Steam Distillation of Lemongrass Oil
I. Introduction
When a mixture of cyclohexane and toluene is distilled, the boiling point of these two miscible liquids is between the
boiling points of each of the pure components. By contrast, if a mixture of benzene and water (Immiscible liquids) is
distilled, the boiling point of the mixture will be found below the boiling point of each pure component. Since the two
liquids are essentially insoluble in each other, the benzene molecules in a droplet of benzene are not diluted by water
molecules from nearby water droplets, and hence the vapor pressure exerted by the benzene is the same as that of
benzene alone at the existing temperature. The same is true of the water present. Because they are immiscible, the two
liquids independently exert pressures against the common external pressure, and when the sum of the two partial
pressures equals the external pressure, boiling occurs. Benzene has a vapor pressure of 760 mm Hg at 80°C, and if it is
mixed with water, the combined vapor pressure must equal 760 mm Hg at some temperature below 80°C. This
temperature, the boiling point of the mixture, can be calculated from known values of the vapor pressure of the separate
liquids at that temperature. Vapor pressures found for water and benzene in the range 50 to 80°C are plotted in the figure
to the right. The dotted line cuts the two curves at points
where the sum of the vapor pressures is 760 mm Hg;
hence this temperature is the boiling point of the mixture
(69.3°C).
Practical use can sometimes be made of the fact that
many water-insoluble liquids and solids behave as
benzene does when mixed with water, volatilizing at
temperatures below their boiling points. Thus
naphthalene, a solid, boils at 218°C but distills with
water at a temperature below 100°C. Since naphthalene is
not very volatile, considerable water is required to entrain
it, and the conventional way of conducting the distillation
is to pass steam into a boiling flask containing
naphthalene and water. The process is called steam
distillation. With more volatile compounds, or with a
small amount of material, the substance can be heated
with water in a sample distillation flask and the steam
generate in situ:
Some high-boiling substances decompose before the
boiling point is reached and, if impure, cannot be purified
by ordinary distillation. However, they can be freed from
contaminating substances by steam distillation at a lower
temperature, at which they are stable. Steam distillation
also offers the advantage of selectivity, since some
water-insoluble substances are volatile with steam an others are not, and some volatilize so very slowly that sharp
separation is possible The technique is useful in processing natural oils and resins, which can be separated into
steam-volatile and non-steam-volatile fractions. It is useful for recovery of a non-steam-volatile solid from its solution in
a high-boiling solvent such as nitrobenzene, boiling point 210°C; all traces of the solvent can be eliminated and the
temperature can be kept low.
The boiling point remains constant during a steam distillation as long as adequate amounts of both water and the organic
component are present to saturate the vapor space. Determination of the boiling point and correction for any deviation
from normal atmospheric pressure permit calculation of the amount of water required for distillation of a given amount
of organic substance. According to Dalton's law, the molecular proportion of the two components in the distillate is
equal to the ratio of their vapor pressures p in the boiling mixture; the more volatile component contributes the greater
number of molecules to the vapor phase. Thus,
Hvordan læger bruger matematik ?
Moles of Water
p water
=
Moles of substance p substance
The vapor pressure of water pwater at the boiling temperature in question can be found by interpolation of the data in the
table below, and that of the organic substance is, of course, equal to 760 – pwater.
Vapor Pressure of Water in Millimeters of Mercury (mm Hg)
t(°C)
60
61
62
63
64
65
66
67
68
69
p(mm Hg)
149.3
156.4
163.8
171.4
179.3
187.5
196.1
205.0
214.2
223.7
t(°C)
p(mm Hg)
70
71
72
73
74
75
76
77
78
79
233.7
243.9
254.6
265.7
277.2
289.1
301.4
314.1
327.3
341.0
t(°C)
80
81
82
83
84
85
86
87
88
89
p(mm Hg)
355.1
369.7
384.9
400.6
416.8
433.6
450.9
468.7
487.1
506.1
t(°C)
90
91
92
93
94
95
96
97
98
99
p(mm Hg)
525.8
546.0
567.0
588.6
610.9
633.9
657.6
682.1
707.3
733.2
Hence the weight of water required per gram of substance is given by the expression:
Weight of water per gram of substance
=
18 × p water
MW of substance × (760 - p water)
From the data given in the figure above for benzene-water, the fact that the mixture boils at 69.3°C, and the molecular
weight 78.11 for benzene, the water required for steam distillation of 1 g of benzene is only 227 x
x
= 0. 10 g.
Nitrobenzene (bp 210°C, MW 123. 11) steam distills at 99°C and requires 4. 0 g of water per gram. The low molecular
weight of water makes water a favorable liquid for two-phase distillation of organic compounds. On a small scale, steam
is generated in the flask that contains the substance to be steam distilled by simply boiling a mixture of water and the
immiscible substance.
Isolation of Limonene
Lemongrass is a perennial fast-growing aromatic grass, growing to about 1 meter (3 feet) high with long, thin leaves.
Originally growing wild in India, it produces a network of roots and rootlets that rapidly exhaust the soil. In India it is
known as 'choomana poolu' and is also referred to as 'Indian Verbena' or 'Indian Melissa oil'. In Ayurvedic medicine
Lemongrass is used to help bring down fevers and treat infectious illnesses. It is a valuable ingredient in perfumes and
citrus-type soaps and is also an insect deterrent.
