laboratory preparation of ester

Preparations of Ester

Introduction

Ester is produce when organic acid R-COOH is heated with alcohol, R-OH, in the present of strong acid.

Below is a general reaction for esterification of an organic acid with alcohol;

R-COOH + HO-R  ⇔ R-CO-OR + H2O

Production of ester by heating organic acid with alcohol in the present of strong mineral acid is called esterification.

On the above general reaction, we can observed that, reactant on the left hand side both has R group attach to COOH and HO, this R group represent hydrocarbon chains which determine parent of the organic compound. Remember that the mineral acid in the reaction act as catalyst, it do not take part in chemical reaction, but help to faster the rate of chemical reaction. For example; when acetic acid, CH3COOH heated with ethyl alcohol, CH3CH2OH, in the present of mineral acid as catalyst. The reaction yield ester and water. Below is the equation;

CH3COOH + CH3CH2OH  ⇔  CH3 –COO-CH2– CH3 + H2O

On the above equation, the reactant, acetic acid and ethyl alcohol react to give ethyl acetate and water as the product, CH3 –COO-CH2– CH3 + H2O, ethyl acetate is the ester product obtain completion of the reaction. Also know that esterification is an equilibrium reaction, meaning that the reaction did not on it own proceed to completion. Often time, production of ester is highly volatile and may be extracted from the system by distillation process. But in same case, ester may be distilled easily, at such it may be necessary to add a desiccant the system to equilibrate. By adding a desiccant on the, it will remove or chelate water and then force the equilibrium to move to the right.

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Identification of ester

Ester is often identify from other organic chemical base on the very pleasant, fruitlike odours which they have. The odours, fruit flavouring and flowers found in some plant is a result of the present of ester of in them. Fruit or flower entirely made up of a few drop of ester, yielding a nice odours. Usually some complex mixture of substance have ester component, which when take as a the entirely compound produce aroma assign to the entire material. The laboratory preparation  of ester in a large relative amount of ester may be observed to produce a pronounced chemical odour, and such it may required a lot of effort to recognize fruit or flower that own the aroma.

Table Common ester

Ester Aroma Constituent
Benzyl acetate peaches Benzyl alcohol/acetic acid
n-propyl acetate pears n-propyl alcohol/acetic acid
Octyl acetate Oranges n-octyl alcohol/acetic acid
Methyl butyrate Apples Methyl alcohol/butyric acid
Ethyl butyrate Pineapples Ethyl alcohol/butyric acid
Isobutyl propionate Rum Isobutyl alcohol/propionic acid
Methyl anthranilate Grapes Methyl alcohol/2-aminoboenzoic acid
Isoamyl acetate Bananas Isoamyl alcohol/acetic acid

Practical

Material

  1. Hot plate
  2. 4ml disposable plastic pipette
  3. Eye google
  4. 250ml of beaker
  5. Watch glass
  6. Water bath

Reagent required

  1. 50% sulfuric acid
  2. Alcohol
  3. Organic acid
  4. Methyl salicylate
  5. Sodium hydroxide

Safety precaution

Before commencing the practical, protective I google must be worn

All organic acid to be use in these practical are flammable, as such heating must be carried out using hotplate. No flame should allowed into the laboratory.

Sulfuric shall be use as a catalyst for esterification, the acid is very dangerous, it has the power to burn skin in a bad way, if spilled on your skin wash immediately, because it delay the acid will have the chance causing burn.

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There is chance that the vapour of the ester produce may be harmful, as such when determining the odours of the ester produce, never deeply inhale the vapours directly, instead swift small quantity of the vapours from the product formed toward your nose.

Sodium hydroxide is very corrosive to eye and skin so wash immediately with plenty amount of water if spilled on the eye or skin.

Procedure

Prepare water bath in a 250ml beaker on a hot plate in the exhaust hood. Note that most of the chemical prepare in this method are highly flammable, as such no flame is allowed into the laboratory during practical work. The water bath temperature must be regulated at 70oC and maintained at that point.

Synthesis of ester

Mix 3-4 drop or approximately 0.1g is if the acid is in the solid form with 3-4 drop of indicated alcohol o clean watch glass. Then add I drop of 50% sulfuric acid to the mixture on the watch glass, with caution. Use the tip of plastic pipette to stir the mixture on the watch glass. Then suck as much as possible of the mixture into the pipette, after turn the pipette such that the tip of the pipette is upward and place it in a warm water bath, allow it to heat for 5 minute.

Then remove the pipette from water bath and discard the mixture in the pipette into a beaker containing a worm water, with caution swift the vapour coming out from the beaker toward yours nose.

Also be inform that ester odours is highly concentrated, as such several sniffs me be required before proper identification of ester produced.

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PROCEDURE FOR WIDAL TEST

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