We'll be preparing acetic anhydride the

classic way using acetyl chloride and sodium acetate the single most important

thing in this reaction is that your sodium acetate must be anhydrous and

completely dry we've baked ours in an oven at 200 degrees C for four hours in

total occasionally stirring the powder the textbook process is to actually melt

the solid prior to performing the reaction however we wanted to see if we

could get a reasonable yield through just oven drying weigh out 80 grams of

anhydrous sodium acetate and place this into a dry 500 ml flask we've added a

thermometer to track the temperature now stopper the flask and place it into an

ice bath and get the contents as cool as you can measure out 60 grams of chilled

acetyl chloride

about 55 mils of liquid as you can see you'll need ventilation here now slowly

add a small amount of the chilled acetyl chloride to the sodium acetate in the

ice bath

give the mixture a stir and allow the acetyl chloride to absorb and react this

produces quite a lot of heat so go slowly and don't allow the temperature

to rise above about 60 degrees C otherwise you'll risk losing acetyl

chloride it's easy to overshoot we added a little too much and quickly hit

70 degrees so go slow and allow the mixture to cool between additions once

the acetyl chloride is completely added swirl the flask and you'll notice some

more heat generated the contents of the flask turns into a liquid slurry now set

up for simple distillation

liquid so a thermometer in the still head isn't necessary we've used a

calcium chloride drying tube to protect the distillate the first sign of liquid

boiling is around 110 degrees C since our sodium acetate is unlikely to be

100% dry there will almost certainly be some acetic acid present in our mixture

pretty soon a strong distillation starts and a solvent front creeps up the

adapter tube it's always very satisfying to watch this distillation is pretty

rapid to begin with and slowly tails off but keeps going for quite a while as

distillation proceeds the slurry in the flask turns more and more solid

quite a lot of liquid distal off so don't stop the heat too early

it took a good hour and a half before the distillation died down to a very low

level in the contents of the flask turned the yellow burnt color

here's our final distillate it has a slight yellow color as you can see

definitely contains a city can hydrate but also smaller impurities such as

acetic acid and maybe some ketone

as well. To purify our crude product, we'll fractionally distill it. We're using a 40

centimeters Vigro Cologne as you can see here. We've chosen to fractionally distill because

acetic acid and acetic anhydride have boiling points which are around 30 degrees apart.

You could possibly use a simple distillation on a strong heat source such as an oil bath,

but you'd need to go very slowly and carefully. Soon after heating strongly the liquid in the

flask boils and we get another nice solvent front climbing up the column. We used aluminum foil to

insulate the column to make the distillation more efficient. When the vapor reached the top of the

column, we initially got a temperature of about 100 degrees C and slowly cooled.

At this point we got a slow distillation of a small amount of initial product. This had a faint

yellow color and an acetic acid aroma and ultimately we discarded this. Once the temperature

rose in the still head to above 130 degrees C, we switched the receiving flask and then began to

collect the distillate as our main product. Use ventilation because of the high temperature

and we noticed that even the tiniest amount of anhydride vapor in the air is quite irritating

to the eyes even if you can't smell it. Note that we performed the initial reaction twice and then

combined the products for this distillation so the volumes you can see here are double.

Complete distillation took around two and a half hours in total and the boiling point

was very consistent throughout.

At the end the boiling flask contained some tar-like residue.

Here's our final product, 130 grams of pure colorless acetic anhydride. Given that we

used doubled the first reaction, this corresponds to an average yield on our first reaction

of 83% based on acetyl chloride used. The product doesn't smell strongly of acetic acid.

In fact, you initially think it has no aroma at all and then suddenly it hits you, a very

pungent and irritating aroma. Another common question is how it mixes and reacts with water.

Let's add a little bit and find out. You can see at the bottom of the beaker that there is a

separate bubble of acetic anhydride with a small bubble of air trapped on top. This is cold water

and the reaction is slow. We can see something happening, but it's a slow process.

Stay tuned for some interesting experiments which make use of our acetic anhydride.

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