Purification of an Ester

Introduction

ESTERS

are compounds derived from the reaction of a organic acid with an alcohol.

Acid + Phenol/alcohol --> ester + water

R--C=O + R'--OH ----> R--C=O + H2O | | C-OH C-O-R'

Esters are the compounds that give fruits their characteristic flavours and odours. ie. methyl salycilate is "Oil of Wintergreen".

http://www.petrik.com/PUBLIC/library/misc/glossary_of_org_chem.htm#ESTERS

Aim

To synthesise the ester 3-methylbutyl ethanoate( isopentyl acetate) and to identify the product by its boiling point.

Method

The ester was prepared by the lab assistant in the fume cupboard by reacting 3-methylbutan-1-ol (iso-pentanol, 25mL), and acetic anhydride (24mL plus 3 drops of concentrated sulfuric acid). A strong exothermic reaction occurs. Once the reaction mixture is cooled in the fume cupboard (after about 10mins-15mins), the process of isolating the ester by solvent extraction and purifying it by distillation began.

The first step of purifying the ester was to add 100mL of water to the reaction mixture and transfer it to a 250mL separating funnel. A further 50mL of water was used to make sure all of the reaction mixture had been transferred to the separating funnel. The contents were swirled as well as making sure the pressure was vented using the venting procedure which was demonstrated, that is swirl the contents holding the lid with one hand and venting the gas pressure by using the other hand by opening the nozzle. The contents were allowed to settle and then the water layer was discarded off. The crude ester was then extracted with 5% Sodium Bicarbonate to remove any acidic impurities. Any layer formed was discarded again. The ester was then again extracted with 20mL of saturated NaCl solution and again the aqueous layer drained off.

The crude ester was then transferred into a dry 100mL conical flask. Magnesium Sulfate was used as a drying agent to remove any water traces still left. The flask with the magnesium sulfate was left to stand for about 10 minutes.