Synthesis, Isolation and Purification of Esters in a Direct Esterification Reaction Chemistry Tutorial

Key Concepts

• Esters have the general formula :

  	O ||
  R -	C	- O -	R'

  in which R and R' represent alkyl groups⁽¹⁾ and the functional group is the -CO-O- group.

• Esters (alkyl alkanoates) can be synthesized or prepared by reacting an alkanol with an alkanoic acid in the presence of an acid catalyst⁽²⁾:

  alkanol		+	alkanoic acid		H+   →	alkyl alkanoate (ester)	+	water
  R'-O	-H	+	HO-	OC-R	H+   →	R'-OOC-R (ester)	+	H2O

• Fischer Esterification is the name given to the acid-catalysed reaction between an alkanoic acid (carboxylic acid) and an alkanol (alcohol)⁽³⁾.
• Synthesis or preparation of esters in the laboratory involves 3 steps:

  Step 1: Synthesis of the ester

  Step 2: Isolation of the ester

  Step 3: Purification of the ester

• Esters of the low molar mass alkanoic acids and alkanols have fragrant, fruity odours, and are used in perfumes and artificial flavourings.

  Esters are also used as solvents.

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Synthesis of an Ester in the Laboratory Using Fischer Esterification

An ester can be synthesised in the laboratory by heating a reaction mixture consisting of the alkanol (alcohol), the alkanoic acid (caboxylic acid) and a small amount of concentrated sulfuric acid catalyst in a vessel fitted with a water-cooled condenser (Liebig condenser) to prevent loss of volatile material.

The steps in the procedure for the synthesis of the ester ethyl acetate (ethyl ethanoate) are given below:

1. Add 15 mL of alcohol (eg, ethanol) and 10 mL carboxylic acid (eg, glacial acetic acid), to a 50 mL round bottom flask.
2. Slowly add about 1 mL of concentrated sulfuric acid and a few boiling chips which will prevent "bumping"⁽⁴⁾.
3. Assemble the glassware as shown in the diagram below:

   

4. Turn on the heating mantle to achieve a gently boiling of the reaction mixture.
5. Reflux the mixture for 30 minutes.
6. Turn off the heat and allow the mixture to cool.
   Note: your round bottom flask now contains some ester (the desired product), as well as water, unreacted alcohol, unreacted carboxylic acid, and sulfuric acid.

Safety Considerations for the synthesis of esters:

• Alcohols are flammable, keep clear of flames.
• Liquid carboxylic acids are corrosive, avoid skin contact.
• Concentrated sulfuric acid is corrosive, avoid skin contact.

Improving the Yield of Ester Prepared by Fischer Esterification

The esterification reaction is reversible, reactants are in equilibrium with products as shown in the chemical equations below:

When the reaction reaches equilibrium there is still a large amount of reactants left in the mixture resulting in a poor yield of the ester.

The yield of ester can be improved by increasing the concentration of one of the reactants (either the alcohol or the carboxylic acid).
By Le Chatelier's Principle an excess of one reactant will drive the reaction to the right, increasing the production of ester, and therefore increasing the yield of ester.
In our experiment we are using an excess of carboxylic aicd (acetic acid which is also known as ethanoic acid).

From the balanced chemical equation, you will also appreciate that the presence of water in the reaction mixture will drive the equilibrium to the left, favouring the formation of reactants rather than ester.
So, another way to improve yield is to remove the water as it forms.
Concentrated sulfuric acid can be used for this purpose because it reacts rapidly with water to form a hydrated form of sulfuric acid, effectively removing the water from the reaction mixture⁽⁵⁾.

Increasing the Rate of the Direct Esterification Reaction

The esterification reaction is quite slow.

Heating the reaction mixture will speed up the rate of reaction.
The experimental technique we are using is known as "heating under reflux". As the volatile components in the reaction mixture vaporise, the hot vapours rise up into the condenser. Cool water running around inside the water-jacket around the condenser removes the heat from the hot vapours, cooling them down and allowing them to condense on the inside of the condenser. This cooled liquid runs back down the inside of the condenser and is returned to the reaction mixture.
Heating the reaction mixture under reflux prevents the loss of volatile reactants and products.

Concentrated sulfuric acid is used as a catalyst to speed up the rate at which the ester is formed⁽⁶⁾.

Isolation of the Ester Produced by Direct Esterification

The ester we synthesised above is impure.
The round bottom flask now contains a mixture of organic and inorganic substances as shown below:

These substances are all miscible with water, except the ester.

