Posted by: Mark Foreman | January 28, 2012

Esters and mechanisms

Dear Reader,

I want to discuss with you carboxylic acids and their derivatives, now before we get started I want to point out that carboxylic acids appear in some many places in both everyday and not so everyday life.

Acetic acid is in food, it is also used for making the polymer used for emulsion paint (poly vinyl acetate).

Formic acid is in ants

Oxalic acid in rhubarb

Aspirin in headache pills

1-Carboxy-2,2-dimethyl-3-(2-methylprop-1-enyl) cyclopropane is a key part of the pyrethrin insecticides, while things like 1-carboxy-2,2-dimethyl-3-(2,2-dichloroethenyl) cyclopropane is used in some man made versions of this key class of pesticide. The pyrethrins are used in dog shampoo.

Adipic acid is used to make nylon-6,6

Terephthalic acid is used to make Kevlar which is a super strong nylon used for bullet resistant clothing and strong ropes.

I hope that you can now see that carboxylic acids appear in a range of places, now on to some of the reactivity. In recent times vegetable oil has been used as a feed stock for motor fuel, rather than trying to run diesel engines directly on oil it is normal to convert the oils (lipids) into methyl esters. The normal way to do this is to use methanol and a little sodium methoxide to convert one ester into another.

This is a simple reaction; the methoxide anion attacks the carbonyl carbon to form a tetrahedral intermediate which then decomposes to form the methyl ester. Below is the mechanism of the reaction.

Conversion of a typical lipid (glycerol ester) into a methyl ester

We should draw the arrows onto the mechanism, lets get to meet our best friend: the curved arrow. Now be careful of the curved arrow there are some ways in which it can be misused. Lets use a simple reaction of a ketone being protonated as one to explain the use of the arrow.

Now for goodness sake do not use the curved arrow to indicate the movement of atoms, the following is wrong.

This is very wrong, please do not ever use a curved arrow to move an atom !

To be frank with you I do not want to see this misuse of the curved arrow again. What you must do is to draw the movement of electrons, the electrons make up the bonds (bonding pairs) and the lone pairs so the curved arrow will move the bonds. By altering the bonds the atoms of the molecules will then move or change in some ways. The following curved arrow is better.

A better arrow, but not quite right yet

The arrow is not quite right, it is showing the electrons going to the atom and not to the bond which they are going to form. It is best to draw the curved arrow forming a bond rather than going to the proton. So below is the perfect curved arrow.

Much better

The only problem is that it can be less clear as to which atom the curved arrow is forming a bond to. One solution which I like (I have no idea is it is strictly correct is to show a dashed bond where the new bond will be).

Perfect and clear

Now back to the formation of FAME (Fatty Acid Methyl Ester) biodiesel, the first pair of arrows form the tetrahedral intermediate and then the second two form the methyl ester. Here is the mechanism with the curved arrows.

Now with the curved arrows

It is important to bear in mind that the attack of the methoxide is reversible, the tetrahedral intermediate can expel the methoxide but this takes us back to the start. The reformed methoxide and the glycerol ester of the fatty acid can then reencounter each other and have a second attempt at reacting.

About FAME, FAME biodiesel is an exciting way in which we can get motor fuel without the need for oil from oil wells, FAME biodiesel is made by a transesterification of vegetable oil with methanol.

While FAME is a great fuel it is not a perfect fuel, if you want to read about some of its shortcomings then see this article. The ene reaction with singlet oxygen and the free radical chemistry is not some thing which will be in the exam, but the paper does explain how biodiesel can go bad in storage.

A disadvantage of the normal methoxide route is that if used cooking oil with plenty of free carboxylic acids is used then the methoxide catalyst is used up and plenty of sodium carboxylate (soap) builds up in the reaction mixture. This is because the carboxylate anion does not react with methanol to form the ester.

One alternative is to use an acid catalyst instead of the methoxide (basic catalyst). Any strong mineral acid will normally do OK. For example you could use sulfuric acid to make the ester. Below is the mechanism for the conversion of one ester into another by alcohol with an acid catalyst.

If we try to use used cooking oil to make our FAME biodiesel in the acid catalysed method then the reaction will work OK. What will happen is that the free carboxylic acid (FFA, Free Fatty Acids) will react with the methanol according to the following mechanism. As before the first step is the protonation of the carboxylic acid which is then followed by attack of the methanol to form the methyl ester. Here is the mechanism of what happens.

Carboxylic acid being converted to a methyl ester

You should note that the mechanism looks very similar to that of the conversion of one ester to another, I think that the two mechanisms share the same inner core. It is important to note that as a byproduct of these biodiesel forming processes we will get glycerol (propane-1,2,3-triol). I will tell you more about the glycerol chemistry when we get to the alkene chemistry. But that will come later.

If any of you are thinking of making your own biodiesel and putting it in your car then there is something to watch out for, if you fail to remove the catalyst (typical method is distillation) then you run a grave risk of killing your pride and joy. If you get sodium methoxide or sulphuric acid into the fuel tank of a truck, bus or car then you tend to corrode the metal parts which the fuel touches. It is well known in Northern Ireland that some outlawed organisations make money by selling illegal road fuel. Some of the people washing red diesel fail to get every last trace of the acid out of the fuel, thus the fuel then can damage the cars / trucks or whatever you put it into. So my advice is if you are offered cheap fuel by some one at a make shift illegal petrol station then say “no thankyou” and drive away to a legal petrol station. You might pay more for the tank of fuel at the legal station but you are less likely to have to pay your motor mechanic for a big repair bill.

