Esterification

 Many esters have pleasant odors and some of them can be found in nature   

Compound

Isoamyl acetate Ethyl butyrate Octyl acetate Methyl anthranilate Methyl butyrate Benzyl acetate Methyl salicylate Menthyl acetate

Flavor/Fragrance

Banana Oil Pineapple Orange Grape Apple Peach Oil of Wintergreen Peppermint Esters are often used in fragrances or flavoring agents due to their organoleptics properties Some esters are used as sex pheromones i.e., isopropyl dodecenoates (attracts female beetles, used in alternative pest control) or alarm pheromones i.e., isoamyl acetate (honey bee)) Ester of

p

-aminobenzoic acid are used as local anesthetics with a short to moderate half-life (benzocaine (ethyl), procaine (2-(diethylamino)ethyl)), propoxycaine, etc.



Esters can be obtained by a broad variety of reactions



Fischer esterification (used in Chem 30BL)

  H + O RCO 2 H + R'OH R C OR' + H 2 O  This approach works well for primary and most secondary alcohols, but

not

acid alcohol for tertiary alcohols because of their high tendency to eliminate water instead of forming an ester ester 

Acyl chloride

 Works for tertiary alcohols as well because of non-acidic conditions  O OH SOCl 2 -HCl, SO 2 O Cl ROH/Pyridine -PyH + Cl O OR

Schotten-Baumann Esterification

 Accessibility of SOCl 2

Anhydride

  Accessibility of anhydrides Often also requires an acidic catalyst i.e., aspirin synthesis O O O ROH/H + O O OR OH ROH/H + -H 2 O O O OR OR

  

Example 1: Aspirin (Bayer AG,1899)

It uses salicylic acid, acetic acid anhydride and a mineral acid as catalyst (usually conc. phosphoric acid) The phenol group acts as the alcohol in the reaction O OH OH O O [H + ] O O O OH

Aspirin

O Serin group in cyclooxygenase is blocked and therefore the prostagladin synthesis suppressed CH 2 OH CH 2 O O O + HO HO  Aspirin is considered a pro-drug for salicylic acid, which was long known (Hippocrates, bitter willow bark extract, 5 nausea th century BC) to work against inflammations and fever but it caused vomiting and

   Intramolecular esterification afford lactones These reactions can usually be carried out under mild conditions

Example 2: GHB (

g

-hydroxybutyric acid)

O HO COOH H + O

Lacton

mild conditions GHB GBL     It is used as date rape drug:

Liquid Ecstasy

It is colorless, odorless and has a slightly salty taste It is very dangerous because the effect of the drug differs greatly More than 200 deaths and more than 5700 overdoses have been attributed to this drug since 1990. The major problem is that humans can have very different reactions to this drug particular in connection with the consumption of alcohol.

  

Example 3: Biodiesel

It has gained a lot of interest lately due to the ever increasing cost of gasoline over the past decades It uses renewable resources i.e., plant oils, algae, grease, etc.

O O O R 2 R 1 O OH R 1 O O NaOH O + 3 CH 3 OH + R 2 O O O R 3 OH OH O R 3 O    Methanol Triglyceride Methyl esters of f atty acids Glycerin

Trans-esterification

: It converts oils into methyl esters that are much more volatile than oils due to their lower molecular weight It is important that water is absent during the reaction to avoid the formation of soap (Na + RCOO , where R= C 17 H 35 , etc.) Food vs. Fuel debate (i.e., 80 gal/acre for soy and sunflower)   42 billion gallons of diesel in 2008 in the US: 525,000,000 acre (21 % of the US) If the gasoline is also included (135 billion gallons) about 85 % of the area is needed!



In the lab, an unknown carboxylic acid is reacted with an unknown alcohol (both assigned by the TA)

RCO 2 H + R'OH H + R O C OR' + H 2 O acid alcohol ester 

Problems:

     The reaction is an equilibrium reaction with poor yields if a

1:1

-ratio of the reactants is used The carboxylic acid is a poor electrophile (neutral) The alcohol is a poor nucleophile (neutral) The reaction is very slow at room temperature Any water in the reaction mixture lowers the yield significantly



