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Esters and Esterification

Key Concepts

  • Esters have the general formula :
      O
    ||
       
    R - C - O - R'

    in which R and R' represent alkyl groups1.

  • Esters (alkyl alkanoates) are prepared by reacting an alkanol with an alkanoic acid in the presence of an acid catalyst2:
    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.

  • Preparation of esters in the laboratory involves 3 steps

    1. Synthesis of the ester

    2. Isolation of the ester

    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.

Synthesis of an Ester in the Laboratory

An ester can be synthesised in the laboratory by heating a reaction mixture consisting of the alkanol, the alkanoic 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.

  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 on the right.

  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.
refluxing apparatus

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

Improving Yield of Ester

The esterification reaction is reversible, reactants are in equilibrium with products:

  reactants   products
general
word equation:
alkanol + alkanoic acid alkyl alkonoate
(ester)
+ water
examples: ethanol + acetic acid
(ethanoic acid)
ethyl acetate
(ethyl ethanoate)
+ water
propan-1-ol
(1-propanol)
+ acetic acid
(ethanoic acid)
propyl acetate
(propyl ethanoate)
+ water
butan-1-ol
(1-butanol)
+ acetic acid
(ethanoic acid)
butyl acetate
(butyl ethanoate)
+ water

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 mixture5.

Increasing the Rate of the 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 formed6.

Isolation of the Ester

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

organic substances inorganic substances
ester (desired product)

alcohol (excess reactant now present as an impurity)

carboxylic acid (excess reactant now present as an impurity)

water (undesired product of the reaction)

sulfuric acid (used to catalyse the reaction)


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.

  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 (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 present7.
    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 to separate 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.

substance density / g mL-1 relative
density
water 1.0 more dense
ethyl acetate
(ethyl ethanoate)
0.9 less dense

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 carbonate8) is used to neutralise any remaining acid:

sodium carbonate + acid → water soluble salt + carbon dioxide gas + water
net equation: CO32-(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 mixture9.

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:

reactants boiling point / oC
(approximate at 1 atm)
  ester
(product)
boiling point / oC
(approximate at 1 atm)
water 100   ethyl acetate
(ethyl ethanoate)
77
acetic acid
(ethanoic acid)
118      
sulfuric acid 142      

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.

  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 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 77oC
The table below gives the approximate boiling points of some other acetate esters:

ester methyl acetate
(methyl ethanoate)
ethyl acetate
(ethyl ethanoate)
propyl acetate
(propyl ethanoate)
butyl acetate
(butyl ethaoate)
boiling point
oC
56 77 101 124

Animated 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

reactants   products use
alcohol + carboxylic acid H+
water + ester
methanol + acetic acid
(ethanoic acid)
H+
water + methyl acetate
(methyl ethanoate)
solvent
  H
|
   
H- C -O- H
  |
H
   
+
  O
||
  H
|
 
HO- C - C -H
      |
H
 
H+
H2O +
  H
|
  O
||
  H
|
 
H- C -O- C - C -H
  |
H
      |
H
 

methanol + butanoic acid H+
water + methyl butanoate apple flavour
  H
|
   
H- C -O- H
  |
H
   
+
  O
||
  H
|
  H
|
  H
|
 
HO- C - C - C - C -H
      |
H
  |
H
  |
H
 
H+
H2O +
  H
|
  O
||
  H
|
  H
|
  H
|
 
H- C -O- C - C - C - C -H
  |
H
      |
H
  |
H
  |
H
 

ethanol + formic acid
(methanoic acid)
H+
water + ethyl formate
(ethyl methanoate)
rum essence
  H
|
  H
|
   
H- C - C -O- H
  |
H
  |
H
   
+
  O
||
 
HO- C -H
     
H+
H2O +
  H
|
  H
|
  O
||
 
H- C - C -O- C -H
  |
H
  |
H
     

ethanol + acetic acid
(ethanoic acid)
H+
water + ethyl acetate
(ethyl ethanoate)
solvent
  H
|
  H
|
   
H- C - C -O- H
  |
H
  |
H
   
+
  O
||
  H
|
 
HO- C - C -H
      |
H
 
H+
H2O +
  H
|
  H
|
  O
||
  H
|
 
H- C - C -O- C - C -H
  |
H
  |
H
      |
H
 

ethanol + butanoic acid H+
water + ethyl butanoate pineapple flavour
  H
|
  H
|
   
H- C - C -O- H
  |
H
  |
H
   
+
  O
||
  H
|
  H
|
  H
|
 
HO- C - C - C - C -H
      |
H
  |
H
  |
H
 
H+
H2O +
  H
|
  H
|
  O
||
  H
|
  H
|
  H
|
 
H- C - C -O- C - C - C - C -H
  |
H
  |
H
      |
H
  |
H
  |
H
 

pentan-1-ol
(1-pentanol)
+ acetic acid
(ethanoic acid)
H+
water + pentyl acetate
(pentyl ethanoate)
banana flavour
  H
|
  H
|
  H
|
  H
|
  H
|
   
