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Production of Esters

Esters: Production of Esters

As was discussed earlier in this tutorial one way to form an ester is through the reaction of an alcohol and a carboxylic acid. This process is called an acid-catalysed condensation or esterification of a carboxylic acid.

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The acid-catalysed condensation of a carboxylic acid to form an ester.

Fact:

In the formation of an ester one oxygen atom comes from the alcohol molecule, while the carbonyl group comes from the carboxylic acid. This is known because it is possible to label (using radioactive nuclides) the atoms of the reactants and see where they end up in the products.

In the general form above an alcohol (red) and a carboxylic acid (orange) combine to form an ester and water. A specific example is given in the figure below for the formation of butyl propanoate and water.

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The esterification of butanol and propanoic acid to form butyl propanoate, water is also formed in this reaction.

This reaction can also be written as:

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A more general example is:

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It is important to be able to identify what ester a specific alcohol and carboxylic acid will form. Remember that the first part of the ester name takes its prefix from the alcohol with the suffix -yl. The second part of the ester takes its prefix from the carboxylic acid with the ester suffix -oate.

Example: Determining Ester Names

Question

What is the name of the ester that will form from hexanol and propanoic acid

Which compound forms the first part of the ester name and which forms the second part of the ester name?

The alcohol forms the first part of the ester name and takes the suffix -yl. The carboxylic acid forms the second part of the ester name and takes the suffix -oate.

Determine the first part of the ester name

The alcohol is hexanol, therefore there are 6 carbons and this will be hexyl.

Determine the second part of the ester name

The carboxylic acid is propanoic acid, therefore there are 3 carbons and this will be propanoate.

Combine the first and second parts of the ester name

The ester will be hexyl propanoate.

It is also important to be able to determine which compounds were used to form an ester.

Example: Determining Starting Materials of Esters

Question

What compounds did the ester octyl heptanoate come from?

What types of compounds are used to form esters?

Esters are formed from alcohols (which become the first part of the ester name) and carboxylic acids (which become the second part of the ester name).

Determine the prefix for the alcohol

The first part of the ester name comes from the alcohol (-ol). Therefore the prefix is oct-.

Determine the prefix for the carboxylic acid

The second part of the ester name comes from the carboxylic acid (-oic acid). Therefore the prefix is hept-.

Determine the compounds use to form the ester

Octyl heptanoate was formed from octanol and heptanoic acid.

A few examples of esters are given below.

Ester name

Smell

Uses

methyl

methanoate

ether

quick-dry

finishes

insecticide

pharmaceuticals

ethyl

methanoate

rum

lacquers

safety glass

fumigating foods

propyl

methanoate

pears

solvent

flavour

fragrance

methyl

ethanoate

glue

glues

paints

nail polish

remover

ethyl

ethanoate

apple

glues etc

solvents

decaffeination

propyl

ethanoate

pear

solvent

flavour

fragrance

butyl

ethanoate

banana

or apple

lacquers

flavour

 

pentyl

ethanoate

banana

or apple

lacquers

solvents

resins

methyl

butanoate

apple or

pineapple

flavour

fragrance

 

ethyl

butanoate

pineapple

flavour

fragrance

plasticiser

propyl

hexanoate

blackberry

or cheese

flavour

solvent

fragrance

Table: The uses of some esters.

The following experiment will help you to prepare esters. Use what you have learned in this section to answer the questions that follow.

Optional Experiment: Preparation of Esters

Aim

To prepare and identify esters.

Apparatus

  • Methanol (\(\text{CH}_{3}\text{OH}\)), ethanol (\(\text{CH}_{3}\text{CH}_{2}(\text{OH})\)),

    pentan-1-ol (\(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}(\text{OH})\)), methanoic acid (\(\text{HCOOH}\)),

    ethanoic acid (\(\text{CH}_{3}\text{COOH}\)), sulfuric acid (\(\text{H}_{2}\text{SO}_{4}\))

  • Marble chips (or small, clean stones)

  • Five tall test tubes or beakers, a water bath, a hot-plate (or bunsen burner), a thermometer, rubber bands, paper towel, five bowls of cold water

Method

Warning:

Concentrated acids can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid. Always add the acid to the water and avoid sniffing the acid.

Remember that all alcohols are toxic, methanol is particularly toxic and can cause blindness, coma or death. Handle all chemicals with care.

  1. Place the marble chips (or clean stones) in the test tubes and label them A – E.

  2. Add \(\text{4}\) \(\text{ml}\) methanoic acid to test tubes A and B.

  3. Add \(\text{4}\) \(\text{ml}\) ethanoic acid to test tubes C, D and E.

    986e690fadbd03bb7bbb5b47512fc4b0.png

  4. Add \(\text{5}\) \(\text{ml}\) methanol to test tubes A and C.

  5. Add \(\text{5}\) \(\text{ml}\) ethanol to test tubes B and D.

  6. Add \(\text{5}\) \(\text{ml}\) pentanol to test tube E.

    57ce53d4bc0307dbf33caaac32d3571e.png

  7. Slowly add \(\text{2}\) \(\text{ml}\) \(\text{H}_{2}\text{SO}_{4}\) to each test tube.

  8. Soak the paper towel in cold water and attach it to the sides near the top of each test tube with a rubber band (do not close the test tube, just wrap the paper around the sides near the top).

    077c3e2916bd1d81918e68742e82afe7.png

  9. Heat the water bath to \(\text{60}\) \(\text{℃}\) (using the hot-plate or bunsen burner) and place the test tubes in it for \(\text{10}\)-\(\text{15}\) \(\text{min}\).

  10. After \(\text{10}\)-\(\text{15}\) \(\text{min}\) cool each test tube in cold water. Label the bowls of cold water A – E and pour the contents of test tube A into bowl A, etc.

    8fb522020303a34554556e511d48908d.png

  11. Observe the surface of the water and note the smell in each bowl.

Questions

  1. What is the purpose of the \(\text{H}_{2}\text{SO}_{4}\)?

  2. What is the purpose of the wet paper towel?

  3. What did you observe on the top of the water in each bowl at the end?

  4. Fill in the details in a table like the one below:

    Carboxylic acid

    Alcohol

    Product

    Smell

    methanoic acid

    methanol

      

    methanoic acid

    ethanol

      

    ethanoic acid

    methanol

      

    ethanoic acid

    ethanol

      

    ethanoic acid

    pentanol

      

Discussion and Conclusion

An ester is the product of an acid-catalysed condensation between an alcohol and a carboxylic acid. Esters have identifiable aromas (like the fragrant smell and odour of fruit) and are used in perfumes. If you didn’t smell anything it is likely that your water bath was set at too high a temperature and the ester degraded.

Esters are less water soluble than the carboxylic acid they were formed from and appear as an oily substance on water. Before being cooled however some of the ester would have been a vapour, so the wet paper towel would help to prevent a loss of the product.

Refer to the table above and see if the smells match those listed there. Remember to waft the smell towards you using your hand, do not sniff the fumes directly!

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