Synthesizing Esters: An Experiment to Give Fruits Characteristic Flavors

Introduction

Esters are derived from carboxylic acids. A carboxylic acid contains the –COOH group. In an ester, the hydrogen is this group is replaced by a hydrocarbon group of a kind. The hydrocarbon group can be an alkyl group such as methyl or ethyl or one containing a benzene ring such as phenyl. Low molecular weight esters are commonly used as fragrances and found in essential oils and pheromones. Esters can be obtained from reaction of alkyl halides or acid anhydrides with alcohols or by reaction of salts of carboxylic acid with alkyl halides. ( Byju’s, May 31, 2016)

The small esters have almost the same boiling point as the ketones and aldehydes with the same number of carbon atoms. Aldehydes and ketones are polar molecules. Therefore, they have dipole-dipole interactions and Van der Waals dispersion forces. Their boiling point are not as high as an acid which have the same number of atoms with them because they do not form hydrogen bonds. The small esters are fairly soluble in water but solubility decreases with chain length. The reason of solubility is although esters cannot hydrogen bond with themselves, they can hydrogen bond with water molecules. ( Jim Clark 2004 )

Objective

To synthesis esters which give fruits their characteristic flavours.

Apparatus needed

Water bath, test tubes, droppers

Chemicals needed

Salicylic acid, methyl alcohol, concentrated sulphuric acid , acetic acid, n-propyl alcohol, octyl alcohol, butyric acid, n-butyl alcohol

Procedures

Part A : Synthesis of Methyl Salicylate

1. 0.5g salicylic acid was weighted.
2. The salicylic acid was transferred to another large dry test tube and 1.5ml of methyl alcohol was added.
3. 3 to 4 drops of concentrated H2S04 was added carefully to the mixture (in the hood) and I was mixed again by tapping the bottom of the tube with fingers.
4. This mixture was placed in the hot water bath (90˚C) for 10 min to complete the reaction.
5. After the reaction is complete, the test tube was removed from the water bath and it was allowed to cool.
6. After it has cooled, the fumes was wafted carefully by passing your hand over the top of the tube towards your nose. The minty aroma of wintergreen can be smelled.
7. If you have difficulty smelling it, the content of the test tube can be poured into a beaker containing hot water or sodium carbonate solution. The aroma of the methyl salicylate should be apparent upon addition of hot water or sodium carbonate solution.
8. The observation was observed.

Part B: Synthesis of Orange

Carboxylic acid Alcohol Ester Fragance

Acetic acid Octyl alcohol n-octyl acetate Orange

1. n-octyl acetate was prepared by putting 6 drops of octyl alcohol in a clean, dry 10mL test tube.
2. 2 drops of glacial acetic acid was added.
3. 1 drop of concentrated sulfuric acid was added, the tube was agitated to mix the contents, and the test tube was placed in the boiling water bath for 3 minutes.
4. When heating is completed, the test tube was removed from the water bath, and 20 drops of water was added to the test tube contents. The test tube was shook well.
5. The odor of the product was cautiously noted and the results was recorded.

Part C : Synthesis of Strawberry

Carboxylic acid Alcohol Ester Fragrance

Butyric acid Ethyl ethyl butyrate Strawberry

1. Ethyl butyrate was prepared by putting 6 drops of ethyl alcohol in a clean, dry 10mL test tube.
2. 2 drops of butyric acid was added carefully.
3. 1 drop of concentrated sulphuric acid was added, the tube was agitated to mix the contents and the test tube was placed in the boiling water for 3 minutes.
4. When the heating is completed, the test tube was removed from the water bath, and 20 drops of water was added to the test tube containing the ester. It was agitated to mix.
5. The odor of the product was cautiously noted and the results were recorded.

Results

Odour/smell of esters formed in reactions between(Joseph Hiltner 2011):

Salicylic acid and Methyl Alcohol : Wintergreen Odour

Acetic acid and Octyl Alcohol : Orange Odour

Butyric Acid and Ethyl Alcohol : Strawberry Odour