THE CHEMISTRY BEHIND ESTERS, ACYL CHLORIDES, AMIDES AND EQUILIBRIA #2

Hello, great readers. Here is a continuation from where I stopped in my last post on Esters. Have a quick read of the post here and kindly join me on this. I will start by explaining the Le Chatelier’s principle and then go ahead to discussing the consequences of it on the esterification reaction. Afterwards, I will give an insight into the reactions of esters and their applications.

^{Image by djedj from Pixabay}

LE CHATELIER’S PRINCIPLE

Le Chatelier’s principle states that the position of the equilibrium of a system changes to minimize the effect of any imposed change in conditions. Le Chatelier’s principle describes the effect that a change in the pressure, temperature or concentration of a substance in an equilibrium mixture will have on the position of equilibrium. Le Chatelier’s principle applies to any reaction that is in equilibrium and allows chemists to manage and manipulate equilibrium reactions. I will describe here the consequences of Le Chatelier’s principle on the esterification reaction.

Effect of concentration changes on the esterification equilibrium

Changing the concentration of a reactant or a product does not change the numerical value of the equilibrium constant, Kc. However, it does change the position of equilibrium. As an example, let’s look at what happens to the equilibrium mixture formed by the reaction of an alcohol with a carboxylic acid when extra alcohol is added. Temporarily, the reaction is not in equilibrium, but equilibrium is quickly restored by reaction of the alcohol to form the ester. A new equilibrium mixture is produced that has the same equilibrium constant.

This effect is an example of how Le Chatelier’s principle operates in practice. The change is the increase of alcohol concentration, which disturbs the equilibrium of the mixture. However, once a new state of equilibrium is established, some of the extra alcohol is changed into ester.

Industrially, this means that if an ester is to be made starting from an expensive alcohol, it is more economical to use an excess of the cheaper carboxylic acid. This ensures that the equilibrium is shifted to the right side and that as much as possible of expensive alcohol is converted into the ester.

Another way to manage the reversible reaction is to remove one of the products the moment it has formed. This prevents the reaction from achieving equilibrium, but it will nevertheless ‘keep trying’ to, as long as the product continues to be removed from the reaction mixture. In esterification, water is the easier product to remove.

Effect of temperature changes on the esterification equilibrium and K_c

Although the equilibrium constant does not change when concentrations of the reactants and products change, it will change as the temperature of the equilibrium is changed. For an exothermic reaction the Kc becomes numerically smaller as the temperature increases, and for an endothermic process the Kc becomes numerically larger as the temperature increases. Esterification is an exothermic process: when a carboxylic acid and an alcohol react, energy is transferred to the surroundings. So, if the temperature of the esterification is raised, the equilibrium shifts to absorb energy, that is, in the direction of the endothermic reaction (the left side). As the temperature is increased, the yield of ester decreases, though the rate of production of ester increases. For esterification in industry, a compromise temperature is used so that the rate and the yield are both acceptable, and it is normal for the alcohol and the carboxylic acid to be refluxed together in the presence of concentrated sulfuric acid catalyst. The sulfuric acid catalyst has no effect on the position of equilibrium but reduces the time to reach equilibrium.

^{The reversible reaction N2O4(g) ⇌ 2NO2(g) is endothermic, so the equilibrium position can be shifted by changing the temperature. When heat is added and the temperature increases, the reaction shifts to the right and the flask turns reddish brown due to an increase in NO2. This demonstrates Le Chatelier's Principle: the equilibrium shifts in the direction that consumes energy. When heat is removed and the temperature decreases, the reaction shifts to the left and flask turns colorless due to an increase in N2O4: again, according to the Principle. en:User:Greenhorn1 Public Domain}

APPLICATIONS OF THE ESTERIFICATION REACTION

The importance of esters as a source of pleasant-smelling substances and to flavourings has been mentioned earlier in the previous post. Almost all these esters are manufactured using the esterification reaction just described with the formation of the ester linkage.

There are several examples of the esterification reaction used to link two carbon chains to form a molecule that has a particular shape associated with biological activity. Two such molecules, benzocaine and procaine, are anaesthetics based on 4-aminobenzoic acid. Among other uses, there are ester solvents (including ethyl ethanoate), ester plasticisers in PVC and ester synthetic fibres (polyesters).

Esters and the food flavourings industry

The simple esters (that is, those with only a few carbon atoms per molecule) tend to have pleasant odours. The characteristic flavours of fruits and the fragrances of flowers often result from esters – commonly a subtle blend of esters and other odoriferous compounds.

When I talk about ‘flavour’ I mean a combination of taste and odour by receptors on the tongue and in the nose. Almost always, it is a combination of the substances that our various receptors detect that allows us to recognise a particular flavour.

The table below shows the 11 chemicals, and the amount of each, that chemists have put together to imitate the flavour of pineapple. Notice that many of the esters listed are still known by their traditional (pre-systematic) names – the result of long-established use in the food industry. Almost all the esters in this formulation are made by the reaction of the appropriate alcohol and carboxylic acid.