We will need to isolate the part of the mixture that contains the ester.
We can do this by taking advantage of the ester's low solubility in water, and, the fact that the ester will be less dense than water so it will float on top of a layer of water.
We use a piece of glassware called a separating funnel to isolate the less dense ester layer from the aqueous layer as described in the steps below:

1. With the stopcock closed, pour the cooled mixture into a separating funnel containing about 20 mL of water.
   Stopper the funnel and shake.
   Allow the layers to separate.
2. With the separating funnel suspended above a conical fask (erlenmeyer flask) and the stopper removed, open the stopcock and allow the more dense aqueous layer to flow into the conical flask.
   Close the stopcock.
3. Repeat the process above by adding more water and then separating the layers.
   This should remove most of the water soluble species (that is the acids and the alcohol).
4. Add 5 mL sodium carbonate solution to neutralise acid still present⁽⁷⁾.
   Swirl the flask until the production of carbon dioxide gas is no longer vigorous.
   Repeat this process until no more carbon dioxide gas is produced.
5. Allow the mixture to stand so that it separates into layers, then remove the denser aqueous layer by opening the stopcock.
   Close the stopcock once the aqueous layer has been run out.
6. Wash the remaining organic layer with about 20 mL of water, stand to allow the layers to separate, then open the stopcock once more to run out the aqueous layer.
   Close the stopcock once the aqueous layer has been removed.
7. Add a small amount of anhydrous magnesium sulfate to a clean, dry conical flask.
   Run the remaining organic contents of the separating funnel into the conical flask.
   Stopper the conical flask and allow to stand for 15 minutes.
8. Decant the contents of the conical flask into a clean, dry, 50 mL round bottom flask.

Initial washing with water will remove most of the water soluble species which includes any excess alcohol (ethanol), carboxylic acid (acetic acid), and sulfuric acid.
The ester is not very soluble in water so will separate into a separate layer.
The ester is less dense than water so the ester layer floats on top of the aqueous layer.
The density of water and ethyl acetate ester is given in the table below:

After washing there will still be traces of acid left in the organic, ester, layer.
An aqueous solution of sodium carbonate (or alternatively sodium hydrogen carbonate⁽⁸⁾) is used to neutralise any remaining acid:

sodium carbonate + acid → water soluble salt + carbon dioxide gas + water

net equation: CO₃^2-(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)

As sodium carbonate is added, bubbles of carbon dioxide form. When all the acid has been neutralised, adding sodium carbonate will no longer produce bubbles of carbon dioxide gas.
The final rinsing with water should remove traces of the water soluble salt formed.

Anhydrous magnesium sulfate is added to the remaining mixture because it is a "drying agent", it will remove water from the mixture⁽⁹⁾.

Purification of the Ester

The organic layer we decanted into the round bottom flask contains our impure ester.

The ester can be purified by distillation because the various components possible in the mixture have different boiling points.

The boiling points of the components which could still be in the mixture are given in the table below:

The ester we have prepared, ethyl acetate (ethyl ethanoate) has the lowest boiling point of all the possible components in the mixture.
Therefore ethyl acetate will be the first fraction collected as the distillate.
A sharp boiling point is an indication of the purity of the ester.

The procedure for the distillation of the mixture containing the ester is outlined below:

1. To the mixture in the round bottom flask as prepared above, add boiling chips to prevent "bumping".
2. Set up the distillation equipment as in the diagram on the right.
   Notes:
   (i) the mixture in the round bottom flask is best heated using a heating mantle, but an oil bath could also be used. The method of heating is not shown in the diagram.
   (ii) the reservoir (bulb) of the thermometer needs to be level with the intake to the condenser to ensure that the temperature being recorded is that of the vapour that is being condensed in the condenser, and not the temperature of the vapour that is being condensed in the fractionating column and therefore returning to the mixture in the round bottom flask.
3. Gently heat the mixture.
4. Collect the distillate in a clean, dry flask.
   Discard any distillate collected before the temperature of the vapour reaches a constant.
   Do not discard the distillate that collects at a constant temperature that is approximately the same as that expected for your ester.
   Replace the collection flask with another flask as soon as you think the temperature of the vapour is increasing, and stop heating the mixture.
5. Do not continue heating the mixture to dryness.
   There should always be some mixture left in the flask when you turn off the heat.

Note the appearance of your purified ester. If it looks cloudy there is probably water present.

Note the smell of your ester by using your hand to gently waft ester vapour to your nose.
NEVER place you face directly over the flask to note the odour!

Note that the fraction that you collect will depend on the relative boiling points of the components in the mixture.
If you have not prepared ethyl acetate, the boiling point of the ester will not be 77°C
The table below gives the approximate boiling points of some other acetate esters that you could synthesise in the laboratory:

Animated YouTube Tutorial

Examples of Esters

The name of an ester produced by direct (Fischer) esterification can tell us how the ester has been produced.

The first part of the name comes from the name of the alkanol used, with the suffix "anol" replaced by the suffix "yl".

alkanol becomes alkyl

The second part of the name comes from the name of the alkanoic acid used, with the suffix "oic acid" replaced by the suffix "oate".

alkanoic acid becomes alkanoate

Name of the ester is two separate words: alkyl alkanoate

Some examples of the synthesis of different esters, their names and usefulness, is given in the table below: