Esterification

October 27 – 28, 2011

Esters

Volatile esters typically smell ”fruity”

Fischer esterification

Conversion of a carboxylic acid and an alcohol to an

ester under acid catalysis

This is a condensation reaction- 2 molecules make one

molecule, and a small molecule is lost (H2O)

Reaction is in equilibrium and does not go to completion

Mechanism

Isolation flowchart

Benzoic acid

Methanol

Methyl benzoate

Sulfuric acid

Methanol

Sulfuric acid

Benzoic acid

Methyl benzoate

Methyl benzoate

Benzoic acid

Sodium benzoate

1. Reflux

2. Cool and dilute with

CH2Cl2

3. Extract with H2O

Extract with NaHCO3

CH2Cl2

CH2Cl2 H2O

HCl

H2O H2O

Purification of product and

starting material

1. Vacuum filter

2. Wash with cold water

3. Air dry

4. Obtain mass and MP

1. Dry over MgSO4

2. Gravity filter

3. Evaporate solvent

4. Distill

5. Obtain mass

Methyl benzoate Benzoic acid

• The entire procedure must be finished in one lab session

• Be well organized

Multiple ways to define yield Percent yield

Percent conversion - a measure of how much starting material was

consumed regardless of whether it was destroyed or converted to

product or side product

Process yield - a measure of how much of the starting material became

desired product [yield corrected for recovered starting material]

Equilibrium constant

At the end of the reaction, you will recover both methyl benzoate and

benzoic acid to determine Keq

How can we drive the reaction forward? Le Chatelier’s principle.

Keq calculation You will determine Keq (equilibrium constant) 2 ways –

• based on benzoic acid consumed

• based on methyl benzoate created

Benzoic

acid

Methanol Methyl

benzoate

Water

Initial Bi Mi 0 0

Change - x - x + x + x

equilibrium Bi - x Mi - x x x

Assume Bf = moles of benzoic acid at equilibrium, so Bf = Bi – x and x = Bi – Bf

Substitution gives:

Keq based on

methyl benzoate

recovered

Keq based on benzoic

acid recovered

Note: because all 4

components are in the same

volume, you can use moles

rather than concentration

Some Helpful Tips

Interpreting Esterification 1H NMR Spectra:

• For the starting material, the peak at 10.9 ppm

can be considered as integrating to 1

• broad peaks often have low integration values

• For the product, the peak at 3.898 ppm integrates

to 3 (in case you’re using an old version of the

lab manual)

More NMR examples