Download 39 ESTERIFICATION: PREPARATION OF BENZYL ACETATE

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ESTERIFICATION: PREPARATION OF BENZYL ACETATE
MICROSCALE EXPERIMENT IV
O
R
C
O
R'
The ester group is an important functional group that can be synthesized in a number of
different ways. The low molecular-weight esters have very pleasant odours and indeed are major
components of the flavour and odour aspects of a number of fruits. Although the natural flavour
may contain nearly a hundred different compounds, single esters approximate the natural odours
and are often used in the food industry for artificial flavours and fragrances (see Table on p. 40).
Esters can be prepared by the reaction of a carboxylic acid with an alcohol in the
presence of a catalyst such as concentrated sulfuric acid, hydrogen chloride, p-toluenesulfonic
acid, or by using the acid form of an ion-exchange resin, for example:
O
CH3
C
acetic acid
O
O
H
CH3OH
methanol
HCl
CH3
C
O
CH3
H 2O
methyl acetate
This Fischer esterification reaction reaches equilibrium after a few hours of refluxing.
The position of the equilibrium can be shifted by adding more of the acid or of the alcohol
depending on cost or availability. The mechanism of the reaction involves initial protonation of
the carboxyl group, attack by the nucleophilic hydroxyl group of the alcohol, a proton transfer,
and loss of water, followed by deprotonation to give the ester. Because each of these steps is
completely reversible, this process is also, in reverse, the mechanism for the hydrolysis of an
ester.
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TABLE. Fragrances and Boiling Points of Esters
Ester
O
Isobutyl formate
Boiling
Point (°C)
Formula
Fragrance
CH3
HCOCH2CHCH3
98
Raspberry
102
Pear
102
Apple
121
Pineapple
137
Rum
142
Banana
206
Peach
210
Orange
222
Wintergreen
O
n-Propyl acetate
CH3COCH2CH2CH3
O
Methyl butanoate
CH3CH2CH2COCH3
O
Ethyl butanoate
Isobutyl
propanoate
CH3CH2CH2COCH2CH3
O
CH3
CH3CH2COCH2CHCH3
O
Isoamyl acetate
CH3
CH3COCH2CH2CHCH3
O
Benzyl acetate
CH3COCH2
O
Octyl acetate
CH3COCH2(CH2)6 CH3
O
COCH3
Methyl salicylate
OH
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Other methods are available for the synthesis of esters, most of them more expensive but
readily carried out on a small scale. For example, alcohols react with anhydrides and with acid
chlorides:
O
O
O
CH3CH2OH + CH3C O CCH3
Ethanol
O
CH3COCH2CH3 + CH3COH
Acetic anhydride
Ethyl acetate
Acetic acid
O
O
CH3COCH2CH2CH3 + HCl
CH3CH2CH2OH + CH3CCl
1-Propanol
Acetyl chloride
n-Propyl acetate
Pyridine is usually added to react with the hydrogen chloride.
A number of other methods can be used to synthesize the ester group. Among these are
the addition of 2-methylpropene to an acid to form t-butyl esters and the reaction of a silver salt
with an alkyl halide:
O
CH3
CH2
CCH3 + CH3CH2COH
2-Methylpropene
(isobutylene)
O
O CH3
H+
CH3CH2COCCH3
CH3
t-Butyl propanoate
Propanoic acid
(propionic acid)
CH3
O
CH3CO-Ag+ + BrCh2CH2CHCH3
Silver acetate
CH3
CH3COCH2CH2CHCH3
1-Bromo-3-methylbutane
Isoamyl acetate
As noted above, Fischer esterification is an equilibrium process. Consider the reaction of
acetic acid with 1-butanol to give n-butyl acetate:
O
CH3COH + HOCH2CH2CH2CH3
H+
O
CH3COCH2CH2CH2CH3 + H2O
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The equilibrium constant is given by
K eq =
[n − BuOAc][H 2 O]
[n − BuOH][H 2 OAc]
For primary alcohols reacting with unhindered carboxylic acids, Keq . 4.
If equal
quantities of 1-butanol and acetic acid are allowed to react, at equilibrium the theoretical yield of
ester is only 67%. To upset the equilibrium we can, by Le Châtelier’s principle, increase the
concentration of either the alcohol or acid, as noted above.
If either one is doubled, the
theoretical yield increases to 85%. When one is tripled, it goes to 90%. But note that in the
example cited, the boiling point of the relatively nonpolar ester is only about 8°C higher than the
boiling points of the polar acetic acid and 1-butanol, so a difficult separation problem exists if
either starting material is increased in concentration and the product is isolated by distillation.
Another way to upset the equilibrium is to remove water. This can be done by adding to
the reaction mixture molecular sieves, an artificial zeolite, which preferentially adsorb water.
Most other drying agents, such as anhydrous sodium sulfate or calcium chloride, will not remove
water at the temperatures used to make esters.
Reference:
Organic Chemistry (5th edn., 2000) by J. McMurry; Sections 21.3, 21.6.
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Experimental Procedure Preparation of Benzyl Acetate from Acetic Anhydride and Benzyl
Alcohol
O
O
O
CH2OH2 + CH3C O CCH3
O
CH2OCCH3 + CH3COH
To a round bottomed flask add 108 mg of benzyl alcohol and 102 mg of acetic anhydride and a
boiling chip. Reflux the mixture for at least 1 hour, cool the mixture to room temperature, and
chromatograph the liquid as follows: Assemble the column as
shown in the Figure, being sure it is clamped (with one clamp) in
a vertical position. Close the valve and fill ¼ of the column with
dichloromethane
— DICHLOROMETHANE
IS
VERY
VOLATILE — to the bottom of the funnel. Prepare a slurry of
10 mL silica gel in 4 mL of dichloromethane in a small beaker.
Stir the slurry gently to get rid of air bubbles and gently swirl,
pour and scrape the slurry into the funnel. After some of the
silica gel has been added to the column, allow the solvent to
drain slowly into an Erlenmeyer flask. Use this dichloromethane
to rinse the beaker containing the silica gel. As the silica gel is
being added, VERY GENTLY tap the column with a pencil so
the adsorbent will pack tightly into the column. Continue to tap
the column while cycling the dichloromethane through the column once more and then add a
sample of anhydrous potassium carbonate to the top of the silica gel. The potassium carbonate
will remove water from the esterification mixture as well as react with any carboxylic acid
present. Run the solvent down to the surface of the potassium carbonate.
Using a Pasteur pipet, add the sample to the column and let it run into the adsorbent,
stopping when the solution reaches the top of the potassium carbonate. The flask is rinsed twice
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with 0.5 mL portions of dichloromethane that are run into the column, with the eluent being
collected in a tared sample bottle. The elution is completed with 4 mL more dichloromethane.
Evaporate the dichloromethane on the steam bath in the hood. Since the dichloromethane
boils at 40°C and the product at 206°C, separation of the two is easily accomplished. Determine
the weight of the product and calculate the yield.
The ester should be a perfectly clear,
1
homogeneous liquid. An IR spectrum and a H NMR spectrum of benzyl acetate are supplied.
Analyze it for the presence of unreacted alcohol and acetic anhydride. (These spectra can be
found on page 45 - 47.)
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