Today we're going to do another experiment with our favorite
aromatic aldehyde, vanillin. As an aldehyde, vanillin is fairly reactive and
can easily be oxidized to the corresponding carboxylic acid, but it can
also be reduced by some reducing agents to the corresponding alcohol, vanilla
alcohol. We want to do a few experiments with the product, so we're going to make a
reasonable quantity. We're starting here with 25 grams of crystalline vanillin
and 99% pure. As you can see from the structure, vanillin is a phenol
derivative, and although itself isn't soluble in water, it can form a sodium
salt which is. So this will be our first step. We've got a large beaker set up
with 200 mils of water in it. As you will see later on, we recommend using as large
a beaker as you can get. We've got a nice big magnetic stirred bar in here.
As well, so the first thing we'll do is get this on a stirrer and start stirring.
To form our vanillin sodium salt, we're going to use sodium hydroxide. So here's
7.3 grams, which is a 1.1 molar ratio to the vanillin, leaving a little bit
excess. So let's dissolve this in the water.
Warming a little bit as it dissolves, but not too much.
So now let's add the 25 grams of vanillin powder to our sodium hydroxide solution.
You can see an immediate yellow color forming, and the vanillin starts to dissolve.
Okay so our vanillin is now all converted into the yellow colored water soluble sodium salt.
Chill this down to fridge temperature now.
Time for our reducing agent.
Here we've weighed out 7 grams of sodium borohydride.
It's a crystalline white solid as you can see.
This isn't very OTC and so we had to borrow it for this experiment.
These are the lengths we go to to bring you videos on Kemplier.
One note about this stuff.
In alkaline solutions is slightly unstable, but stable enough to do a reaction.
In acid conditions though it reacts very violently and releases huge quantities of hydrogen gas.
Let's start adding it to our vanillin sodium salt solution.
As you can see, it's just dissolving gently.
Because we used a slight excess of sodium hydroxide, it should be fairly stable.
We add slowly over a period of about 20 minutes.
Keep the mixture chilled.
The borohydride will give off a little bit of hydrogen gas, but it shouldn't be too much.
With a slow addition the temperature shouldn't rise too fast.
We moderate the temperature just by placing an ice cube into the mixture as it's stirring.
Let's have a look at how this reaction works.
Sodium borohydride contains a very interesting anion, BH4-, otherwise known as the tetrahydrodorate anion.
And this is a source of hydride anions.
You'd normally think of hydrogen as a proton, a positively charged hydrogen atom.
But a hydride anion is a negatively charged hydrogen atom.
And it's a strong reducing agent.
But more importantly in this case, because it's negatively charged it can act as a nucleophile.
So the carboxyl group of the aldehyde is attacked by this hydride anion, forming an internidia, as you can see here.
And this then picks up a proton from a water molecule, forming our product, the corresponding alcohol.
It's very super cool and neat.
The problem is that borohydride is impossible to make yourself.
And it's expensive and hard to get.
So if you ever spy a bottle of it, you'll need to create a distraction.
Now for an important tip.
The key to this reaction succeeding is to keep the reaction mixture cool whilst very slowly adding the borohydride.
The easiest way is to simply add the borohydride.
Simply start with a chilled solution of aniline and sodium hydroxide.
And then occasionally add an ice cube to the reaction mixture.
So long as you can always see a little bit of ice present.
And just add another ice cube when the previous one disappears.
Then you should be okay.
Okay, we managed to add all the borohydride in 20 minutes.
And keep the mixture chilled with ice cubes.
The final volume is about 350 mils.
The mixture isn't really bubbling much.
Just a very small amount of hydrogen gas being given off.
This is good.
Too many bubbles means that our reducing agent is decomposing.
We leave this stirring now for an extra 30 minutes.
And allow the ice to melt and the mixture to warm very slightly.
And here we are 30 minutes later.
A few more bubbles coming off now.
We're now going to cover this up and place back in the fridge for an hour.
So it chilled right down again.
This is important for the neutralization step.
So now we're going to add hydrochloric acid to neutralize the sodium hydroxide.
Decompose any remaining borohydride.
And convert the sodium salt into our product.
Here's 40 mils of concentrated hydrochloric acid.
We probably needed about 30 to 35 in practice.
Let's get this diluted down and chilled down with a bath.
Add a bit of ice.
About 100 mils in volume.
And we'll now place this in the fridge as well.
To chill down for an hour alongside the reaction mixture.
Okay the reaction mixture has nicely chilled down.
And so is our diluted hydrochloric acid.
We're going to have to perform the neutralization very slowly now.
As excess borohydride species still present in the mixture.
Turn into hydrogen gas.
So no rush.
Just keep slowly adding the acid dropwise.
About a quarter to addition now.
So let's check the pH.
Still very strongly alkali.
So we've got a long way to go yet.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
off the mixture don't be tempted to add the acid more quickly or you will screw
it up let's just add an ice cube to keep the mixture cool pretty soon we've got
the white frothy precipitate starting to form in the mixture this is our product
vanilla alcohol use a stirring rod to keep this under control quick pH check
still strongly alkaline so keep going slowly that was a close one the bubbles
are a pain in the ass
so you can see why we recommend using a very large beaker for this very nearly
doing this in a Nile red hydrazine style but not quite
still strongly alkaline
okay we're at pH 7 now you know this step of precipitate is now better
distributed and there are less bubbles adding acid but this stage no longer
generates foam
strongly acidic now so we're done go down too much acid to the mixture just
no more toxic sanitizer
enough to get to a low pH. Now we're going to chill this down again in the
fridge for a few hours in order to make sure that the product has crystallized
and precipitated out completely.
And here we are. Lots of fluffy white precipitate. Let's filter this off.
Wash the beaker out with 50 mils of ice-cold water. Wash the crystal carefully
with a few portions of ice-cold water.
And then dry for an extended period on the pump.
Here's the dry product. 23.9 grams of vanilla alcohol. And the yield is a
fantastic 94% based on starting vanillin.
We could recrystallize this from warm ethyl acetate if we want to, but for the
moment we're going to use it in follow-up reactions as it is.
Interestingly there's no aroma of vanillin at all in the product,
maybe the tiniest trace in the background but hardly anything.
So this process works really well. Remember to keep the reaction mixture
and the neutralization stage chilled down. If you do it to warm, then on
neutralizing you get a milky suspension on cooling, and a strange
glue the oil separating out. We screwed it up first time we tried and we're not
totally sure what this is but it's probably a quinone compound. A bit like the one she was using.
shown here, or perhaps an addition compound. But it's not vanilla alcohol.
And given that sodium borohydride is hard to get, it's a bit of a waste of a valuable
reagent. Speaking of which, let's waste some.
We know you want to see what happens when pure borohydride reacts with 50% sulfuric
acid. So here goes.
It's vigorous but a bit disappointing.
Okay let's do something a bit more hardcore.
68% nitric acid. This is a strong oxidizing agent so it should be a bit more fun.
Let's rock the house.
Let's see.
A bit better.
Still not spectacular.
But we wouldn't want to put it on our corner.
flakes. Thanks for watching and stay tuned for more reactions.