The main chemical components of Lemongrass oil are: Citral, Farnesol, Nerol, Citronellal and Myrcene.
All are examples of a very large group of natural products called terpenes. They are responsible for the characteristic
odors of plants such as eucalyptus, pine, mint, peppermint, and lemon. The odors of camphor, menthol, lavender, rose,
and hundreds of other fragrances are due to terpenes, which have 10 carbon atoms with double bonds or rings and
aldehyde, ketone, or alcohol functional groups.
In nature these terpenes all arise from a common precursor, isopentenyl pyrophosphate. At one time they were thought
to come from the simple diene, isoprene (2-methyl-1,3-butadiene), because the skeletons of terpenes can be dissected
into isoprene units, having five carbon atoms arranged as in 2-methy1butane. These isoprene units are almost always
arranged in a "head-to-tail fashion."
Some Terpenes:
CHO
OH
O
Camphor
Menthol
Citral
Limonene
II. Procedure
In the present experiment citral is isolated by steam distillation of lemon grass oil, which is used to make
lemongrass tea, a popular drink in Mexico. The distillate contains 90 % citral and 10 % neral, the isomer about the 2,3bond. Citral is used in a commercial synthesis of Vitamin A. The citral and neral is isolated from the water by shaking
the mixture with t-butyl methyl ether. The citral dissolved in the t-butyl methyl ether, which is immiscible with water,
and the two layers are separated. The t-butyl methyl ether is dried (it dissolves some water) and evaporated to leave
citral. Store the citral in the smallest possible container in your dessicator for later analysis, IR and index of refraction.
A. Steam Distillation: Into a 5-mLshort-necked round-bottomed flask place a boiling chip, 0.5 mL of
lemongrass oil (obtain the mass of the lemongrass oil added), and 3 mL of water. Assemble the apparatus as
demonstrated in the lab. Distill as rapidly as possible, taking care that all the distillate condenses. Using a syringe,
inject water dropwise through the septum to keep the volume in the flask constant. Distill into an ice-cooled 15-mL
centrifuge tube until no more oily drops can be detected, about 10 to 12 mL.
B. Extraction (http://orgchem.colorado.edu/hndbksupport/ext/ext.html
look on the web for more information on the theory and practice of extraction) of Citral: Add 2.5 mL of tert-butyl
methyl ether to the cold distillate in the 15-mL centrifuge tube. Put a cap on the centrifuge tube and shake carefully.
The citral will dissolve into the upper ether layer. Draw off most of the ether layer with a Pasteur pipette and place in a
clean reaction tube. Extract the aqueous layer twice more with 1.5 mL portions of tert-butyl methyl ether to remove all
of the citral. Combine all three of the ether extracts in the reaction tube.
To dry (remove all traces of water http://orgchem.colorado.edu/hndbksupport/drying/drying.html) the ether
layer, add about 2 mL of saturated sodium chloride solution to the ether in the reaction tube. Stopper the reaction tube
(use a septum), shake it, and then remove and discard the aqueous layer. (Note! Before discarding the aqueous layer,
make sure it is the aqueous layer! Is it the top or bottom layer?). Add a small scope anhydrous calcium chloride pellets
to the ether in the reaction tube to remove the last traces of water. Stopper the reaction tube again and shake over a
period of 5 to 10 minutes. Read about drying agents on the internet. Remove the dry (no water) ether layer and place in
another clean, dry, and tared (weighed) reaction tube with a boiling stone. Make sure you weigh the reaction tube with
the boiling stone added! Add fresh ether to the reaction tube with the drying agent to remove all traces of citral.
Combine with the ether in the second reaction tube. Evaporate the ether by placing the tube into a gently boiling water
bath in the hood. The last traces of ether are removed by connecting the reaction tube directly to the water aspirator.
The residue should be a clear, fragrant oil. If it is cloudy, it is wet. Determine the weight of the citral, and calculate the
percentage of citral recovered from the lemongrass oil, assuming that the density of citral is the same as that of
lemongrass oil (0.89). Transfer the citral to the smallest possible airtight container, and store in your dessicator for later
analysis: FTIR and refractometer.
1.
Record the vapor temperature during the distillation as each 1 mL of distillate is collected. Plot this data on graph
paper. (Each fraction is 10 mL)
Fraction Vapor Temperature
______ ________________
1
2
3
4
5
Fraction Vapor Temperature
______ ________________
6
7
8
9
10
2.
Comment briefly on how the above data correlates with the theory of steam distillation. (Include the b.p. of pure
citral in your answer)
3.
Mass of lemon grass oil used. _______________ grams
Final yield of purified citral. ________________ grams
% Yield _________________%
Physical appearance and odor of your product. __________________________________
_______________________________________________________________________
4.
Run an I.R. Spectrum of your purified citral. Indicate on the spectrum the presence of specific absorptions that
correspond specific structural components of citral. Comment below on how the I.R. spectrum might be used to
confirm the identity of your product.
5.
The index of refraction of my product was _______________________ at _________°C .
The index of refraction of pure citral is _____________________