Today's experiment is simple but it's an interesting one we're going to try to

perform a reduction of trichloroacetic acid trichloroacetic acid is otherwise

known as TCA and it's present in some skin products and skin peel treatments

it's the product of completely chlorinating acetic acid using chlorine

gas so here's our two starting reagents first up we've measured out 25 grams of

trichloroacetic acid crystals take care with this as the pure acid is a strong

acid and can burn your skin in addition it rapidly absorbs moisture from the

atmosphere so don't leave in the air for long normally a strong acid like this

can react with a suitably reactive metal and form hydrogen gas you've probably

seen a reaction between zinc or magnesium and hydrochloric acid before

where hydrogen is produced so bear this equation in mind because what's going to

happen is quite different in this case

here we've got 11 grams of zinc metal powder we've crushed it up very finely so

that there are no lumps at all in here this is approximately a 1.1 molar ratio

to the acid we're using so the zinc metal is going to act as a reducing

agent but instead of forming hydrogen gas something interesting happens first

of all let's just play with this and see how the reaction occurs in an undiluted

state

amount of trichloroacetic acid in a glass dish you can see it's starting to

absorb moisture rapidly so here's a little powdered zinc pretty rapid and

exothermic reaction white clouds of vapor is trichloroacetic acid so unless

we want to make the world's most evil smoke bomb ever invented we'd better

look at doing this in a dilute solution so let's set something up here's a 250

mil beaker and we've set this up on a magnetic stirrer and placed a stir bar

in so we can stir the contents well we'll start off with 50 mils of cold

water

we'll add the trichloroacetic acid and get it into solution

it dissolves rapidly it's extremely water-soluble we know that the reaction

is highly exothermic so we've got some ice cubes standing by as well in order

to help tame it a bit will add two to start off with so now for the next step

we'll add this very slowly

interestingly the ice now melts rapidly and the zinc seems to largely dissolve

in the mixture so we slowly continue addition

you can see a few specks of zinc at the bottom but it reacts rapidly and

dissolves if we add without stirring you can hear the reaction take place and

there are a few bubbles of possibly hydrogen gas evolved so we slowly

continue the addition

you can see there's no effervescence no gas-produced

about halfway through and the mixture is starting to get warm as you can see we

don't want the mixture to be very soft because we have already added a few

further with the dichloroacetic acid so we're going to add a bit more ice and

continue addition we searched and we can't find a mechanism for this but it's

possible that the zinc metal is chemically adsorbing the hydrogen as

it's generated by reaction with the acid this hydrogen then acts to reduce the

group of the acid under harsher conditions excess zinc will apparently

reduce further to chloroacetic acid and even to acetic acid if you perform the

reaction at a hot temperature you can start to detect the acetic acid aroma

total addition took 20 minutes and we needed another two ice cubes making six

in total

so here we are with all the zink in we left this stirring for another 20

minutes to ensure that the zinc reacted completely

ok so here we are 20 minutes later there's still a small amount of

unreacted solid in there as you can see it's still slightly warm

so we'll cover this up and chill it down so that it's nice and cool ready for

the next step

here we go so first of all let's do a quick test if our reaction did proceed

as expected the organic acids present in here should now exist as the soluble

zinc salts so let's test the pH not strongly acidic anymore but still

slightly acid as we'd expect from a very strong acid coupled with a weak base

to convert the zinc salt back to the organic acid we're going to add a

strong acid which will shift the equilibrium so here's 10 mils of 50% by

volume sulfuric acid let's add this to the chilled mixture

this rapidly dissolves any outstanding cloudiness and solids in the

mixture hopefully now our organic acids are much more likely to be in their

acidic form let's just test the pH very strongly acidic so let's now have a go

and see if we can extract some products out of this mixture chloroacetic acids

are very soluble in water but they are also soluble in diethyl ether so we'll

use some AHA yeah I'm not sure if that's true or not so let's see how we can do this now and see if we can extract some products out of this mixture chloroacetic acids are very soluble in water but they are also soluble in diet a leader so we'll use some

