Warning: Dichromates are carcinogenic.
Sulfuric acid is corrosive.
Wear gloves when handling them.
Greetings fellow nerds.
In a previous video i made various tertiary alcohols by the grignard reaction.
Now i purified them with a combination of acid washings, distillations, dean stark dryings and fractional distillations.
But if you go too fast or don't pay close enough attention to the temperature, you can still end up with impure products.
So to get some idea of the impurities i'm going to do some very basic reactivity testing with my alcohols.
Unfortunately, i kinda failed.
Oh well, we still get pretty colors so might as well post a video and learn something.
The first is the dichromate test and to do it we'll need some dichromate test solution.
Start with 100mg of potassium dichromate.
You can also use ammonium or sodium dichromate, it doesn't matter as long as it has dichromate ions.
Then add in 10mL of water.
Now add in 1mL of concentrated sulfuric acid and shake until everything is dissolved.
You can use more sulfuric acid if you desire increased speed.
While amateurs mostly use low grade drain cleaner acid.
It helps a lot in this case to use the highest quality sulfuric acid you can get.
When doing any sort of analytical work you should use high purity test solutions,
so the impurities in the solutions don't interfere with whatever you are testing.
And there is our test solution.
So let's use it.
Now i've set up a whole array of organic and one inorganic compound to test by adding about five drops to each vial.
Now in each vial i'll add half a milliliter of the dichromate test solution.
Now dichromate is a strong oxidizer, especially when it's acidic as high acidity raises the redox potential.
When it reacts with a reducing agent the normally hexavalent chromium atoms become trivalent chromium atoms.
In water trivalent chromium is green in color so the color change from orange to green gives us an indication that a redox reaction is happening.
So how does this help us determine the purity of a compound?
If the compound itself doesn't react then the dichromate test should remain orange.
But if it changes color then that means the compound is impure or it might not even be the desired compound at all.
Likewise if the compound normally does react but the test comes up negative,
then you definitely know you don't have the right compound.
Oh by the way i'm putting the sodium metabisulfite back over here because it was releasing sulfur dioxide gas.
I wanted it to air out a bit so the gas doesn't waft over and interfere with the vials beside it.
Anyway, you can already see it reacting with the dichromate and changing color.
Sodium metabisulfite is a well-known reducing agent in chemistry and as you can see it works very quickly even at cold temperatures.
It also gives you a sense of the degree of color change you can expect.
Now i doubt you'll be testing very many inorganic reducing agents
but as you can see the dichromate test is not limited just to organic ones.
Okay, i've added in the test solution to all the samples.
Now all we do is wait.
As you can see our primary alcohols of hexanol and methanol give a positive test, as well as our secondary alcohol of isopropanol.
The primary alcohols react to give carboxylic acids while the secondary alcohols react to give ketones.
Tertiary alcohols like the ones we made in our previous video on grignards can't form these products so they should resist oxidation.
They should remain unreacted and this forms the basis of our test.
Now I'm timelapsing this over the course of two hours but you don't need to wait that long.
The reaction is greatly accelerated by heating and you can confirm results in just a few minutes.
Alternatively, using a larger amount of acid also helps.
Now because the rate and degree of change can be variable depending on acid content and temperature,
i recommend simultaneously running the test with a few reference samples of known purity and reactivity like i am here.
This way you can compare them and know if your actual test sample should be reacting or not over the time scale you are observing.
Now this test is cheap and easy to perform without any specialize equipment but its usefulness is very limited by itself.
All it does is tell you if something can be oxidized by acidic dichromate.
But it does not tell you very much about what that substance actually is.
As we can see, ketones, esters, tertiary alcohols and mineral oil all give negative results.
While primary and secondary alcohols as well as sodium metabisulfite all give positive results.
This test alone can't tell the difference within those groups.
Nonetheless this test is a good start and still useful when you know exactly what you're looking for.
In this case we're looking for reactive impurities in our tertiary alcohols.
And interestingly enough as we wait, we're seeing our 2-methyl-2-octanol getting a stronger color change than the other tertiary alcohols.
It looks like it's slightly impure or else it wouldn't do that.
I probably went too fast during my distillation and overlapped with other distillates like hexanol.
Alternatively there maybe an azeotrope with hexanol i don't know about.
As we wait even longer, we can see our ester, ethyl propionate also starting to change color.
Under these acidic conditions the ester molecule is being hydrolyzed into its component carboxylic acid and alcohol.
The alcohol in turn gives a positive result upon oxidation by the dichromate.
Now if we wait a very long time, a few hours at least, almost everything reacts.
As said before the tertiary alcohols will resist oxidation, but they are not immune.
Under acidic conditions they can isomerize into oxidizable compounds.
This underscores the need to have a reference reaction going so you can compare reactivity to a known substance and see if they're the same.
Now dichromate testing alone is simple so i wanted to try another reaction.
A very simple one i found was the komarowsky reaction.
I have here the same panel of reagents containing 5 drops each and also adding in about 20mg of vanillin.
We then add in about 10 drops or so of concentrated sulfuric acid and let it sit.
The benzylic aldehyde of the vanillin reacts with the samples to produce strongly colored organic products.
Exactly what they are is different for each one and even the literature isn't entirely clear on the subject.
Unfortunately, as a technique to figure out what we have, this is a failure.
Everything is different and there doesn't appear to be much consistency between functional groups.
For example methanol, a primary alcohol is green, but hexanol, also a primary alcohol is red.
Maybe the issue is size, as isopropanol is also red.
But if that were true then how come t-amyl alcohol is purple while 7-hexyl-7-tridecanol is red? I don't know.
Even stranger is that mineral oil, which is normally inert to most reactions, is turning red, i have no idea why.
I was hoping the kamarowski reaction would be more straightforward
but as a means of identifying an unknown compound, this is not very good.
It's not totally useless though.
You could run the test with an unknown sample and again with a known reference sample and see if they match.
If they do then that gives you a little more evidence your unknown sample is the same as the reference sample.
But if you have no suspicion what your unknown sample is, then all this does is give you pretty colors.
Oh well, at this point i've done enough testing.
I should go to my actual objective of trying to make alkali metals.
Hopefully i'll have something to report soon.
Thanks for watching.
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