Esterification

Concepts Instructor Prep Student Protocol

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Esters are a class of organic molecules that can have a fruity or flowery aroma. The structure of an ester is a carbonyl with an alkyl or aryl group on one side and an oxygen bound to another alkyl or aryl group on the other side.

Depending on the alkyl or aryl groups, the ester can take on many different characteristics. For example, the reaction of glycerol and three fatty acids, which are carboxylic acids with long alkyl chains, results in a triglyceride, which has three ester groups. The long alkyl chains from the fatty acids give triglycerides a very high molecular weight. In contrast, simple esters have a low molecular weight and small functional groups.

The smells and flavors of fruits and flowers are attributed to simple esters. Even small changes to the structure of a simple ester greatly affect its fragrance. For example, changing a hydroxyl group to an amine changes the scent of this ester from wintergreen to grape. Similarly, propyl acetate smells like pears, while butyl acetate, which has only one more carbon in its chain, smells like apples.

One common way to make an ester is Fischer esterification, where a carboxylic acid and an alcohol react in the presence of an acid catalyst to form the ester and water. The R group and the carbonyl come from the carboxylic acid, and the alkoxy or aryloxy group with the R' comes from the alcohol. This esterification reaction is reversible. With a 1 to 1 mixture of the carboxylic acid and the alcohol, it tends to reach equilibrium with about a 70% yield of the ester at best.

However, Le Chatelier's principle allows us to increase the yield of the ester beyond that. Le Chatelier's principle states that any system at chemical equilibrium that is subjected to a change in concentration, pressure, temperature, or volume will adjust to a new equilibrium that counteracts the change.

So, if we increase the concentration of one of the reactants in a reversible reaction, the equilibrium will shift in the direction that decreases its concentration. This results in a higher yield of the ester product at equilibrium. Thus, we can improve the yield of esterification by using a 3:1 or 1:3 molar ratio of carboxylic acid to alcohol.

In this experiment, a Fischer esterification will be performed using alcohol in excess and sulfuric acid as the catalyst. In this reaction, the carboxylic acid reactivity is enhanced by sulfuric acid, which protonates the oxygen of the carbonyl. The alcohol is a nucleophile that attacks the carbon of the carbonyl to form an intermediate. Next, the hydrogen of the alcohol is transferred to a nearby hydroxyl. The carbonyl then reforms, eliminating a water molecule. Finally, deprotonation results in a neutral ester.

In this lab, you'll perform a Fischer esterification reaction with an unknown alcohol and carboxylic acid in a molar ratio of 3:1. You'll then identify the ester using its odor, determine the yield, and identify the two unknown reagents based on the structure of the ester.

At the end of this lab, students should know...

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What is the general structure of an ester?

An ester consists of a carbonyl with an alkyl or aryl group on one side and an oxygen bound to another alkyl or aryl group on the other side.

What effect does alkyl chain length and different groups have on a simple ester?

Simple esters, or those with short alkyl chains, have smells and flavors of fruits and flowers. The structure of the alkyl chain influences the smell of the ester. Changing the length or even the groups present in the simple ester can drastically change the scent produced by the ester.

How is an ester formed?

In general, an ester is made from an acid in which at least one hydroxyl group (OH) is converted into an oxygen that is bonded to either an alkyl or aryl group. Most esters are formed from the reaction of a carboxylic acid and an alcohol, though other forms of esters exist, such as the phosphodiesters that form the backbone of DNA.

What is Le Chatelier's principle, and how can it affect the percent yield of an ester?

Le Chatelier's principle states that an equilibrium reaction can be driven in one direction by changing the concentration, pressure, temperature, or volume of the reagents or products. With a 1:1 mixture of carboxylic acid and alcohol, the esterification reaction reaches equilibrium with about 70% yield of the ester. By increasing the concentration of one of the reactants, either the carboxylic acid or the alcohol, the esterification reaction can be driven to reach equilibrium with about 90% yield of the ester.

How does Fisher esterification produce an ester?

The acid catalyst protonates the carboxylic acid via the double-bonded oxygen. This leads to an electron-poor carbon. The oxygen from the alcohol acts as a nucleophile, bonding with this carbon and losing a hydrogen in the process. The acid catalyst once again protonates the intermediate product, resulting in a restructuring of the product, where a water molecule leaves and the carbonyl group reforms. Deprotonation occurs to produce the final neutral ester.

