Why does esterification need an acid catalyst?

The π bond of the carbonyl group can act as a base to a strong inorganic acid due to the distortion of the electrons from the electronegativity difference between the oxygen atom and the carbon atom and also the resonance dipole. The cation produced in the reaction with sulfuric acid will have resonance stabilization.

Table of Contents Show

• HSC Chemistry Syllabus
• Production of Simple Esters, Reflux, Isolation and Purification
• Fisher Esterification
• Role of Concentrated H2SO4
• Reaction Conditions for Esterification
• Why is Reflux Important?
• Isolation of Ester
• Purification of Ester
• BACK TO MODULE 7: ORGANIC CHEMISTRY

Step 1: Formation of cation

Step 2: The methanol can act as a nucleophile to a carbocation. Remember that there are many methanol molecules in the solution...it is always in excess in this reaction.

Step 3: The protonated ether can leave as methanol but that will not accomplish anything. A proton can be transferred to one of the hydroxyl groups and thus make it a good leaving group.

Step 4: The alcohol oxygen atom from the hydroxy group can donate a pair of electrons to the carbon atom making a π bond and eliminating water. The water will not be a viable nucleophile that will reverse the reaction because its concentration will be low compared to the concentration of the methanol.

Step 5: The water will be in too low a concentration to reverse the reaction.

Richard Banks (Boise State University)

Footnotes:

(1) For this tutorial, we will only be concerned with examples where R is an alkyl group (that is a group derived from the alkane series). And, we only be dealing with esters derived from straight chain carboxylic acids, so the name of the ester will be two words. It should be noted that the names of more complex esters will be made up of more than 2 words.

If more than organyl group (alkyl, aryl etc) is present, they are cited in alphabetical order.

(2) During the esterification reaction, the C-O bond in the carboxylic acid is broken. Evidence for this comes from experiments using isotopically labeled reactants.

If some of the alcohol used contains the 18O isotope, the atoms of this isotope are all found in the ester product and NOT in the water.

(3) There are other ways to produce esters besides the direct (Fischer) esterfication of a carboxylic acid with an alcohol. (a) Alcoholysis of acid chlorides, anhydrides, or nitriles, produces esters.

(b) Reaction of the salt of a carboxylic acid with an alkyl halide or sulfate will produce an ester.

(4) If a liquid is largely free of air, and the glass flask is clean and very smooth, the liquid may superheat, a condition in which the temperature of the liquid rises above its boiling point. In a superheated liquid it is hard to form vapour bubbles resulting in irregular expulsion of bubbles of vapour, or, bumping.

Boiling chips (pieces of porous pot) provide an additional source of minute air bubbles which act as a nucleus for building bubbles of vapour in the liquid, allowing the liquid to boil quietly.

(5) An even better way to improve ester yield is to use anhydrides in the synthesis instead of carboxylic acids. For example, the reaction between ethanol and acetic anhydride is irreversible, and goes to completion within minutes.

acetic anhydride + ethanol → ethyl acetate + acetic acid

(6) Although sulfuric acid plays a vital role in the esterification reaction mechanism, it is beyond the scope of this tutorial. Suffice it to say that since the sulfuric acid that is used during in the reaction is re-produced at the end of the reaction mechanism, sulfuric acid is acting as a catalyst for the reaction.

(7) For a saturated sodium carbonate solution, dissolve 4.5 g sodium carbonate in 15 mL of distilled water.

(8) If you are an organic chemist you will name HO-CO-O-Na+ as sodium hydrogen carbonate, but if you are an inorganic chemist you will probably name it sodium hydrogencarbonate.
Another common name for the same compound is sodium bicarbonate.

(9) Anhydrous magnesium sulfate is used as a drying agent in preference to fused or "anhydrous" calcium chloride which can combine with some esters.
Note the use of the term "drying agent". No chemical change is involved in using a drying agent, the drying agent is used to remove excess water molecules from the mixture.
In a dehydration reaction, however, there is a chemical change. Atoms of hydrogen and oxygen are removed from reactant molecules and produce water as product.

