I have blue litmus paper, and if I put it in an acid it turns red. I also have red litmus paper, and if I put it in a base, it turns blue. I know the question about litmus was asked and answered and I like the answer: How does the litmus pH indicator work? However, damp litmus paper also becomes bleached in presence of chlorine gas. I believe it has nothing to do with the mechanism described in aforementioned question. I wonder what the mechanism is. Wikipedia actually suggests:

For instance, chlorine gas turns blue litmus paper white – the litmus dye is bleached, because of presence of hypochlorite ions. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.

I also found an equation which describes formation of hypochloric acid (source): $$\ce{Cl2 (g) + H2O (l) -> HClO (aq) + HCl (aq)}$$

The problem that I have now is as follows: We know that litmus paper gets discolored because of contact with hypochloric acid. But what is actually the chemical reaction for that?

  • $\begingroup$ I'm not sure about this (plus it doesn't give sufficient details) so I am not posting this as an answer: Litmus is a mixture of dyes extracted from lichens, and most of these dyes/indicators have colour due to a extended pi conjugated system. The acid-base environment alters the pi conjugated system and causes a shift in absorbance, causing colour change. The HOCl is the bleach in the reaction, and it probably functions by destroying the extended pi conjugated system. $\endgroup$ – getafix Mar 23 '16 at 4:41
  • $\begingroup$ Azolitmin is known to be one of the major components of litmus: azolitmin I have no access to SciFinder at the moment, but I do recall hydantoin imides to be exquisitely sensitive to oxidizing agents, and I don't see azolitmin being an exception. $\endgroup$ – user28178 Mar 23 '16 at 5:53
  • $\begingroup$ @tovok Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. I have converted your answer into a comment for now. With a bit more rep, you will be able to post comments on any question/answer. $\endgroup$ – Martin - マーチン Mar 23 '16 at 8:20
  • $\begingroup$ I just came across a mention of the easiness with which the double bond is disrupted by halogens, in the 10th grade textbook by Rudzitis (Russian). Nice. $\endgroup$ – CowperKettle Mar 24 '16 at 9:48

I can supply some details now, and hopefully this ought to qualify as an answer.

As I mentioned earlier litmus is a mixture of 10-12 dyes (CAS number: 1393-92-6).

The acid-base indicator properties of litmus are primarily due to 7-hydroxyphenoxazone chromophore (pictured below) enter image description here

The answer you linked to discusses the acid-base indication mechanism in some detail, so I shall skip over that.

Anyway, what this serves to establishing that it is indeed a extended $\pi$-conjugated system that we are dealing with in the chromophore.

Now, $\ce{HOCl}$ would bring about halohydrination (basically an electrophilic addition) across the $\pi$ bonds, thus disrupting the conjugated system.

Halohydrins are compounds that contain an $\ce{-OH}$ and $\ce{-X}$ groups on adjacent carbons.

This image describes the general mechanistic scheme in a simpler case: enter image description here

Anyway, after said addition reaction has taken place in the chromophore, the $\pi$-conjugated system is disrupted, and thus, we see an absence of colour.

  • $\begingroup$ Thank you, getafix! Did I understand you correct, that there is no reaction with HOCl, but instead there is a reaction with Cl2, followed by reaction with H2O? $\endgroup$ – Sleepy Hollow Mar 23 '16 at 12:15
  • $\begingroup$ Well, the chlorine+water is furnished by the HOCl (so to speak). The reaction of chlorine you put down with water in your question is reversible. $\endgroup$ – getafix Mar 23 '16 at 12:26
  • 3
    $\begingroup$ @SleepyHollow - the reaction is probably with the hypochloride ion (ClO minus), since HClO dissociates in water. Here's a nice description of electrophilic addition, showing how the breakdown of the pi-bond occurs and what follows that. $\endgroup$ – CowperKettle Mar 24 '16 at 9:51

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