Acyl chlorides

In this new post, I’m discussing Acyl Chlorides.

You’ll probably have come across them as Ethanoyl Chloride or Benzoyl Chloride

What you might not know however is that molecules like these are major constituents of tear gas. 

Protestor catches Tear Gas canister in Tahir Square, Cairo

Exploding tear gas canister on the fly!!

CS and CN gas (or Mace) contain the following molecules:

CS gas

Tear gas was first used when the French fired canisters into German trenches at the start of WW1

Tear gas and acyl chlorides are examples of molecules called lachrimators

Molecules in tear gas and molecules like them causes burning of the eyes and skin, tears, and a sensation of suffocation.

Their use is aimed at driving people from their cover and disrupting their protest.

The gas also provokes fear and insecurity playing on the human survival instinct. 

It has been claimed that it is easier for men to face bullets than invisible debilitating gas.

You can read more about tear gas here.

Preparation of acyl chlorides

Acyl chlorides can be prepared from the corresponding carboxylic acid using a suitable chlorinating agent e.g. PCl₅ phosphorus pentachloride or SOCl₂ thionyl chloride

Or

The formation of this intermediate then enables the formation of many different carboxylic derivatives in very high yields and atom economies. 

So for example:

Ethanoyl chloride and ammonia produce acetamide (ethanamide) a quick reaction at room temperature

Ethanoyl chloride and phenylamine produce N-phenylethanamide, an anti-coagulant.

Ethanoyl chloride and ethanol (or any carboxylic acid and any alcohol) form esters at high yield.

Ethanoyl chloride is easily hydrolysed using water to form the corresponding carboxylic acid.

Summary of carboxylic acid derivative chemistry

“Unsaturated carbon in a suitable environment undergoes substitution  via a two step process  consisting of addition followed by elimination.” ROC Norman

Substitution at a carbonyl group involves a nucleophile. 

Carboxylic acid derivatives where the carbonyl group is attached and adjacent to an electronegative atom are substituted via an addition–elimination mechanism.

Among the common carboxylic derivatives the order of reactivity is acid chloride > acid anhydride > ester > amide.

Acid chlorides react rapidly with water, anhydrides slowly and esters and amides not at all.

Here is a typical nucleophilic substitution mechanism:

Amides

I wonder if we know just how common amides are? 

Let me ask you: do you like cooked chicken, roast beef or lamb, grilled fish or are you having a barbeque somewhere in the world as I write this?

Then you are eating amides in the form of protein. 

The human body proteins are essentially polyamides like keratin in hair, collagen in muscle, haemoglobin in blood.

You and I could be described as bundles of protein, bundles of amides.

So a study of amides bears paying some attention, do you not think?

I’m going to describe essential structure and formation of amides and discuss some more relevant and contentious examples. 

Structure and Formation

An amide is a carboxylic acid derivative formed from the reaction between an acid chloride and ammonia.

This is a nucleophilic addition-elimination reaction.

Here is the mechanism:

The amide group is –CONH₂

The structure can be written: 

Here is ethanamide:

Amides also form condensation polymers.

The following polymers are polyamides:

Nylon, Kevlar, all proteins, but don’t confuse nylon with the Nylon publication:

Nylon

Synthesis of a typical nylon:

Nylon 6:10 is called 6:10 because the two monomers have 6 and 10 carbon atoms respectively. 

One monomer is decandioyl dichloride, the other 1,6–diaminohexane.

 

Notice the peptide bond (–CONH–) in the polymer repeat unit.

This example reaction forms the chemistry of what is sometimes called the Nylon Rope Trick that you can see here on You Tube.

You need to listen carefully to the spiel because the professor makes a couple of mistakes.

The diamine is in the lower aqueous solution put in the flask first and the solution he adds to the flask is the acid dichloride dissolved in hexane. 

He has it the other way round, wrongly. 

If you want to repeat this experiment for yourself you can find it here on the RSC site

Nylon has great strength.

The polymer chains can hydrogen bond to each effectively through the peptide linkages. 

The long alkyl groups also bond quite well through van der Waals forces. 

There is a fine myth about Nylon as to how it got its name. 

You can read about that myth here on the Timeless Myths website.

I won’t spoil the party for you. 

There are no spoilers here!!

Kevlar

Let’s look now at Kevlar

Here is the formation of Kevlar:

Kevlar is a constituent of bulletproof vests.

You can see that the polymer chains have many phenyl rings.

These rings bond the chains strongly together through the use of many van der Waals forces. 

The peptide linkages also bond strongly through Hydrogen bonding like this:

The result is a very tough impenetrable material. 

Can you work out what the monomers of Kevlar would have to be?

Kevlar uses

Proteins

All proteins are polyamides but their monomers are not diamines and acid dichlorides.

Amino acids are the protein monomers.

And interestingly there are some amino acids that are essential amino acids that we must take in our diet because they are not made in our bodies.

They are histidine, isoleucione, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

You can read more about essential and non–essential aminoacids here

Here’s how amino acids polymerise:

The convention is to draw them starting with the amino group on the left. 

Note also the peptide bond that unites both amino acids.

The condensation reaction eliminates a small molecule of water (not shown) from between the two amino acids.

A string of amino acid residues linked to one another using peptide bonds is the primary structure of a protein. 

Amides as drugs

N-phenylethanamide is an anti–coagulant.

It prevents the clotting and coagulation of red blood cells. 

Its formation involves reaction between phenylamine and ethanoyl chloride or ethanoic anhydride.

Substituted versions of this molecule have been used in the treatment of arthritis.

Aspartame

Aspartame is an artificial sweetener or sucrose substitute.

It is a dipeptide and here is its structure:

This internet clip shows the peptide bond highlighted in a green box.

 

This clip shows that the molecule is a dipeptide formed from two amino acids aspartic acid and phenylalanine.

It also shows that it is the methyl ester of phenylalanine.

So far all rather innocuous and conventional.

OK let's use it as an artificial sugar substitute.  That's what Splenda is formed from:

But……

You see this on the internet and this:

 

It's just that “the lady doth protest too much methinks” to quote Shakespeare’s Queen Gertrude in Hamlet. 

Are we really to believe that all these conditions are solely attributable to aspartame. 

What about the dangers of sucrose?

But of course, in today’s suspicious world everyone is thought to have a hidden agenda and cannot therefore be trusted. 

And what is there that we eat that does not cause us issues?