Print Setup Guide

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Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

Preparation of the Laboratory

Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

First, put on a lab coat, safety glasses, and nitrile gloves. Note: Some of the reagents are volatile or have unpleasant odors, so you will handle them in a fume hood.
Ensure that you have glass and organic waste containers and that each sink has paper towels.

Follow the table to designate codes for the carboxylic acids and alcohols used in this lab. The carboxylic acids are coded A through C, and the alcohols are coded D through I. The students will use a 3:1 molar ratio of alcohol to carboxylic acid in the reaction.

Code	Compound	MW (g/mol)	Density at RT (g/cm³)	Moles per vial	Grams per vial	cm³ per vial	Number of vials
A	Acetic acid	60.05	1.049	0.07	4.2035	4.0	3
B	Butyric acid	88.11	0.953	0.07	6.1677	6.5	3
C	Anthranilic acid	137.14	-	0.07	9.5998	-	1
D	1-Propanol	60.10	0.803	0.21	12.6210	15.7	1
E	Isoamyl alcohol	88.15	0.810	0.21	18.512	22.8	1
F	1-Butanol	74.12	0.810	0.21	15.565	19.2	1
G	Ethanol	46.07	0.789	0.21	9.6747	12.3	1
H	Methanol	32.04	0.791	0.21	6.7284	8.5	2
I	1-Octanol	130.23	0.830	0.21	27.348	32.9	1

Prepare the three carboxylic acids: acetic acid, butyric acid, and anthranilic acid. Each group will need 0.07 moles of their assigned carboxylic acid in a vial.
Tare a 10-mL glass vial on an analytical balance in a fume hood. Use a Pasteur pipette to measure 4.2035 g of acetic acid, which will be about 4 mL. Cap the vial and label it ‘A’. Prepare two more labeled vials of acetic acid in the same way.
Weigh 6.1677 g of butyric acid, which will be about 6.5 mL, in a tared 10-mL vial. Cap the vial and label it ‘B’. Prepare two more vials of butyric acid in the same way.
Use a small spatula to weigh 9.5998 g of solid anthranilic acid in a tared 10-mL vial. Cap the vial and label it ‘C’. You should now have a total of seven vials of carboxylic acid.
Prepare the 7 alcohols. Each group will need 0.21 moles of their assigned alcohol in a vial. Use the table as a guide to prepare and label one vial of each alcohol. Prepare and label two vials of methanol.
You should now have a total of seven vials of alcohol. Keep the reagent vials in the instructor's hood for now and put away the stock bottles of the carboxylic acids and alcohols.

Assign each student group one of these seven combinations of reagents.

Ester code	Carboxylic acid	Alcohol	Ester product	Scent
AD	Acetic acid	1-Propanol	Propyl acetate	Pear
AE	Acetic acid	Isoamyl alcohol	Isoamyl acetate	Banana
AI	Acetic acid	1-Octanol	Octyl acetate	Orange
BF	Butyric acid	1-Butanol	Butyl butyrate	Pineapple
BG	Butyric acid	Ethanol	Ethyl butyrate	Strawberry
BH	Butyric acid	Methanol	Methyl butyrate	Apple
CH	Anthranilic acid	Methanol	Methyl anthranilate	Grape

Prepare or obtain seven vials of the fruit scents and label them with the fruit names as references.
Set out the following glassware and equipment at each student lab station (we suggest that students work in pairs):
Put bottles of saturated sodium bicarbonate and 3 M sulfuric acid in a dedicated fume hood, along with a roll of plastic paraffin film and scissors.

Set out filter paper and Pasteur pipettes in a central area so that every student group can easily access them.

1Lab stand

23-prong clamps

1Stir plate

1Lab jack

1Heating mantle

1Temperature controller

2Pieces of tubing (for condenser)

1Joint clip

1Tube of vacuum grease

1Box of laboratory wipes

110-mL graduated cylinder

1600-mL beaker

150-mL round-bottom flask and flask stand

1Standard condenser

1Small stir bar

1Glass rod

1Package of pH paper

2Pasteur pipette bulbs

Just before the lab, fill a large insulated cooler with crushed ice. Place the cooler and an ice scoop in a central area in the lab.
When the students arrive, show them how to safely waft vapors before giving them their assigned reagents.

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