Steam Distillation

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Experiment
 2:
 Isolation
 of
 Eugenol
 from
 Cloves
 

 

Background;
 Readings
 on
 Vapor
 pressure,
 Raoult’s
 Law
 from
 TRO:
 
A mixture of the essential oils, eugenol and acetyleugenol, will be steam distilled from cloves. These compounds are isolated from aqueous distillate by extraction into dichloromethane. The dichloromethane solution is shaken with aqueous sodium hydroxide, which will react with eugenol, to yield the sodium salt of eugenol in the basic aqueous layer, and acetyleugenol in the organic layer. The basic aqueous layer can be acidified to re-extract eugenol from it. And the organic layer can be dried and concentrated to yield acetyleugenol The principle of steam distillation is based on the fact that two immiscible liquids will boil at a lower temperature than the boiling points of either pure component, because the total vapor pressure of the heterogeneous mixture is simply the sum of the vapor pressures of the individual components (i. e. PT = PoA + PoB, where Po is the vapor pressure of the pure liquids). This leads to a higher vapor pressure for the mixture than would be predicted for a solution using Raoult’s Law(applies for miscible mixtures) (that is PT = Po(A)n(A) + Po(B)n(B), where n is the mole fraction of the component in the mixture). The higher total vapor pressure leads to a lower boiling point for the mixture than for either single component. The boiling point of eugenol, an oil found in cloves, is 248 °C, but it can be isolated at a lower temperature by performing a co-distillation with water. Steam distillation allows separating substances at lower temperatures which is useful since many organic compounds tend to decompose at high temperatures which regular distillation would require. For steam distillation to be successful, the material to be isolated must be insoluble in water. Usually  these

 compounds
 have
 a
 low
 vapour
 pressure.
 After
 mixing
 them
 with
 water,
 however,
 the
 mixture
  will
 distil
 when
 the
 sum
 of
 the
 two
 vapour
 pressures
 reaches
 atmospheric
 pressure.
 It
 follows,
 then,
 that
 this
  must
 happen
 below
 the
 boiling
 point
 of
 water.
 
  Note that by steam distillation, as long as water is present, the high-boiling component vaporizes at a temperature well below its normal boiling point without using a vacuum. Since eugenol is not soluble in water, the concentration of the eugenol in the vapor over the boiling eugenol– water suspension does not depend on concentration of the eugenol. The relative amounts of eugenol and water in the vapor simply depend on the vapor pressures of the pure materials. The vapor pressure of water at 100 °C is 760 torr, and the vapor pressure of eugenol at 100 °C is approximately 4 torr; (Note, the suspension boils when it’s vapor pressure is equal to the external pressure. Since both the eugenol and the water are contributing to the vapor pressure of the suspension, the suspension will boil before either pure substance would normally boil.) Since the distillate will contain both water and eugenol, the eugenol must be extracted from the water using an organic solvent. Once the eugenol is extracted into an organic solvent,the organic layer is separated from the aqueous layer and dried. The eugenol is finally isolated by evaporation of the organic solvent. When  the

 sum
 of
 the
 separate
 vapor
 pressures
 equals
 the
 total
 pressure,
 the
 mixture
 boils
 and
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  P
 =P(A)
 +
 P(B)
  Where
  PA
 is
 vapor
 pressure
 of
 pure
 water
 A
  PB
 is
 vapor
 pressure
 of
 pure
 B
 

 

 
  1

Then
 the
 vapor
 composition...
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