Addition of bromine to ethyne
We put bromine water in a cylinder filled with acetylene gas and close it again. We make sure that only a little dilute bromine water is added. Then we shake well.
Only a small amount of bromine is added. This is surprising, since a high level of bromine consumption is to be expected due to the triple bond. The low tendency towards addition is due to the fact that the carbon atoms in acetylene are more electronegative than in ethene and therefore do not like to give up their electrons to electrophilic substances like the bromine cation Br+, which initiates the attack. One can also say that this is due to the energetically particularly favorable charge distribution of the electrons of the triple bond. Because they are arranged in a rotationally symmetrical way.
In contrast, the reaction between ethyne and chlorine is more spectacular.
Experiment: reaction between ethyne and chlorine
Put a few pieces of calcium carbide (F) and about the same amount of potassium permanganate (O) in a beaker (50 ml) and pour 10 ml of concentrated hydrochloric acid (C) over it.
From potassium permanganate and hydrochloric acid, chlorine is formed in radical form, which reacts with the ethyne that is formed at the same time.
KMnO4 + 8 HCl —> KCl + MnCl2 + 5/2 Cl2 + 4 H2O
The escaping gas mixture ignites itself again and again with the formation of sooty flames.
Acetylene also reacts with soda-alkaline potassium permanganate solution (Baeyer sample).
Experiment: Baeyer sample with ethyne
Soda-alkaline potassium permanganate solution (w(KMnO4) = 0.1%) is placed in a cylinder filled with acetylene. In contrast to the addition of bromine, the mixture reacts surprisingly quickly.
This one is in fact a real redox reaction. In fact, acetylene is a good reducing agent. Its related oxidation equation is
C22- —> 2 C + 2 e–
Silver ions should also act as oxidizing agents. In addition to metallic silver and carbon, a silver-carbon compound, silver carbide or better: silver acetylide Ag2C2 is also formed. The name suggests that this is a salt.