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Department of Chemistry, University of Missouri – Kansas City, 2007 – CHEM 322L
10. Fisher Esterification: Preparation
of Banana Oil
M. Jones: Reactions of Esters: Addition - Elimination, Chapter 19.7, pgs 1049- Fischer Esterification, Figures 19.32 and 19.33, pgs 1049-
This procedure has been adapted from the microscale procedure described in the third edition of Macroscale and Microscale Organic Experiments by Kenneth L. Williamson (Houghton Mifflin, Boston, 1999).
Background
Esters. Esters have the structure of -COOR. Instead of the alcohol portion of the carboxylic acid (-COOH), there is an ether portion. The low boiling, volatile esters are known for their "fruity" smell and flavor. They are used in artificial flavorings. Some typical esters, which are used as artificial flavors are listed in the table below ( Operational Organic Chemistry , John W. Lehman, Third Edition, Prentice Hall, 1999). The "taste" of a candy or food is a special science, food chemistry. It is an art to create are combinations of different flavorings that imitate a "natural" flavor.
Table 1. Name Structure Flavor
1- propyl acetate CH 3 CO 2 CH 2 CH 2 CH 3 pear
octyl acetate CH 3 CO 2 (CH 2 ) 7 CH 3 oranges
isopentyl acetate CH 3 CO 2 CH 2 CH=C(CH 3 ) 2 "Juicy-Fruit"
isopentyl acetate CH 3 CO 2 CH 2 CH 2 CH(CH 3 ) 2 banana
isobutyl propionate CH 3 CH 2 CO 2 CH 2 CH(CH 3 ) 2 rum
ethyl butyrate CH 3 CH 2 CH 2 CO 2 CH 2 CH 3 pineapples
In this experiment, you will be performing a Fischer esterification. The general mechanism is pictured in Figure 1.
R
O
O H
R
O
O R
ROH 2 ROH
Figure 1. The overall reaction for Fischer esterification.
The overall mechanism for a general acid and alcohol is depicted in Figure 2.
R
O
O H
H OR
H
R
O
O H
H R OH
OH
OR H
R OH
OH
OR
H OR
H
R OH
OH (^2)
OR
R
O
OR
H R
O
OR
L
ROH 2 ROH ROH (^) ROH
ROH
P A^ D
P (^) D
Figure 2. The general mechanism for Fischer esterification.
The overall mechanism follows the normal PADPLD. The acid catalyst is involved in the first step and regenerated in the last step. The first step is protonation of the carbonyl oxygen, followed by addition of the alcohol to form a tetrahedral intermediate. The third and fourth steps involve a proton transfer to form water, a good leaving group. After the water leaves in the fifth step, there is just the deprotonation of the carbonyl oxygen to form the neutral species.
In your reaction, the synthesis of banana oil – isopentyl acetate, you will be using isopentyl alcohol (3-methyl-1-butanol) and acetic acid (ethanoic acid, see Figure 3).
H 3 C
O
O H
- HO (CH 2 ) 2 CH(CH 3 ) 2 H 3 C
O
O (CH 2 ) 2 CH(CH 3 ) (^2)
Acetic acid (ethanoic acid) MW 60.1, d 1. bp 118 o^ C
Isopentyl alcohol (3-methyl-1-butanol) MW 88.1, d 0. bp 129 o^ C
Isopentyl acetate (3-methyl-1-butyl acetate) MW 130.2, d 0. bp 142 oC
H 2 SO 4
Figure 3. The overall reaction for this experiment.