Mechanism of Fischer esterification

  R C OH + H + R OH C OH R H OH C OH O R' R O – C OH + R OH C OH – H + (charge separation) activated carbonyl (no charge separation) better electrophile R OH C OH O R' + H + R O C OR' – H + R + OH C OR' –H 2 O R •• OH C O H H 2 O is a good leaving group H OR' In the neutral state, the resonance structure with the charge separation is a very minor contributor making the carbonyl function of the carboxylic acid a poor electrophile The situation changes in the protonated form of the carboxylic acid in which the carbonyl carbon bears a larger positive charge (~0.2 units in the case of acetic acid), which makes it a better electrophile



Le Châtelier Principle

 If equimolar amounts of the carboxylic acid and the alcohol were used, the theoretical yield would be low i.e., 67 % (K eq =4) K eq = R O C OR' H 2 O ­ 1–10 RCO 2 H R'OH     One or all products have to be removed from equilibrium  An excess of one the reactants has to be used  The carboxylic acids cannot be used in excess because all of them are solids  The reaction in the lab uses an excess of the alcohol   The alcohol doubles up as the solvent and as a reactant in the reaction  Usually about 4-10 fold molar excess in the literature (a five-fold molar excess is used in the Chem 30BL lab i.e., 10 mmol of the carboxylic acid are reacted with 50 mmol of the alcohol)



A very strong mineral acid is used as catalyst

  The carboxylic acid is neutral and a weak electrophile The mineral acid protonates the carbonyl carbon atom and increases its electrophilic character 

It is very important to reflux the mixture properly to increase the rate of the reaction i.e., for every 10

o

C temperature increase, the rate of the reaction about doubles (

Arrhenius equation

)

rate



A

*

e



E a RT

  

Which compounds are present in the reaction mixture after the reaction

Water Extraction:

is completed?

 organic phase RCO 2 H R'OH RCO 2 R' H 2 SO 4 1. mix RCO 2 R' RCO 2 H H 2 SO 4 be a small amount), sulfuric acid (used as the catalyst) aqueous phase RCO 2 H ROH H 2 SO 4 All of the alcohols (log K ow = -0.77 (MeOH), -0.24 (EtOH), 0.05 ( (water just added)

iso-

PrOH),

t >> 0

0.25 (PrOH)) and the sulfuric acid (log K ow = -2.20) are soluble in water The carboxylic acid and the sulfuric acid can be extracted with a weak base i.e., NaHCO 3 while the ester and traces of the alcohol remain in the organic layer. They are separated by a fractionated vacuum distillation later.

NaHCO 3 Extraction: organic phase aqueous phase (5% NaHCO 3 ) RCO 2 H R'OH RCO 2 R' H 2 SO 4

t = 0

(5% NaHCO 3 just added) 1. mix R'OH RCO 2 R' R'OH Na + RCO 2 – Na + HSO 4 -

t >> 0

 Sodium hydroxide

cannot

be used for the extraction step because it would destroy the ester (saponification) due to its higher nucleophilicity R O OR' + OH R

base-catalyzed ester hydrolysis e.g. conversion of f ats to soap

O O + R'OH



Dissolve the carboxylic acid in the alcohol in a 10 mL round-bottomed flask (both assigned by your TA)

 

Add a few drops of concentrated sulfuric acid Reflux the mixture for at least 60 minutes (the longer the better)

 How much of the acid is used for the reaction?

1.0 g

 How much alcohol should be used here?

5 mol equivalents

 Why is a 10 mL round bottomed flask used here?

 How much is appropriate here?

3-4 drops

 What does this imply in terms of equipment and setup?

1. Air condenser with wet paper towel 2. Stir bar 3. Drying tube with CaCl 2

 Cool the mixture down  Add ice-cold water to the reaction mixture  Remove the organic layer     How can this be accomplished quickly?

Ice-bath

Which container should be used here?

Centrifuge tube

Why is the water added?

How much water should be added?

Until a phase separation is observed usually 4-8 mL

  What should the student observe/not observe here?

The formation of a solid is bad

Which one is the organic layer here?

Usually the bottom layer=ester

  Extract the aqueous layer with diethyl ether Combine

all

organic layers  Extract the combined organic layers with sodium bicarbonate solution  Why is the aqueous layer extracted with ether?

To collect the suspended and dissolved ester

     How much ether should be used here?

2 x 3 mL

Which layers does this referred to?

Ester + two ether layers

Why is this step performed?

