Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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dichloromethane : CH2Cl2
iodoform : CHI3
chlorine : Cl2
Class:
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elem=C,Cl,I
organic
Summary:
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Iodoform reacts exothermically with chlorine, one of the reaction products
being iodine. The other products must be substitution products, with iodine
completely or partly replaced by chlorine.
When excess chlorine is used, then iodine monochloride and iodine trichloride
are formed as well. These products react with iodoform as well, resulting in
formation of iodine.
Description:
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Prepare some very dry chlorine gas. In this experiment, chlorine is prepared by
pouring 10% hydrochloric acid on calcium hypochlorite and this chlorine is
dried over phosphorus pentoxide. The chlorine next is sucked into a large 60 ml
syringe to which a small PVC tube is connected.
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Experiment 1:
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Put some dry iodoform in a dry test tube and push a small whiff of chlorine
from the syringe over the iodoform: Immediately the yellow solid becomes dark
brown.
Pour some dichloromethane over the dark brown material: All of the material
dissolves (with some difficulty). The solution becomes beautifully purple.
Experiment 2:
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Put a small quantity of iodoform in a dry test tube and carefully and slowly
pass chlorine from the syringe over the iodoform: Initially, the yellow solid
becomes dark brown. When more chlorine is passed over the solid, then it
liquefies and also becomes quite warm. A dark brown liquid is formed. When even
more chlorine is passed over the dark liquid, then a yellow solid is formed,
especially at places where the dark liquid forms a thin layer on the glass of
the test tube.
Pour some dichloromethane on the dark blob of liquid and the yellow material
around it: The blob of dark brown liquid dissolves in the dichloromethane, but
with some difficulty. Quite some shaking is needed before all of it has
dissolved. The resulting solution is a clear brown liquid with a yellowish hue.
Experiment 3:
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Add an excess amount of iodoform to the final result of experiment 2: The
iodoform dissolves and while the iodoform dissolves, the liquid at first
becomes much darker brown, but from a certain point in time, the color shifts
from brown to red and when even more iodoform dissolves, the liquid becomes
purple.
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Explanation of results:
- Iodoform reacts with dry chlorine gas. When excess iodoform is present, then
all chlorine atoms are connected to a carbon atom and free iodine is formed.
Iodine atoms in the CHI3 molecule are replaced by chlorine atoms.
- When excess chlorine is used, then the excess chlorine reacts with the iodine
to form iodine monochloride (a brown liquid) and when even more chlorine is
used, then yellow iodine trichloride is formed (this has nearly the same color
as iodoform). The brown liquid, formed in this reaction is a mix of ICl and
(most likely) CHCl3.
- When dichloromethane is added, then the blob of dark liquid only dissolves
with difficulty. The organic part of this liquid (the CHCl3) quickly is
extracted into the dichloromethane and the remaining ICl also dissolves, but
much more slowly.
- When excess iodoform is added to the brown solution, then the chlorine in the
ICl and ICl3 replaces iodine atoms in CHI3, resulting in formation of free I2
and formation of CHI2Cl, CHICl2 and CHCl3.
In the experiments, no fuming in contact with humid air could be detected at
all. This is a strong indication that no hydrogen is replaced from the CHCl3
molecule. If hydrogen were replaced, then either HI or HCl would be formed and
these definitely would result in fuming and a frosty and humid appearance of
the rim of the test tube.