Biochemistry: Reaction Mechanisms of Esters. Esters are among the most widespread of all naturally occurring compounds. Many simple esters are pleasant-smelling liquids that are responsible for the fragrant odors of fruits and flowers. For example, methyl butanoate is found in pineapple oil, and isopentyl acetate is a constituent of banana oil. The ester linkage is also present in animal fats and in many biologically important molecule.

1. Reaction
Mechanism
of Esters

2. What are ESTERS?
 Esters are among the most widespread of all naturally
occurring compounds.
 Many simple esters are pleasant-smelling liquids that
are responsible for the fragrant odors of fruits and
flowers. For example, methyl butanoate is found in
pineapple oil, and isopentyl acetate is a constituent of
banana oil.
 The ester linkage is also present in animal fats and in
many biologically important molecules.ion

3. Reaction Mechanism of Esters
1. Conversion of Esters into Carboxylic Acids
Hydrolysis
Fisher Esterification
2. Conversion of Esters into Amides
Aminolysis
3. Conversion of Esters into Alcohols
Reduction
Grignard Reaction

4. HYDROLYSIS
An ester is hydrolized, either by aqueous base or
by aqueous acid, to yield a carboxylic acid and
an alcohol.

5. Mechanism of base-induced
ester
Nucleophilic addition of hydroxide ion
to the ester carbonyl group gives the
usual tetrahedral alkoxide intermediate.
Elimination of alkoxide ion then
generates the carboxylic acid.
Protonation of the carboxylate ion by
addition of aqueous mineral acid in a
separate step then gives the free
carboxylic acid.
Alkoxide ion abstracts the acidic proton
from the carboxylic acid and yields a
carboxylate ion

6. FISCHER ESTERIFICATION
Mechanism of acid-
catalyzed ester.
The ester is first acticated
toward nucleophilic attack
by protonation of oxygen
atom, and nucleophilic
addition of water then
occurs. The transfer of a
proton and elimination of
alcohol yields the carboxylic
acid.
Mechanism of acid-
catalyzed ester. The forward
reaction is hydrolysis; the
backward reaction is fischer
esterification.
Protonation of the carbonyl
group activates the ester.
The ester is activated for
nucleophilic attack by water
to yield a tetrahedral
intermediate.
The transfer of a proton the
converts the OR' into a good
leaving group.
Expulsion of alcohol yields the
free carboxylic acid product
and generate the acid
catalyst.

7. HYDROLISIS FISHER ESTERIFICATION
Nucleophilic addition of
hydroxide ion to the ester
carbonyl group gives the usual
tetrahedral alkoxide
intermediate.
Elimination of alkoxide ion
then generates the carboxylic
acid.
Protonation of the
carboxylate ion by addition of
aqueous mineral acid in a
separate step then gives the
free carboxylic acid.
Alkoxide ion abstracts the
acidic proton from the
carboxylic acid and yields a
carboxylate ion
Protonation of the
carbonyl group activates
the ester.
The ester is activated for
nucleophilic attack by
water to yield a
tetrahedral intermediate.
The transfer of a proton
the converts the OR' into
a good leaving group.
Expulsion of alcohol yields
the free carboxylic acid
product and generate the
acid catalyst.

8. Esters react with amonia and amines to yield
amides. the reaction is not often used, however,
because it's usually easier to prepare an amide
by starting with an acid chloride.
AMINOLYSIS

9. The mechanism of ester reduction is similar to
that of acid chloride reduction in that a hydride
ion first adds to the carbonyl group, followed by
elimination f alkoxide ion to yield an aldehyde
gives the primary alcohol.
REDUCTION

10. Esters react with 2 equivalents of a Grignard reagent to
yield a tertiary alcohol in which two of the substituents
are identical. The reaction occurs by the usual
nucleophilic substitution mechanism to give an
intermediate ketone, whihc reacts further with the
Grignard reagent to yield a tertiary alcohol.
GRIGNARD REACTION

11. Economic
Value of
Esters

12. The table above displays common esters used for flavors and
fragrances. It shows the ester, what it smells like, and what alcohol
and acid it is made with. For instance ethyl butyrate smells like
pineapple and is prepared from the alcohol ethanol and the acid
butanoic acid.
1. FRAGRANCES AND ARTIFICIAL FLAVORS
Ÿ TABLE 1: Common Esters Used in Flavors and Fragrances

13. 2. SOAPS
 Saponification is when esters are reacted with a base in an
aqueous solution.
 Fats and oils are naturally ocurring esters.
 To produce soap, an animal fat or a vegetable oil is boiled with a
strong base, usually NaOH. This results in a soap that consists of a
mixture of sodium salts of long-chain carboxylic acids. These long-
chain carboxylic acids are called fatty acids which forms during
the saponification reaction.

14. R E F E R E N C E S
• All figures in this material are adopted from McMurry (2010) -
Organic Chemistry 8th Edition
• Table 1. Common Esters used for Flavors and Fragrances - PDF
• https://goo.gl/images/vLWQo4