Esterification oil of wintergreen.doc

7/30/2019 Esterification oil of wintergreen.doc

1/8

Fischer Esterification: Synthesis of Oil of Wintergreen

Introduction

Natural flavors and fragrances tend to be complex mixtures banana has at least two dozen components, for

example but often one or a very few of the components offer a pretty good duplication. Interestingly, only avery few functional groups are commonly found in pleasant flavors and fragrances, and of those, esters are

predominant. Examples shown in Figure 1 include banana, pineapple, apple, cherry, plum, apricot, strawberry,

pear, honey, wintergreen, and even rum and wine. Some esters ethyl acetate, butyl acetate, pentyl acetate,and isopentyl acetate are examples are components of many natural fragrances. *

O

O

O

O

O

O

O

O

O

O

O

OO

O

O

O

H O

O

O

O

OH

O

O

O

O

O

O

O

O

butyl formate

plumbutyl isobutyrate

pineapple

butyl propanoate

apricot

butyl pentanoate

applebutyl isopentanoate

apple

butyl acetate

pineapple

allyl phenylacetate

honey

allyl benzoate

cherry

ethyl formate

rum

ethyl acetate

"wine"

isopentyl salicylate

strawberry

methyl salicylate

wintergreen

OH

O

O

methyl butyrate

appleethyl isopentanoate

apple

phenethyl acetate

honey

O

O

benzyl butyrate

pear

O

O

O

O

pentyl acetate

banana

isopentyl acetate

banana

Figure 1. Representative esters and the flavor type of each. *

Written by John McCormick, Department of Chemistry, University of Missouri-Columbia.

*Source: Fenaroli, Gioanni,Fenarolis Handbook of Flavor Ingredients, Furia, Thomas E., and Nicolo Bellanca, Eds. The

Chemical Rubber Co., Cleveland, 1971, pp 643 672.


2/8

When you see food flavorings labeled imitation, you know that flavor came from a chemical company that

used some specific chemical processes to make the flavoring. And if the key chemical is an ester, its a good bet

that it was made from an acid by esterification the reaction used to transform an acid into an ester. By the


3/8

way, it is interesting that while esters generally smell great, the carboxylic acids they are made from often have

sharp (think vinegar) or bad (think sour milk) odors. In this laboratory, you will do an esterification reaction

(Equation 1) that transforms salicylic acid into methyl salicylate (oil of wintergreen).

(1)

OH

COH

O OH

COCH3

O

salicylic acid methyl salicylate

Synthesis of Esters

There are several procedures to make esters from the carboxylic acids. Among them, one Fischer

esterification has been used by chemists for well over a century because it gives highs yields and lends itself to

large-scale commercial use as well as very small scale research use. As indicated in Equation 2, Fischer

esterification is based on an equilibrium reaction (actually, a series of reactions), which means that something

has to be done to cause the equilibrium to strongly favor product formation. If the goal is a high-yield

conversion of the carboxylic acidinto the ester, there are two options to shift the equilibrium toward esterformation: use a large excess of the alcohol, or remove one of the products as the reaction proceeds. When the

esterifying alcohol is cheap and abundant, chemists use it in large excess, as you will in this lab. Alternatively,

because water is one of the two products and there are good ways to remove water, this also is a commonly used

strategy.

RCOH

O

+ ROH RCOR

O

H2O+(2)H+

Mechanism of Fischer Esterification

The mechanism (Equation 3) of the Fischer esterification reveals that it is a typical nucleophilic substitution at

acyl C=O: a substitution reaction that takes place by an initial addition of alcohol to the carbonyl group to form

a tetrahedral intermediate (II) and a subsequent elimination of water, which re-forms the carbonyl group. Thenet result of this addition-elimination process substitutes OR for OH. The mechanistic details shown in

Equation 3 emphasize the catalytic role of acid, first in increasing the rate of addition of alcohol to the acid (I)

and then in increasing the rate of loss of water from intermediate II to form the ester (III).