But back to carboxylic acids and their derivatives, we have used the chemistry to run our school bus, taxi or even our car but there is more to carboxylic acid chemistry than just FAME diesel. Now we need to consider the acid halides, I would like at this point to explain to you how most of the world’s acetic acid is made from methanol and carbon monoxide using a miracle process called the Monsanto process (or the more advanced British version). I will save the miracle organometallic chemistry for another day, what happens is that rhodium converts methyl iodide and carbon monoxide into an acetic acid derivative called acetyl iodide. Here is a picture of acetyl iodide.

Acetyl iodide, a key part of the Monsanto process

When we name an acid chloride (or an acid iodide, bromide or fluoride) what we do is the following.

Acetic acid, the carboxylic acid is changed in name into

Acetyl chloride, which is the acid chloride.

You should practise at home changing the names of the following carboxylic acids into the acid chlorides

Benzoic acid

Oxalic acid

2-Ethoxy ethanolic acid

Hexanolic acid

Please for goodness sake do not practise converting samples of the carboxylic acids into the acid chlorides at home as I do not want you to fill your homes with clouds of stinging toxic fumes. I would like you now to practise with the changing the names of the following acid halides into the carboxylic acids.

Acetyl chloride

2,2,4-trimethylhexanoyl bromide

2-bromo-3-chloro-4-fluoro-5-methyloctanoyl chloride

Lastly I would like you to draw out on a sheet of paper all seven of the carboxylic acids and all seven of the acid halides.

Now back to the chemistry, the acid halides are likely to be the most reactive of the carboxylic acid derivatives. They all react with water to form the carboxylic acid and the hydrogen halide. I would like you to draw the mechanism which is the final step in the Monsanto process which forms the acetic acid from the acetyl iodide

Reaction of water with acetyl iodide

By the way the hydrogen iodide in the Monsanto process is then used to make methyl iodide through a SN2 reaction, we will get onto this reaction soon. But we are not done with the carboxylic acids yet. One modification of the Monsanto process is to run at very low water levels, this allows the process to make something called an acid anhydride. The carboxylic acid anhydrides are important chemicals for the production of a range of things. One of the advantages of the acid anhydride is that it can be used to make esters without forming the super acidic hydrogen halide as a biproduct, instead it just forms the carboxylic acid as a biproduct when the acid anhydride is reacted with an amine or an alcohol. Here we have benzoyl anhydride reacting with methanol to make benzoic acid and methyl benzoate.

Benzoyl anhydride and methanol reaction

An example of an acid anhydride is acetic anhydride which is related to acetic acid. It is called an acid anhydride as the addition of a molecule of water to a molecule of an acid anhydride will convert the acid anhydride group into two carboxylic acid groups. Please draw out at home the reaction of acetic anhydride with water and you will soon see how the water converts the acid anhydride into the carboxylic acid.

Acetic anhydride is used for the production of aspirin (a nice legal drug), heroin (a nasty illegal drug) and for the production of cigarette filters (a part of the delivery system for a nasty but legal drug). Now I do not want to encourage my readers to either smoke cigarettes or do heroin abuse I will only be following you the reaction for forming aspirin. We start below with 2-hydroxybenzoic acid. This is reacted with acetic anhydride to form aspirin and acetic acid.

Synthesis of aspirin from 2-hydroxybenzoic acid

Before we get any further I would point out that while some illegal organic chemistry does occur in the world, I refuse to give out details of how to do it. So please do not bother me with requests for the exact details of how to do things like make heroin or MDMA in your kitchen, basement or shed.

A large range of mild to medium pain killers use carboxylic acids, for example ibuprofen is a carboxylic acid drug which was introduced to the market by Boots the British drug and pharmacy company. These tend not to be made in the same way as aspirin. Here for your information is ibuprofen.

The wonder pain killer from Boots

Before we leave the acid anhydrides I would like to point out that cyclic acid anhydrides such as maleic anhydride exist. These relate to dicarboxylic acids, one use of maleic anhydride is in the production of a form of polyester paint known as alkyd paint. These are typically made from glycerol (or some other triol), the cyclic acid anhydride of benzene-1,2-dicarboxylic acid (also known as phthalic anhydride) and an unsaturated fatty acid. I will let you work out how the alkyd paint is formed in this reaction.

A short part of the alkyd paint polymer before it sets by cross linking

Typically the triol and the benzene-1,2-dicarboxyl anhydride are reacted together with an acid catalyst. The mixture will be heated and any water which forms will be distilled out of the mixture. Then the fatty acid will be added and the heating continued to give the final product. Now I think you should be able to work out the mechanism by which this product is formed.

What happens when you paint with this type of paint is that the coating of the paint reacts with the oxygen of the air to form cross links between the fatty acid chains. Sometimes a little maleic anhydride is used to increase the number of alkene groups in the paint which can form the cross links. Now I hope you have had fun with the carboxylic acid derivatives, we can fix our headaches with them, drive the bus with them and paint the house with them !


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