When three fatty acids react with glycerol, they form a triglyceride — the most common form of fatty acids. Triglycerides have a very high molecular weight and can be highly branched. Their function and properties are dependent on the length and types of alkyl chains incorporated into their structure. One of the most common industrial uses for triglycerides is in the production of soaps and detergents.
Esters are formed from an esterification reaction, with simple esters being formed through Fisher esterification. This reaction converts a carboxylic acid and alcohol into an ester with water as a by-product. Fisher esterification is a reversible reaction that proceeds very slowly. An acid catalyst, typically in the form of sulfuric acid, is added to increase the rate of the reaction while also acting as a dehydrating agent. The mechanism of action for a Fisher esterification begins by the carbonyl oxygen attacking the sulfuric acid and deprotonating its OH group.
This leads to a positively charged carbonyl oxygen. To better stabilize the positive charge, one electron pair of the carbonyl double bond is pushed to the carbonyl oxygen while simultaneously creating an electron-poor carbon center. Next, the hydroxy group of the alcohol acts as a nucleophile and attacks the electron-poor carbon — the electrophile — forming an intermediate.
Then, the positively-charged hydroxyl group from the alcohol is deprotonated by the hydroxyl group of the carboxylic acid, stabilizing the oxygen atom of the alcohol. In the last step, the carbonyl oxygen is deprotonated by the conjugated base, and the free electron pair is moved towards the carbonyl center while the protonated carboxyl group leaves as a water molecule. However, as with any equilibrium process, the reaction can be driven in one direction by changing the concentration, pressure, temperature, or volume of the reagents.
When the concentration of one of the reactants is increased, the equilibrium shifts in the direction that will decrease its concentration.
Thus, by increasing the concentration of one reactant, the equilibrium produces more of the product, thereby resulting in a higher yield of the ester product.
One common method utilized in organic chemistry labs is to provide an excess of one of the reagents, typically the alcohol.
As mentioned earlier, Fisher esterification can utilize sulfuric acid as a catalyst. How are compounds in organic chemistry named? What is the general formula of a carboxylic acid? What is a carboxyl group? What is an alkane? What is the formula of the ethyl group? What is a functional group in organic chemistry? To be honest, if you are that unsure about the conventions used in reaction mechanisms, you probably shouldn't be reading this page anyway - you will find it distinctly scary!
In the first step, the ethanoic acid takes a proton a hydrogen ion from the concentrated sulphuric acid. The proton becomes attached to one of the lone pairs on the oxygen which is double-bonded to the carbon.
The transfer of the proton to the oxygen gives it a positive charge, but it is actually misleading to draw the structure in this way although nearly everybody does! The positive charge is delocalised over the whole of the right-hand end of the ion, with a fair amount of positiveness on the carbon atom.
In other words, you can think of an electron pair shifting to give this structure:. So which of these is the correct structure of the ion formed? None of them! The truth lies somewhere in between all of them. One way of writing the delocalised structure of the ion is like this:. The double headed arrows are telling you that each of the individual structures makes a contribution to the real structure of the ion.
They don't mean that the bonds are flipping back and forth between one structure and another. The various structures are known as resonance structures or canonical forms. There will be some degree of positive charge on both of the oxygen atoms, and also on the carbon atom.
Each of the bonds between the carbon and the two oxygens will be the same - somewhere between a single bond and a double bond. Note: You will find a more pictorial look at a similar case to this in a page discussing the acidity of organic acids.
Amongst other things, that page looks at the structure of ions like the ethanoate ions which also have delocalised charges. For the purposes of the rest of this discussion, we are going to use the structure where the positive charge is on the carbon atom.
The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen of the ethanol molecule. Note: You could work out precisely why that particular oxygen carries the positive charge on the right-hand side.
On the other hand, you could realise that there has to be a positive charge somewhere because you started with one , and that particular oxygen doesn't look right - it has too many bonds.
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