RCOH

O

+

ROH

RCOR

O

H2O

+

(3)

H3O+

RCOH

OH+

+ +RCOH

OH

RCOH

OH

ROH+

+

RCOH

OH

RO

H3O+

H2OI

II

II

III

H2ORCOH

OH

RO

RCOR

OH

+

+

+

RCOR

OHH3O+

H2O

RC

OH

RO

OH2

+

H3O+

H2O

+


4/8

Analysis of Products by Gas Chromatography

Once you obtain your product, how do you know that it has the expected structure and that it is pure? Well, IR

spectroscopy comes to mind as useful: The OH group, easily observed in the IR spectrum, of the starting

carboxylic acid should be absent in the IR spectrum of the product. In addition, thefingerprint region of the IR

should be an exact match with that of an IR taken of an authentic sample of your product. However, if your

product is contaminated by something else (including a small amount of starting carboxylic acid), the fingerprintregion will have additional peaks and therefore will be deceptive.

Gas-liquid chromatography, often simply called gas chromatography (GC), offers an alternative analytical

technique that lets you assess not only whether your product has the expected structure but also just how pure

your product is. Compared to the other two types of chromatography that you have seen thin layer

chromatography (TLC) and column chromatography GC is much more sensitive (It will detect a far smalleramount of material.) and is much more effective for separating molecules, even those having nearly identical

structures. Appendix A is devoted to a brief overview of gas chromatography, which you will use to analyze

your final product.

Distillation

Figure 2 shows the apparatus you should use for distillation. Be sure to use a thermometer adaptor. (Ask your

TA if you are uncertain what this is.) Also be sure your distillation apparatus is NOTa closed system! When

heated, the expanding gas in a closed system will blow it up!

condenser

clamp

ring stand

lab jack(to raise & lower heat)

round-bottomflask

wate

rin

wate

rout

heatingmantel

distillation head

thermometer

thermometeradapter

bent adapter

Erlenmeyerflask

clamp

ring stand

ring stand

clamp


5/8

Experimental Procedure

Safety Notes!

Solutions of H2SO4 are extremely corrosive. Avoid contact with skin and clothing. In case of

accidental contact, rinse the affected area with copious amounts of cold water.

Salicylic acid and methyl salicylate are toxic and may be absorbed through the skin. Avoid breathingdust, ingestion, and contact with the skin. In case of accidental skin contact, flush with copious

amounts of water.

Dichloromethane is toxic and has been categorized as a carcinogen. Avoid inhalation, ingestion, and

contact with skin. In case of accidental contact, wash with copious amounts of water.

Synthesis of Methyl Salicylate

Weigh 3.45 g of salicylic acid and 15 mL of methanol in a 50-mL round bottom flask. Swirl to dissolve the

salicylic acid, using a hot water bath to heat the mixture as needed to completely dissolve the acid. Then add to

the coolmethanol solution 5 mL of conc. H2SO4in small portions, taking care to see that the mixture does not

get too hot to touch the flask. You may observe formation of a white precipitate. Add an acid-resistant boilingchip and reflux the reaction mixture for 1 hour.

Use a cold-water bath to cool the reaction mixture to room temperature. Add 25 mL of H2O to a separatory

funnel and then, using a funnel, pour your reaction mixture into the separatory funnel. Add 10 mL of

dichloromethane to the emptied reaction flask, swirl to rinse it, and pour it into the separatory funnel. Carefully(frequent venting!!) shake the separatory funnel to extract your product, allow the layers to separate, and collect

the dichloromethane layer (Which layer is it?) in a 50-ml Erlenmeyer flask. Repeat the extraction two more

times, each time using 10 mL of dichloromethane and collecting all three extracts in the same Erlenmeyer flask.

Drain the aqueous layer into a beaker (larger than 100 mL capacity) labeled Waste aqueous solutions. Pour

the dichloromethane solution of unpurified methyl salicylate into the separatory funnel, and wash twice with 15

mL of H2O, then twice with 15 mL of 5% NaHCO3 solution (Careful!! Vent frequently! You may see

substantial evolution of CO2 as you neutralize any remaining acid.), and once with 15 mL of saturated NaCl

solution. Note: For each wash, you will keep the lower layer and drain the upper, aqueous layer into theWaste Aqueous Solutions beaker. Drain the washed dichloromethane solution into an Erlenmeyer flask. (Be

careful to avoid lettingany H2O drain into the flask.) Add a small amount of Na2SO4 (drying agent) and swirl

