By James Ashenhurst

Conjugation And Color (+ How Bleach Works)

Last updated: October 31st, 2022 |

Conjugation And Color

Why are tomatoes red? Why are carrots orange? Why are egg yolks yellow?   And… why is Vulcan blood green?

spock has green blood vulcan blood what could cause the color just kidding

OK, I’m not going to touch that last one, but as for the first three: great question.

Table of Contents

  1. These Highly Colored Molecules Have Highly Conjugated Pi Bonds
  2. Removing Pi Bonds Changes The Color (Or Removes It Entirely)
  3. How Bleach Works:  By Destroying Pi  Bonds
  4. So How Exactly Is Conjugation Related To Color?

There are actually 15 chemical causes of color, but today we’ll begin to explore the reason for the characteristic colors of tomatoes, egg yolks, carrots, and many other pigments from nature, such as the green color of leaves and the red color of blood (heme). And yes, there is a spectroscopy angle here – but that’s going to wait until the next post.

Before getting into the “why”, I always like to look at the “what”. Specifically, here are some examples of highly coloured molecules from everyday life. Do you notice something that all of these molecules have in common?

representative pigments from nature lycopene tomatoes lutein yellow egg yolks carotene orange carros each are highly conjugated molecules

Yes, they have lots of double bonds.  But having lots of double bonds is not sufficient for a molecule to be strongly colored.

For instance, natural rubber latex can have hundreds or thousands of pi bonds, and yet it is milky white [Note 1]:

natural rubber latex has hundreds or thousands of pi bonds per molecule is a white color because double bonds not conjugated

1. These Highly Colored Molecules Have Many Conjugated Pi Bonds

There’s something special about the way the pi bonds are arranged in lycopene, lutein, and b-carotene, as opposed to natural rubber latex: the pi bonds are conjugated. 

What does that mean? [See: Are these alkenes conjugated?].

Quickie review on “conjugated” versus “non-conjugated” alkenes:

conjugated versus non conjugated dienes comparing 1 3 hexadiene and 1 4 hexadiene separated by one sp3 hybridized carbon not conjugated

  • In 1,3-hexadiene (above) note that there are two adjacent double bonds. That means that there are four consecutive sp2 hybridized carbons whose p-orbitals can line up to form an extended “pi system”. [We’ll explain why this is important in the next post]
  • In 1,4 hexadiene, note that there is an sp3 hybridized CH2 (“methylene”) carbon separating the two double bonds. The CH2 does not have an available p orbital to overlap with either of the adjacent pi bonds, and thus these two pi bonds are said to be “isolated” (or non-conjugated if you prefer).

Let’s revisit lycopene, lutein, and β-carotene. They each have long systems of conjugated pi bonds.

  • Lycopene and β-carotene each have 11 conjugated pi bonds (lycopene also has 2 isolated pi bonds)
  • Lutein has 10 conjugated pi bonds (with an isolated pi bond).

lycopene red has 11 conjugated pi bonds carotene orange 11 conjugated pi bonds lutein yellow 10 conjugated pi bonds

This is also true of chlorophyll and heme, which are more complex examples but the same principles apply.

structure of chlorophyll structure of heme each highly conjugated molecules with distinctive colors

2. Removing Double Bonds Affects The Color

Let’s start with a hypothesis: color is due to the presence of an extended series of conjugated double bonds.

How could we test this idea?

One way would be to perform an experiment that removed the π bonds while leaving the rest of the molecule intact.

We’ve seen multiple examples of these reactions in our section on alkenes. A great candidate is catalytic hydrogenation, which breaks C-C π bonds and forms adjacent C-H bonds by using hydrogen gas (H2) in the presence of a metal catalyst (such as palladium on carbon, Pd/C).

Indeed, when one subjects red lycopene (C40H56) to exhaustive catalytic hydrogenation, one obtains perhhydrolycopene (aka “lycopane”) with formula C40H82 – a colourless oil. 

Bottom line: removing the conjugated π system removes the source of color!

pi bonds of lkylcopene responsible for its color if hydrogenated to give lycopane it is completely colorless

3. Bleach Works By Destroying Pi Bonds

Does this seem too abstract? Do we need a real life application?

Look no further:


Damn that lycopene! How can we get those nasty stains out of our clothes using our new-found chemistry knowledge?

Here’s an idea I’m giving away for free:

Since you now know that lycopene is responsible for the red color of ketchup, and catalytic hydrogenation removes the color, you could make a home device for catalytic hydrogenation of shirts at high pressure and get rid of the stain. Since every household could use one, and there are about 100 million households in the USA, you could sell each unit  for several hundred dollars apiece, and before you know it, you’ll be buying a top hat and a monocle.


Or… you could just use this.

picture of chlorox bleach for removing color

That’s right: Bleach removes the color of grass, ketchup, blood, carrots, and a lot of other common food and vegetable stains by reacting with the π bonds responsible for the color of these molecules. 

Bleach (sodium hypochlorite, NaOCl) reacts with alkenes in a similar way to a reagent we’ve seen before, Cl2 in H2O. When you examine the structure of NaOCl, notice that Cl is attached to the more electronegative atom O. That means that chlorine bears a partial positive charge – it’s electrophilic. Thus, NaOCl will react with nucleophiles like alkenes in a similar way to Cl2 or Br2, forming a bridging intermediate through the three membered ring pathway The 3-membered ring bridge is then attacked at the most substituted carbon by the nucleophilic solvent (water in this case).

how does bleach work naocl sodium hypochlorite reacts with double bonds electrophille with pi bonds

Let’s apply this specifically to lycopene (and by extension other molecules).

