Why Cant You Continuously Distil Esters

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

HSC Chemistry Syllabus

• investigate the production, in a school laboratory, of simple esters – Fisher esterification

– 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.

The reaction is considered a condensation reaction due to the production of a small molecule i.e. water alongside the main organic product. Condensation reactions can produce molecules other than water.

Esterification is

• 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 ConcentratedH₂SO₄

Concentrated H₂SO₄ is an important reagent for Fisher esterification due to two reasons:

• 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 shouldnot 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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Source: https://scienceready.com.au/pages/esterification