the flask. Add additional Na2SO4if necessary until some of the drying agent does not form in a clump. Allow

the mixture to stand 10 minutes, and then filter it through a funnel with cotton (or glass wool) loosely stuffed in

the stem into a tared (You must know its weight!) 50-mL round bottom flask. Remove the dichloromethane

using a rotovap and weigh your crude (unpurified) methyl salicylate. Weigh your sample of crude methylsalicylate and characterize it using both IR spectroscopy and gas chromatography. Please remember that you

need to compare the retention time and purity of your sample to the known standards. Explain any addition

peaks that show up in the chromatogram and in the IR as compared to the standards!!!!

Clean up

Discard all wastes in the appropriately labeled containers. No materials from this experiment go down thedrain! After you have poured all you can into the waste containers, wash glassware containing only residual

amounts (clinging to the sides of the glassware) in the sink.


6/8

Appendix A: Gas Chromatography

You will use gas chromatography (GC) to check the purity of your methyl salicylate (product) prepared by the

Fischer esterification reaction. GC works on the same principles as TLC and column chromatography (see

description in the chromatography lab), but for GC the mobile phase is a gas and the stationary phase is a liquid.

The components of the sample are continually partitioned between the vapor phase and the liquid phase.

Because the molecules that spend a greater proportion of time in the gas phase elute more rapidly, compoundswith higher vapor pressure (lower boiling point) elutes faster than those with lower vapor pressure (higher

boiling point). The chromatography column is housed in an oven so that the temperature may be controlled

anywhere up to about 300 C. Because vapor pressure increases with heating, compounds elute faster when the

temperature of the oven is increased. The length of time that a compound is retained in the column is known as

its retention time (TR), which can be used to help identify the compound, in the same manner as the TLC Rfvalue.

Two of the most common stationary phases used for GC are non-volatile polymers: dimethylsilicone (non-

polar) and polyethylene glycol (moderate polarity).

These liquid phases are coated on the surface of an inert support material and packed in a column made from

glass or metal tubing. A schematic diagram of a gas chromatography system is given in Figure 1. The column

is coiled and kept in an oven where the temperature can be controlled. The mobile phase, called the carrier gas,

(generally helium) flows through the column. The sample is injected (using a syringe) onto the column through

the injection port at the upstream end of the column. Only small amounts of sample are used, usually between

0.5 5 l. A detector at the end of the column generates a signal when a compound elutes this is recorded

Si OO

CH3

CH3

Si

CH3

CH3

n

Dimethylsilicone

Trade names: SE-30, OV-1

CH2O CH2n

Polyethylene glycol

Trade name: Carbowax


7/8

Figure 1. The components of a typical gas chromatograph. The recorder prints the

chromatogram, which has a peak for each component, proportional in area to the amountof that compound present in the mixture. Right: A method for determining peak areas.

He in

Inject sample

Column

Oven

Detector

Recorder

w1/2h

Area = w1/2 x h


8/8

using a strip-chart recorder. Several types of detectors are available. You will be use a gas chromatograph

equipped with a thermal conductivity ("hot-wire") detector. This detector consists of a wire placed across the

end of the column. The electrical resistance of the wire depends on its temperature. When only the helium

carrier gas is passing over the wire, the voltage level is constant, which results in a straight line on the recorder.When a component from the mixture elutes from the column and passes over the wire, it causes a change in the

wire temperature, which causes a change in the voltage, which causes a change in the signal sent to the recorder.Therefore, each component eluting from the gas chromatograph appears as a peak on the recorder trace, called

the chromatogram. The area of each peak is proportional to the amount of that compound if twice as much

sample is injected, the areas of the peaks double. Therefore, thepeak areas can be used as a quantitative

measure of the amount of each component in the mixture.

Your GC chromatogram will show a peak for your ester (product) and for the unreacted alcohol, if any

contaminates your product. In addition, a small amount of air is always injected along with the sample, and thisalso gives a small peak. Since air spends all of its time in the mobile phase, the TR for the air corresponds to the

time required for the helium to pass through the column.

Esterification oil of wintergreen.doc

Documents

Transcript of Esterification oil of wintergreen.doc