Description of experiment
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Needed compounds:
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ligroin : C7H16
hydrogen bromide : HBr
1-hexene : C6H12
sulphuric acid : H2SO4
potassium nitrite : KNO2
Class:
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elem=C,N
redox
Summary:
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Nitrogen oxides do not dissolve in ligroin. Nitrosyl bromide does. When a
compound with unstaturated C=C bonds is added, then the nitrosyl bromide
does not quickly decompose. It does not react with alkenes and the presence
of alkenes does not cause it to decompose quickly.
Description:
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Dissolve some potassium nitrite in water and pour a layer of ligroin on top
of this solution. A two layer system is formed. Next, add some 2 M sulphuric
acid: The liquid in the bottom layer becomes pale blue. This is an indication
of formation of HNO2 and subsequent formation of N2O3. The pale blue liquid
slowly decomposes, giving a brown gas mix. This is a mix of NO and NO2. The
ligroin layer turns pale yellow/brown, the bottom layer remains pale blue.
Above the ligroin layer there is a clearly visible brown gas mix.
Shake the liquids and allow a two layer system to form again: The bottom
layer is pale blue, the top layer still only is pale yellow/brown. Most of
the brown color of the gas mix above the liquids is gone. It apparently
reacted with the aqueous layer.
Add a little amount of 40% hydrobromic acid: The bottom layer becomes very
dark brown, nearly black. The top layer becomes dark red/brown.
Shake the two layers of liquid and allow two layers to form again: The
bottom layer becomes brown/red, much lighter than it was before shaking.
The top layer becomes nearly black. The dark compound is transferred to
the ligroin layer.
Add some 1-hexene to the liquids: The 1-hexene mixes with the top layer.
The size of the top layer increases, it remains very dark brown, even when
the liquid is shaken. The dark compound is not added quickly to the
unsaturated alkene. If the dark compound were bromine, then the liquid
quickly would have become colorless.
After several minutes, the top layer has become somewhat lighter. This
may be due to hydrolysis in contact with the aqueous layer, it may also
be that there is a slow reaction between alkenes and nitrosyl bromide.