How much solution is used here?

1-2 mL

How many extractions should be performed?

Until the CO 2 formation ceases

   Dry the organic layer over anhydrous sodium sulfate Remove the ether and remaining alcohol using the rotary evaporator Perform vacuum distillation    How much drying should be used?

A small amount to start with!

Why is a vacuum distillation performed here? 100 10

Vapor Pressure of Methyl benzoate

64; 5 77; 10 92; 20 151; 200 131; 100 117; 60 108; 40 200; 760 175; 400 1 20 39; 1 70 120

Boiling Point ( o C)

170 What is the setup for the vacuum distillation?

 

Note that the drying tube does not contain cotton or CaCl 2 !

Collect product from Hickman head Acquire an infrared spectrum and the refractive index of the ester. Submit the rest of the sample, even if it is solid or semi solid) for NMR analysis (label vial and sign in the sample as well)  What should the student do if he had a liquid in the Hickman head and also in the flask/vial?

Acquire an infrared spectrum for both liquids and only submit the “ester” for NMR analysis



Infrared spectrum



Benzoic acid

  n (C=O)=1689 cm -1 n (OH)=2300-3300 cm -1 

Methanol

  n (OH)=3347 cm -1 n (C-OH)=1030 cm -1 

Methyl benzoate

  n (C=O)=1724 cm -1 n (COC)=1112, 1279 cm -1 (absence of OH peak!) n (OH) n (OH) n (C=O) n (C=O) n (C-OH) n (COC)



Refractometry

 The refractive index is a physical property specific to a compound  

light

 angle theta Light is refracted when passing through any medium (Snell’s Law) In the lab, it is used to determine identity and purity of a sample using an Abbé refractometer  angle phi con den sed medium Adjust the height of the dark field so that the edge intersects with the crosshair before adding sample to refractometer  after adding sample to refractometer (ideal) after adding sample to refractometer (non ideal) The refractive index is wavelength and temperature dependent ( l =589 nm, T=recording temperature)

X

n

D

= n

T D

+ (T-X)*0.00045

surface

 1

H-NMR spectrum for methyl benzoate

8.0

7.5

7.0

6.5

6.0

9.5

9.0

8.5

2.0

1.5

1.0

0.5

0.0

3.5

3.0

2.5

5.5

5.0

4.5

4.0

O O CH 3 s, 3H OCH 3 8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

d, 2H ortho t, 1H para t, 2H meta 0.0

8.40

8.30

8.20

8.10

8.00

7.90

7.80

7.70

7.60

7.50

7.40

7.30

7.20

7.10

7.00

 100 95 115 110 105 120 13

C-NMR spectrum for methyl benzoate

120 128.40(5;3) 129.70(2;6) 6 O 10 7 5 1 O 8 CH 9 3 115 110 105 100 95 4 2 90 90 3 85 85 80 80 75 70 65 60 55 35 30 25 50 45 40 20 15 166.80(7) 10 5 0 170 160 150 132.80(4) 140 130 120 110 100 76.28(Sol.) 77.08(Sol.) 77.88(Sol.) 90 80 70 51.00(9) 60 50 40 20 15 10 5 0 45 40 35 30 25 60 55 50 75 70 65 170.39(7) 180 170 160 150 128.45(5;3) 130.19(2;6) 133.76(4) 140 130 130.02(1) 120 5 4 110 6 3 100 1 2 90 O 9 7 76.28(Sol.) 77.08(Sol.) 77.88(Sol.) 80 OH 8 70

17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 

What is that?

120 115 110 105 100 95 90 20 15 10 5 50 45 40 35 30 25 85 80 75 70 65 60 55 8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0 170.39

166.30

150 132.40

133.76

128.00

128.45

129.50

130.19

100 76.28

77.08

77.88

65.90

60.80

50 15.20

14.40

18.14

0

        The reaction should be started as soon as possible in order to have a long enough reaction time Dry glassware is very important here The reaction mixture has to be properly refluxed The air condenser has to be properly cooled with a wet paper towel, which has to have an

intimate contact

with the air condenser (no Hickman head here!!) The purer the final product is, the easier the analysis of the NMR spectra will be The student should submit something even if it is a “solid” that just had a pleasant odor to it It is advisable to acquire a refractive index of the alcohol Do not obtain the melting point for the carboxylic acid