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

– concentrated H2SO4 as a catalyst and dehydrating agent

– reflux, isolation and purification of esters

Production of Simple Esters, Reflux, Isolation and Purification

Fisher Esterification

Fisher esterification refers to the reaction between an alcohol and a carboxylic acid which produces an ester and water.

• slow
• reversible 
• exothermic because more energy is released in the formation of bonds in the ester and water molecule than the energy absorbed to break bonds.

The reversible nature of esterification means that water hydrolyse (react with) an ester to form a carboxylic acid and an alcohol.

Role of Concentrated H2SO4

• Concentrated sulfuric acid acts as a catalyst: increases reaction rate by lowering the activation energy. As a result the reaction rate is increased.

• Concentrated sulfuric acid acts as a dehydrating agent: by removing water from the reaction, the equilibrium position is shifted to the product side. As a result, the equilibrium yield of ester is increased. 

Therefore, concentrated sulfuric acid increases both reaction rate and yield.

Reaction Conditions for Esterification

Esterification is conducted under heat with reflux at 140 – 180ºC.

 
Heat is required to meet the activation energy of the reaction and increase reaction rate. Heat should be supplied using a heating mantle (hotplate). Bunsen burner should not be used in esterification because alcohol is flammable.

 
Reflux is the process of condensing gaseous products back into liquid state, allowing them to return to the reaction mixture. This is achieved by passing cool/cold water into the condenser and letting it absorb heat from the gaseous inside the reflux column. It's better practice to let the cold water enter the condenser from the bottom and exit from the top such that it flows against gravity and spends more time in the condenser. When the gaseous molecules lose heat, their temperature eventually falls below the boiling point. 

Esterification is performed in a round bottom flask to promote uniform heating of the reaction mixture. 

Boiling chips are commonly used to avoid superheating and achieve safe practice in school laboratories. Superheating is a phenomenon in which a liquid is heated to a temperature above its boiling point, without boiling. Superheated solutions can flash boil, causing a sudden increase in pressure and often results in flask breakage.

Why is Reflux Important?

Reflux is important for several reasons:

• Allows heat to be used and therefore increases the reaction rate.

• Prevents the loss of volatile substances e.g. ester, alcohol, carboxylic acid by returning them into liquid states (condensation). As a result, the yield of ester is increased.

• Allows esterification to be carried out in an open chemical system by constantly releasing pressure from inside the reaction chamber. This prevents the round bottom flask from shattering from high pressure.

Isolation of Ester

The ester produced from Fisher esterification will be in an equilibrium mixture with unreacted alcohol and carboxylic acid. Sulfuric acid is also present as catalysts are not consumed in chemical reactions.

Ester is isolated from the mixture by using a separating funnel.

NaHCO3 or Na2CO3 (weak bases) is added to the reaction mixture at the end of esterification to neutralise any remaining, unreacted carboxylic acid. This reaction produces soluble salts. Separating funnel separates substances based on their solubility and density. Unreacted alcohol and salts formed from neutralisation are soluble in water whereas esters are typically sparingly soluble at best but more commonly miscible with water. When Na2CO3-treated mixture is added to the separating funnel, two distinct layers will form: Organic layer which contains the ester is formed at the top Clear aqueous layer which contains the alcohol and soluble salts is formed below the organic layer due to its higher density. The aqueous layer can be discarded by opening the stopcock of the separating funnel, leaving behind the organic layer. This step is repeated several times until only the organic layer remains in the funnel.

Figure: separating funnel with a non-aqueous organic layer and an aqueous inorganic layer.

Purification of Ester

After the ester-containing organic layer is separated from the aqueous layer, purification should be carried out. This is because unreacted alcohol and carboxylic acid can be found in the organic layer (particularly those with lower solubility in water). 

Distillation purifies esters by making use of its lower boiling point compared to other substances in the mixture. Temperature of the mixture is raised just above the ester’s boiling to allow for its evaporation. Gaseous ester is returned to liquid state as it goes through a condenser (similar to reflux). Liquid ester is then collected in a separate vessel as the distillate. The distillate can be confirmed to be an ester by smelling its aroma from a safe distance.

Figure: distillation set-up used to purify esters

BACK TO MODULE 7: ORGANIC CHEMISTRY

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