Bleach works by knocking out the pi bonds responsible for the red color of lycopene:

how does bleach work removes common stains by reactin gwith conjugated pi bonds remove pi bonds remove color electrophile

I should point out that it isn’t necessary for bleach to hit every pi bond. Knocking out just a few in that sequence of 11 conjugated pi bonds is enough to remove the red color.

So there you go, folks. Now you know that bleach doesn’t actually clean anything. It just modifies the molecules so that they aren’t coloured anymore. : – )

4. Conjugation And Color: How Exactly Are They Related?

So far, we’ve explained nothing truly fundamental about the source of color. All we’ve done is show a bunch of pretty pictures, a bad GIF, and explained the workings of a household chemical. So let’s get down to business and start answering the “Why”.

Let’s start with a few obvious things:

  • Substances that do not absorb visible light, such as water,  will appear colourless; or, if finely dispersed, white due to scattering of light (e.g. clouds).
  • Substances that absorb at all frequencies of visible light will appear black.
  • Next, and pardon me if this is obvious to you,  the pigment molecules we’ve been talking about don’t emit light.  [Luciferin from fireflies does, under certain conditions, but that’s chemiluminescence and we’re not talking about that here].
  • What we perceive as color is the light that is reflected from these pigment molecules.

We see ripe tomatoes as red because white light is reflected back to our eyes as red light. So some portion of the visible spectrum is being absorbed by lycopene: we see the light it doesn’t absorb.

So if we see something as red, how can we figure out what wavelengths of light are being absorbed?

For simple cases, it’s been known for hundreds of years that when light of a certain color is absorbed, the complimentary color is observed. A common tool for determining this is a color wheel, which places complimentary colors on opposite sides.  Here’s one made by the German poet (and amateur scientist) Johann Wolfgang von Goethe.

color wheel made by johann von goethe showing complimentary colors

  • From the color wheel we determine that the complimentary colour of red is green.  So a good first guess is that lycopene in tomatoes is absorbing somewhere in the green part of the visible spectrum.
  • Similarly, pigments that appear yellow tend to absorb in the indigo area of the visible spectrum.
  • Pigments that appear orange tend to absorb in the blue area of the visible spectrum…. you get the idea.

So what do all those conjugated pi bonds have to do with lycopene absorbing green light?

Excellent question. Now we’re getting into some deep stuff. This is a great topic!  But it will have to wait until the next post for a full treatment.

Next post: UV-Visible Spectroscopy


Note 1. Commenter John points out:

The latex rubber example is a little bit of apples and oranges. The white color is common for any latex/emulsion and it arise from the physical structure of the dispersion – the emulsion particles scatter all light. A better comparison would be a dried latex, but they are typically light yellow due to impurities.


Comment section

10 thoughts on “Conjugation And Color (+ How Bleach Works)

  1. To answer your comment I would like to remmember you that in electrophic addition in alkenes the slow step of mechanism is the addition of electrophile. In the bleach, HOCl decompose to OH- and Cl+. bcose of this the base (OH-) do not attack the poliene before the addition of electrophile C+
    Please feel free to be in touched to clarified your doubts

  2. Hi! Your post is very interesting and I think I can use the info here for my art project. I’m planning to make flowers out of leaves. The goal is to remove all the colors from the leaves but not get a skeleton-like effect. The leaves will be all-white and ready for dyeing. I’m thinking of of using clorox and hydrogen peroxide since they are readily available, unlike other chemicals which would need special govt permission for me to buy here in my country. Are clorox and peroxide enough? I would greatly appreciate your help because this project is very important to me since it can potentially generate livelihood for the less fortunate people in my place. Thank you!

    1. I don’t know that you’d end up with white leaves. You’d certainly eliminate the colors from chlorophyll and the carotenoids, but those aren’t the only pigments in leaves. The most likely event to my mind is that you’d end up with brown leaves.

  3. > But it will have to wait until the next post for a full treatment.

    no link to next / prev posts

    tiny font in the diagram texts; cannot read

  4. I’m still confused about bleach. Here, we are saying that the chlorine is electrophilic, which makes perfect sense…and generally, we say that bleach is a strong oxidant, again suggesting that it will be accepting electrons.
    But we also generally say that bleach is a strong base. And by that definition, it should be donating electrons…this is my confusion. How can it be a base and an oxidizing agent at the same time, when the definitions are seemingly opposite?

    1. Hi Christine – in bleach, the two sides of the molecule have different behaviours. The electrophilic and oxidizing part is the Cl. Bleach can act as a base (although this is less likely) through the hydroxide functionality, i.e. the terminal O(-).

  5. The latex rubber example is a little bit of apples and oranges. The white color is common for any latex/emulsion and it arise from the physical structure of the dispersion – the emulsion particles scatter all light. A better comparison would be a dried latex, but they are typically light yellow due to impurities.

    But this got me thinking: what would happen if you dispersed any of the conjugated molecules from above in an emulsion. Would the whiteness from the scattering dominate the absorption of the conjugation? I bet it would, but I really don’t know.

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