LAB: ESTERIFICATION
Purpose
" To study the method for making esters.
" To create a specific ester from its alcohol and carboxylic reactants.
" To study some of the properties of esters.
Background Theory
Esters are derivatives of carboxylic acids that are found in a variety of natural sources. Simple esters generally have pleasant
odors, and are the main components of many of the odors and flavors of fruits and flowers. Most odors and flavors are the result of
a complex mixture, with one ester predominating. Because esters are easy to synthesize, flavor chemists often use esters, either
one or several, to reproduce or enhance natural flavors. The table on the next page shows some examples of esters that are
primary components of various odors and flavors. You will choose one of these esters to synthesize.
Although esters are often used as flavorings, they are rarely used as scents to be applied on the body. This is because esters are
not highly stable, and can be hydrolyzed back to carboxylic acids and alcohols when exposed to perspiration. Since esterification
is a reversible reaction, the addition of acid, water and heat (perspiration) favors hydrolysis. And, unfortunately, many simple
carboxylic acids have a very unpleasant odor. For example, butyric acid (butanoic acid) is the main component responsible for the
odor of rancid butter. Butyric acid is formed from hydrolysis of ethyl and methyl butyrate (butanoate), esters that smell like
pineapple and apple. Some esters are also bioactive. Isoamyl acetate, which smells like bananas, is also an alarm pheromone
released by honey bees when they sting an intruder. The pheromone attracts other bees and incites them to attack the intruder.
For these reasons, fragrances are more often made from essential oils (phenols, aldehydes, alkenes, etc.).
In this lab you will synthesize an ester from a carboxylic acid and an alcohol, and then purify it using extraction. You will choose
which ester (see the list below) you want to make, and then choose the appropriate acid and alcohol for your synthesis. The
general reaction scheme is shown below:
It is important to note that this is a reversible reaction, so the acid and alcohol reactants and ester products are in equilibrium
(implied by the double arrow). What this means is that once the reaction reaches equilibrium, there could be a large amount of
starting material remaining, resulting in a poor yield of the ester. In order to resolve this problem we make use of Le Chatelier's
principle, which predicts that we can drive the equilibrium to the right (to the products) by having one of the reactants in excess (in
effect, putting pressure on the left side). Usually, a threefold molar excess is enough to drive the equilibrium sufficiently to the right
in an esterification reaction. Either the alcohol or the acid can be used in excess. The choice can be based on cost, availability
and/or ease of purification at the end of the reaction. In this reaction we will add an excess of the acid.
Sulfuric acid (H SO ) is used as a catalyst for this reaction in order to accelerate the rate at which the product is formed. Since a
2 4
catalyst is not consumed during the course of a reaction, you need to use only a small amount of sulfuric acid in order for it to be
effective. Heating is another way to increase the reaction rate. We will heat the reaction using a reflux apparatus (see figure
below). Refluxing is a process where liquid (your reaction mixture) is heated to its boiling point, with a condenser attached, so that
some of the liquid vaporizes, rises part-way up the condenser, and then condenses and falls back down into the reaction flask.
This process allows the reaction to be heated over a period of time, without evaporating away the solvent or reactants.
At the end of the reaction your product will need to be purified, since it will be contaminated with unreacted, excess acid. The
technique used for purification will be extraction using a separatory funnel. Extracting the product mixture with water and sodium
bicarbonate (a base) will remove the carboxylic acid impurity.
Pre-lab:
" Select the specific ester you wish to synthesize.
" Determine which alcohol and which carboxylic acid you will need to synthesize your ester.
" Write out the reaction for your specific synthesis in your notebook. Include the names along with the structures when you write
out your reaction.
" State the expected odor of this ester.
Materials:
Chemicals alcohol (specify) carboxylic acid (specify) concentrated sulfuric acid
5% sodium bicarbonate anhydrous sodium sulfate
Hardware 25 mL round-bottom flask water-cooled condenser boiling stones
test tube small beaker spatula
test tube holder ring stand clamps
separatory funnel specimen container
Safety Precautions
Concentrated sulfuric acid can cause extreme burns if spilled on skin or in eyes. Use with caution and wear safety goggles at all times.
Flask will be hot at end of refluxing, allow glassware to cool before removing.
Procedures:
Option A: Using Reflux Condenser.
1. Preparation of hot water bath and reflux condenser.
" A large beaker was half filled with tap water and heated to 60Ä…10°C. Do not exceed 70°C.
" The reflux condenser was clamped vertically to a ring stand and rubber tubing was connected securely.
" The bottom rubber tube (water in) was connected to the cold water tap and the other tube (water out) was placed in the sink.
2. Initiate Reaction
" 20 drops of the alcohol and 10 drops of carboxylic acid were placed into a 25 mL round-bottom flask.
" 5 drops of concentrated sulfuric acid were added and the chemicals were mixed by gentle swirling.
" A couple of boiling stones were added and the round bottom flask was attached the reflux condenser.
3. Complete reaction
" The cold water tap was turn on so that water was circulating in the condenser with a nice even flow.
" The reaction mixture was heated to about 80°C. The temperature was recorded.
" Heating under reflux was continued for 40-60 minutes.
" When complete, remove the heating mantle and allow the mixture to cool to room temperature.
Option B: Covered Test Tube
" The same hot water bath and volumes of reagents were used as above.
" The reagents were placed in a large test tube instead of the round bottom flask.
" The test tube was held in place in the water bath using a test tube holder that was slid onto a ring stand.
" The test tube was lowered so that the reactant mixture was near the level of the water of the water bath.
" The test tube was covered by a small beaker to prevent chemicals from escaping
" The reactant mixture was heated for 10 minutes then placed immediately in cool fridge for 5 minutes.
4. Purification
" Allow the layers to separate.
" The upper layer is the  organic phase and the lower layer is the aqueous phase. Using a separation flask, the aqueous layer
was removed, neutralized with sodium bicarbonate, and discarded.
" The organic layer was transferred to a clean, dry specimen container, and labelled.