C = C       H
   /     \     /
  H       C = C
         /     \
        H       H

Alkenes are the simplest organic molecules that contain double bonds. Double carbon-carbon bonds are stronger and shorter than single carbon-carbon bonds. Chemically, molecules containing double bonds are more reactive. Of special note to us is that molecules with double bonds absorb lower energy (longer wavelength) light than similar molecules without double bonds.

For example, the molecule pentane:

    H   H   H   H   H
    |   |   |   |   |
H - C - C - C - C - C - H
    |   |   |   |   |
    H   H   H   H   H

Has no double bonds. The longest wavelength of light it absorbs is about 170 nm. On the other hand, consider 1-pentene, with one double bond:

    H   H   H   H     H
    |   |   |   |    /
H - C - C - C - C = C 
    |   |   |        \ 
    H   H   H         H

absorbs wavelengths as long as 184 nm. Jumping to a five-carbon molecule with 3 double bonds:


  H       H   H       H
   \     /     \     /
    C = C       C = C      
   /     \     /     \     
  H       C = C       C - H
         /     \     /     
        H       H   H      

results in absorption of light in the 250 nm range. More complex molecules, with elaborate double bond networks, such as the indigo structure:

 

 

     Indigo

Results in the absorption of light in the visible wavelength range. Absorption of visible light can lead to bright colors, as we saw in lab meeting three. We will come back to these ideas in the coming weeks.

The History of Indigo

The dye indigo has been used for thousands of years. There is evidence that it was used in Thebes possibly as early as 3000 BCE. Indigo has been found in the cloth on Egyptian mummies dated at 2400 BCE, and was used widely in India around 2000 BCE.

The dye we currently know as indigo was historically obtained from the indigo plant, as well as other plants such as woad. The deep blue dye was obtained from the plant in a process known as fermentation. The fermentation produces a colorless substance which only turns blue on exposure to air.

Woad was widely distributed throughout Europe and Asia. Thus, indigo was primarily obtained from the woad plant in Northern Europe. At this time, woad was considered to be a different dye from indigo, even though we now know that the molecule we call indigo is responsible for the blue color in both cases. Unfortunately, the dye obtained from the woad plant was not as deep as that obtained from the indigo plant. In the 17th century, imports of indigo from India competed with the local woad industry in Europe. As indigo was the superior colorant, it quickly surpassed woad. Some European markets survived by entering the indigo trade themselves. In the 19th century, Britain developed the indigo industry in India and eventually dominated the market.

In 1880, the German chemist Adolf von Baeyer prepared indigo synthetically. Synthetic indigo is chemically identical to naturally derived indigo - the only difference is the preparation. von Baeyer's synthesis was too costly to be used commercially, however. It wasn't until 20 years later that the German chemical company BASF was able to develop a cost effective synthesis of indigo. Once they did, they rapidly took over a large portion of the indigo market.

Early Theories of Organic Dyes

The first synthetic dye, Mauveine, was produced by W. H. Perkin in 1856. What followed was a rapidly growing synthetic dye industry. Of course, there was great interest in synthesizing new dyes, but there was little theoretical background to guide chemists on how to go about obtaining desirable colors.

O.N. Witt developed an empirical theory of dye color in 1876. He proposed that each dye contained a color producing chromatogen. This chromatogen consisted of a chromophore and auxochromes. Chromophores were "color bearing groups" and auxochomes were attached to the chromophores and acted as "color increasers".

Witt's theory of color production eventually gave way to new theories of color production because his theory broke down in many cases. For example, it was unable to explain why the purported chromophore in indigo (the center C=C bond and its attached carbons and nitrogens) did not give rise to bright colors in other molecules.

Though Witt's theory later gave way to other more comprehensive theories, today we still refer to certain arrangements of atoms as chromophores. The arrangements of atoms often result in the molecule absorbing longer wavelength light - either in the visible region, or near ultraviolet region of the electromagnetic spectrum.

Many common chromophores contain carbon-carbon double bonds in alternating or conjugated arrangements. For example, the following molecule contains such alternating double bonds:


  H       H   H       H
   \     /     \     /
    C = C       C = C       H
   /     \     /     \     /
  H       C = C       C = C
         /     \     /     \
        H       H   H       H

Organic molecules absorb light in the ultraviolet region of the electromagnetic spectrum. However, organic molecules with conjugated double bonds tend to absorb light at relatively long wavelengths. In some cases, the wavelengths fall in the visible region, resulting in highly colored species.

An Introduction to Resonance

Many molecules containing conjugated double bonds also undergo resonance, another factor important in describing the bonding in such molecules.

Resonance is the concept that arises when two or more Lewis structures are able to be drawn for the same molecule. An example of this is the molecule ozone, which has two possible Lewis structures:


    ..  ..  ..             ..   ..  ..
  : O - O = O :     and   : O = O - O :
    ..                              ..


Because there should be no difference between the outer two oxygen atoms, we can't really conclude that one structure is better than the other. Thus, the true structure is believed to lie somewhere in between the two possible Lewis structures, sort of like an average. This is backed up by experimental evidence - each oxygen-oxygen bond is stronger than a typical oxygen-oxygen single bond, yet weaker than a typical oxygen-oxygen double bond.

Another type of molecule that exhibit resonance is benzene, discussed in section 16.6 of the textbook. Two possible resonance structures of benzene are shown below.

These two are equivalent. However, the true structure lies somewhere in between, with each of double bonds being shared. Benzene's six carbon-carbon bonds are equal in length, and they are shorter than typical single bonds, but longer that typical double bonds. The stick figure at the bottom of the figure is a shorthand notation for a benzene ring. Note that indigo has two benzene rings in its structure.

As we shall see in the next few weeks, resonance and conjugation are two factors which contribute to the bright colors of many organic molecules.

References

  • Nassau, K. The Physics and Chemistry of Color: The Fifteen Causes of Color; 2nd ed.; Wiley: New York, 2001, pp 113-117, 131-133.
  • Christie, R. M.; Colour Chemistry; Royal Society of Chemistry: Cambridge 2001, pp 74-77.
  • Ball, P. Bright Earth: Art and the Invention of Color; University of Chicago Press: Chicago, IL, 2001, pp 200-203.

Chemistry 105 Home
Blackboard Home
Elmhurst Chemistry Home
Elmhurst College Home

 K. Lawler-Sagarin, 2004