H- C - C - C - C - C -O- H
  |
H
  |
H
  |
H
  |
H
  |
H
   
+
  O
||
  H
|
 
HO- C - C -H
      |
H
 
H+
H2O +
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
  H
|
 
H- C - C - C - C - C -O- C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
      |
H
 

pentan-1-ol
(1-pentanol)
+ butanoic acid H+
water + pentyl butanoate apricot flavour
  H
|
  H
|
  H
|
  H
|
  H
|
   
H- C - C - C - C - C -O- H
  |
H
  |
H
  |
H
  |
H
  |
H
   
+
  O
||
  H
|
  H
|
  H
|
 
HO- C - C - C - C -H
      |
H
  |
H
  |
H
 
H+
H2O +
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
  H
|
  H
|
  H
|
 
H- C - C - C - C - C -O- C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
      |
H
  |
H
  |
H
 

octan-1-ol
(1-octanol)
+ butanoic acid H+
water + octyl butanoate orange flavour
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
   
H- C - C - C - C - C - C - C - C -O- H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
+
  O
||
  H
|
  H
|
  H
|
 
HO- C - C - C - C -H
      |
H
  |
H
  |
H
 
H+
H2O +
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
  H
|
  H
|
  H
|
 
H- C - C - C - C - C - C - C - C -O- C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
      |
H
  |
H
  |
H
 


What would you like to do now?

1For this tutorial, we will only be concerned with examples where R is an alkyl group (that is a group derived from the alkane series).
And, we only be dealing with esters derived from straight chain carboxylic acids, so the name of the ester will be two words.
It should be noted that the names of more complex esters will be made up of more than 2 words.
If more than organyl group (alkyl, aryl etc) is present, they are cited in alphabetical order.

2 During the esterification reaction, the C-O bond in the carboxylic acid is broken.
Evidence for this comes from experiments using isotopically labeled reactants.
If some of the alcohol used contains the 18O isotope, the atoms of this isotope are all found in the ester product and NOT in the water.

3 There are other ways to produce esters besides the direct (Fischer) esterfication of a carboxylic acid with an alcohol.
(a) Alcoholysis of acid chlorides, anhydrides, or nitriles, produces esters.
(b) Reaction of the salt of a carboxylic acid with an alkyl halide or sulfate will produce an ester.
(c) Trans-esterifaction of an ester produces a new ester.

4 If a liquid is largely free of air, and the glass flask is clean and very smooth, the liquid may superheat, a condition in which the temperature of the liquid rises above its boiling point.
In a superheated liquid it is hard to form vapour bubbles resulting in irregular expulsion of bubbles of vapour, or, bumping.
Boiling chips (pieces of porous pot) provide an additional source of minute air bubbles which act as a nucleus for building bubbles of vapour in the liquid, allowing the liquid to boil quietly.

5 An even better way to improve ester yield is to use anhydrides in the synthesis instead of carboxylic acids.
For example, the reaction between ethanol and acetic anhydride is irreversible, and goes to completion within minutes.
acetic anhydride + ethanol → ethyl acetate + acetic acid

6Although sulfuric acid plays a vital role in the esterification reaction mechanism, it is beyond the scope of this tutorial. Suffice it to say that since the sulfuric acid that is used during in the reaction is re-produced at the end of the reaction mechanism, sulfuric acid is acting as a catalyst for the reaction.

7 For a saturated sodium carbonate solution, dissolve 4.5 g sodium carbonate in 15 mL of distilled water.

8If you are an organic chemist you will name HO-CO-O-Na+ as sodium hydrogen carbonate, but if you are an inorganic chemist you will probably name it sodium hydrogencarbonate.
Another common name for the same compound is sodium bicarbonate.

9Anhydrous magnesium sulfate is used as a drying agent in preference to fused or "anhydrous" calcium chloride which can combine with some esters.
Note the use of the term "drying agent". No chemical change is involved in using a drying agent, the drying agent is used to remove excess water molecules from the mixture.
In a dehydration reaction, however, there is a chemical change. Atoms of hydrogen and oxygen are removed from reactant molecules and produce water as product.

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