COMPONENTS OF IMITATION PINEAPPLE FLAVOURING

^{Image by andreas N from Pixabay}

Local anaesthetics

Penzocaine and procaine are local anaesthetics, which means that they make only small areas of the body insensitive to touch and pain. They were used a lot until recently – benzocaine as an ointment, drug or aerosol to relieve painful conditions of the skin, mouth and respiratory tract, and procaine for dental injections. Both compounds are usually prepared by esterification reactions.

Benzocaine has the systematic name of ethyl 4-aminobenzoate. It is synthesised from

4-aminobenzoic acid. The acid is esterified with ethanol in the presence of a little concentrated sulfuric acid to speed up the attainment of equilibrium. An excess of ethanol is used to drive the equilibrium to the right, according to Le Chatelier’s principle.

In a very similar process, procaine is also synthesised from 4-aminobenzoic acid. This time the alcohol has a much more complicated structure, but the chemistry behind the reaction is the same.

REACTIONS OF ESTERS

The carbonyl carbon in the ester group is susceptible to nucleophilic attack. The principal nucleophiles that react with it are the water molecule, the hydroxide ion and the hydride ion.

HYDROLYSIS OF ESTERS

In the hydrolysis of esters, water reacts with the carbonyl carbon and an alcohol is eliminated. The reaction is very slow because the water molecule is a poor nucleophile. It is also a reversible reaction, because it is essentially the reverse of the esterification reaction:

RCOOR’+ H2O ⇌ RCOOH + R’OH

It is this reaction with water that prevents esters from being used in some perfumes and deodorants. Pleasant-smelling esters are liable to be hydrolysed by chemicals in perspiration to form carboxylic acids, many of which have an unpleasant smell. They include butanoic acid, which gives the odour we detect in rancid butter. These carboxylic acids are among the components of body odour and are in the scent that dogs pick up when tracking humans.

BASE-CATALYSED HYDROLYSIS OF ESTERS

The base-catalysed hydrolysis reaction involves boiling an ester with aqueous sodium hydroxide to form the sodium salt of the acid and the corresponding alcohol. Fats and oils are natural esters and their alkaline hydrolysis in the basis of making soap (saponification).

During saponification of a natural fat or oil, propane-1,2,3-triol (glycerol) and the the sodium salt of a long-chain fatty acid are formed. The sodium salt is a major constituent of soaps. Its carboxylate ion is negatively charged at one (hydrophilic) end and non-polar at the other (hydrophobic) end.

Phenyl benzoate is hydrolysed to give two ionic products, since the phenol formed is sufficiently acidic to react with sodium hydroxide to give sodium phenoxide.

Acid-catalysed hydrolysis of esters

It is also possible to hydrolyse an ester by refluxing with a dilute acid. Normally, dilute sulfuric acid or concentrated hydrochloric acid is ased as the acid catalyst. This is really the reverse reaction of esterification, and in theory should lead to an equilibrium mixture that contains carboxylic acid, alcohol, ester and water. However, the presence of excess water in the dilute acid drives the reaction to completion and so the ester is virtually all hydrolysed. Notice that in acidic hydrolysis, the carboxylic acid is obtained rather than the carboxylate salt. Ethyl methanoate is hydrolysed to give methanoic acid and ethanol.

Fuels from oils and fats

Both fats and vegetable oils are used by living systems to transfer energy. They are converted into the corresponding carboxylic acids, which are then oxidised in a complicated series of reactions to form carbon dioxide and water

Fats and vegetable oils will combust in excess oxygen to give carbon dioxide and water. This reaction is highly exothermic and releases lots of heat energy. This reaction can be harnessed when vegetable oils are used as alternative biofuels.

Vegetable oils and animal fats are made up of molecules called triglycerides. These are esters of the alcohol propane-1,2,3-triol and long-chain carboxylic acids. Vegetable oils are easily extracted from the seeds of plants, and several of them are used in cooking, the most popular ones being olive oil, sunflower oil, soya oil and palm oil.

^{Biodiesel sample. Shizhao, CC BY-SA 3.0}

The long carbon chain of the carboxylic acid portion of the triglyceride resembles a long-chain alkane, and hence it can be used to make a renewable source of fuel. The vegetable oil as extracted can be used as biodiesel in specially modified diesel engines. However, if used in normal diesel engines, any partially combusted residues clog up the engine and severely reduce efficiency. So the vegetable oil is hydrolysed with an alkali and then acidified, and the carboxylic acid products are isolated. They are then reacted with methanol to form methyl esters. The methyl esters are much more volatile than the original oil and can be used in unmodified diesel engines. In the United Kingdom, over one million tonnes of rapeseed is produced each year. The oil from rapeseed is easily converted into methyl esters known as rape methyl ester (RME). Rape methyl ester can be used as a diesel substitute and offers several environmental advantages over conventional diesel fuel. It does not form sulfur dioxide and emits fewer sooty particles during combustion. It may even be possible to utilize used cooking oils as diesel substitute, because they can be treated in the same way as rape oil to make methyl esters that can be used in normal diesel engines.

Methyl esters of long-chain fatty acids are also being manufactured to make low-fat spreads. This avoids the use of hydrogenated unsaturated fats and the risks of producing E or trans double bonds.

Thanks for reading.

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