of this to see if we can extract the organic components out of the mixture

here's 40 mils of cold diethyl ether it forms a separate top layer so let's get

this into a separating funnel and give the mixture a good shake for a few

minutes so that there's a good chance for any organic acids to pass into the

ether layer we'll then let the layers separate again

we'll drain off the bottom aqueous layer

okay here we go a top ether layer hopefully containing something

we'll dry this using some anhydrous magnesium sulfate and after about 10

minutes it should be dry so we'll decant off the ether

we'll warm this gently now and evaporate off the ether to see what we're left

with be very careful if you evaporate ether because the vapors can be explosive

in air make sure you've got good ventilation and no flames or anything

that could create a spark anywhere near

there's a little bit of ether left and it's quite tricky to get it all

evaporated as it seems to stick in the mixture

so we'll keep on heating and place a thermometer in the flask so we can measure the temperature.

Soon we're up to 90 degrees C and the ether vapors have completely disappeared.

We'll keep on heating now and see if we can drive off any water that might be present.

And now we'll keep on heating in order to see what the boiling point of the remaining liquid is.

There's definitely some vapor coming off above about 130 degrees which condenses in the flask.

And at 170 degrees C we can see the mixture starting to boil.

Here's what we're left with on cooling.

13.5 grams of slightly pale yellow slightly oily liquid.

We seal it up quickly so it doesn't absorb moisture.

So the big question, is this unreacted trichloroacetic acid, or is it dichloroacetic ACD?

First we chilled it in the fridge down to about 4 degrees C.

And as you can see it remained liquid with no visible change in viscosity.

Here's the melting and boiling points of the various possible reaction products.

Trichloroacetic acid should definitely crystallize by now, if it was relatively pure.

Chloroacetic acid should as well, but according to some references it can be very hard to crystallize, even when chilling down further.

We also noticed a little bit of hydrogen chloride vapor coming off the mixture after it was chilled for a while.

This is interesting because the only source of that chloride was from the starting material.

So this is some evidence but it doesn't confirm the identity of the product.

We looked and we can't find any conclusive test for dichloroacetic acid.

We can only say that this is a trichloroacetic acid which distinguishes it from the trichoro or monochloro acids.

At least with the reagents we have.

We could do a titration of an exact weight of it using a standard base solution,

because there is a big enough molecular weight difference between the acids.

However, we don't have a burette so our accuracy won't be good enough.

So let's chill down further.

Two days later in the freezer at minus 15 degrees C.

There is actually a slight viscosity.

But not much of one.

Still very liquid.

So let's do one more test.

How does this liquid product react with zinc metal?

Dichloroacetic acid is supposed to be a lot less reactive than the trichloro acid in terms of reduction.

So here's some finely powdered zinc just like we used in the reaction.

Run away quick little ant.

It's not quite as vigorous as the pure trichloroacetic acid,

but still a very rapid and exothermic reaction.

So our theory is this.

The product we've got is probably a mixture of some unreacted trichloroacetic acid,

a fair bit of dichloroacetic acid,

and probably also some monochloro acid as well.

Hence the difficulty in crystallizing.

The aroma is slightly different to the starting material.

And has a slight fruitiness to it as well.

Perhaps we've inadvertently made some esters somehow if our ether contained a bit of ethanol?

So this is a bit of an inconclusive reaction.

It probably does create a fair bit of dichloroacetic acid,

but the reaction products are very hard to separate in a pure form.

Just a word of caution.

Be very careful with this pure liquid product as it is extremely damaging to your skin.

We did a tiny test and you can feel it.

Burning badly within 2 seconds.

Don't try this unless your name is Niall.

In fact even if it is, please don't.

We like you.

If you can think of a test we can use on this, then please let us know.

Otherwise, stay tuned for more reactions.

Thanks for watching.