ester

Esters are often used in the food industry as aromatic substances. The lower esters are fluids that smell like fruit (ethyl butanoate smells like ananas, penthyl butanoate smells like pear).

Esters follow the normal IUPAC (alkane) nomenclature: the substituent linked to the oxygen is named first, and the ester names have the -oate suffix (or is that -ate?).

An easy way to cook up an ester is to mix some pentanol with the same amount of acetic acid and warm it up. After it boils, you'll be able to sense a rather unpleasant odor.

Preparation of Esters

Esters are the product of a condensation reaction between alchohols and carboxylic acids, meaning that when they react together they form a molecule of water.

The way they can be prepared can be shown by mixing two such substances : Methanol and Ethanoic Acid.

Equipment needed

Test Tubes, Test Tube Rack
Paper Towel
Rubber band
1 Large Beaker
2 Small Beakers
A Tripod
A Bunsen Burner
Cotton Wool
Methanol
Ethanoic Acid
Concentrated Sulphuric Acid
1mol/ l Sodium hydrogencarbonate

*Methanol is toxic! Ethanoic Acid and Sulphuric acid are corrosive!*

Procedure
1. Put some water into the large beaker and sit it on the tripod. Heat the water until it boils then turn off the bunsen. (This is important! you must not leave the bunsen along due to the flammability of the chemicals)
2. Add about 1cm of the Methanol to the test tube and then add the same volume of ethanoic acid.
3. Add about 5 drops of concentrated sulphuric acid to the mixture.
4. Soak the paper towel in cold water, fold it and secure it around the neck of the test tube with the rubber band. the purpose of this is to act as a condenser.
5. Place a loose plug of cotton wool in the mouth of the test tube to stop any chemicals coming out whilst heating is occuring.
6. Place the test tube into the beaker of hot water. Leave for ten minuets.
7. Wake the small beaker and add 20cm² of sodium hydrogencarbonate to it.
8. Once heated, take the test tube from the hot water and remove the cotton wool. Pour the contents of the test tube into the small beaker. The sodium hydrogencarbonate neutralises any remaining sulphuric/ carboxylic acid. (This means the smell of the carboxylic acid is also removed.)
9. The beaker should be gently swirled, the ester forming on the surface of the solution. Methyl Ethanoate.
10. Take a breath of fresh, uncontaminated air. Hold the beaker a little away and waft the scent towards your nose, sniffing only lightly. You should smell a fruity flavour.

What is Actually Going On
When the molecules are placed together they react (With the help of the sulphuric acid and the heating) by losing a hydrogen from the alchohol and a Hydrogen and Oxygen atom from the carboxylic acid.

The Equations
Word Equation:
Ethanoic Acid + Methanol -> Methyl Ethanoate + Water
Chemical Equation:
CH₃COOH + CH₃OH -> CH₃COOHCH₃ + H₂O
Structural Equation

 
     H  O            H                 H  O
     | //            |                 | //
   H-C-C     +   H-O-C-H    --->     H-C-C   H    +  H-O
     |  \            |                 |  \  |         |
     H  O-H          H                 H   O-C-H       H
                                             |
                                             H

The H is lost from the carbonyl (COOH) group from the alchohol and the OH is the hydroxyl group from the acid, shown below.

 
     H  O |               |   H                
     | // |               |   |                 
   H-C-C  |           H-  | O-C-H    
     |    |  \     +      |   |                 
     H    |  O-H          |   H         
                    |
                    v

                  H-O
                    |
                    H                                   

A new bond is formed by the rest of the molecule

    
                                        _ _
     H  O |     |   H               H  |O  | H
     | // |     |   |               |  ||| | |
   H-C-C  |  +  | O-C-H   --->    H-C- |C-O| -C-H
     |    |     |   |               |  |_ _|  |
     H    |     |   H               H         H
                             

This is the new Ester Group :

      _ _
     |O  | 
     ||| | 
     |C-O| 
     |_ _|  

Key =
C = Carbon
H = Hydrogen
O = Oxygen
]
Some Common Esters and their Scents :

Ester                 Scent 

Ethyl Butanoate              Pineapple
Propyl Ethanoate             Pears
2-Methylpropyl Ethanoate     Raspberries

Sources :
Experimental Data Sheets.
Experience.

Esters are a type of organic compound. Often, they are the chemicals that give plants and foods their odors.

Common Esters

• methyl salicylate, C₆H₄OHCOOCH₃ (wintergreen)
  

        H      O
        |    //
   H    C   C    H
    \ // \ / \   |
     C    C   O--C--H
     |   ||      |
     C    C      H
    / \\ / \
   H    C   O--H
        |
        H

• methyl butanoate, CH₃CH₂CH₂COOCH₃ (apples)
  

      H  H  H  O      H
      |  |  |  ||     |
   H--C--C--C--C---O--C--H
      |  |  |         |
      H  H  H         H

• ethyl butanoate, CH₃CH₂CH₂COOCH₂CH₃ (pineapples)
  

      H  H  H  O      H  H
      |  |  |  ||     |  |
   H--C--C--C--C---O--C--C--H
      |  |  |         |  |
      H  H  H         H  H

• propyl ethanoate, CH₃COOCH₂CH₂CH₃ (pears)
  

      H  O     H  H  H
      |  ||    |  |  |
   H--C--C--O--C--C--C--H
      |        |  |  |
      H        H  H  H

• 2-methylpropyl ethanoate, CH₃COOCH₂CH(CH₃)₂ (raspberries)
  

                  H   H
                   \ /
      H  O     H    C--H
      |  ||    |   / 
   H--C--C--O--C--C--H
      |        |   \
      H        H    C--H
                   / \
                  H   H

• octyl ethanoate, CH₃COOCH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃ (oranges)
  

      H  O     H  H  H  H  H  H  H  H
      |  ||    |  |  |  |  |  |  |  |
   H--C--C--O--C--C--C--C--C--C--C--C--H
      |        |  |  |  |  |  |  |  |
      H        H  H  H  H  H  H  H  H

---

Another example of esters is fats. Fats are esters of glycerol (1,2,3-propanetriol, or glycerine).

An Example of a Fat

    H                O
    |                ||
 H--C--O--H    H--O--C--CH2CH2...CH3
    |
    |                O
    |                ||
 H--C--O--H +  H--O--C--CH2CH2...CH3
    |
    |                O
    |                ||
 H--C--O--H    H--O--C--CH2CH2...CH3
    |
    H
  glycerol    three misc. fatty acids

                  ||
                 \||/
                  \/

    H     O
    |     ||
 H--C--O--C--CH2CH2...CH3
    |
    |     O
    |     ||
 H--C--O--C--CH2CH2...CH3
    |
    |     O
    |     ||
 H--C--O--C--CH2CH2...CH3
    |
    H

     triester of glycerol (fat)

---

Credit to Avis Rapax because I didn't know about 2-methylpropyl ethanoate until I saw his writeup

Es"ter (?), n. [A word invented by L. Gmelin, a German chemist.] Chem.

An ethereal salt, or compound ether, consisting of an organic radical united with the residue of any oxygen acid, organic or inorganic; thus the natural fats are esters of glycerin and the fatty acids, oleic, etc.

 

© Webster 1913.