Introduction

Pure liquids generally boil at a constant temperature or over a narrow temperature range (the

total pressure in the system must remain constant). The pressure of 760 torr (1 atm) is the

pressure for the normal boiling point of the liquid. The boiling point of a liquid is affected by the

presence of volatile and nonvolatile impurities, and most of the mixtures of liquids boil over a

wide range.

Simple distillation is used to separate the distillates from less-volatile substances that remain as

pot residue at the completion of the distillation. To be able to separate the volatile substances, the

differences in the boiling point must be greater than 40-50°C. In a homogenous solution with

volatile and nonvolatile components, the nonvolatile impurity will reduce the vapor pressure of

the volatile component. This happens because it lowers the concentration of the volatile

component in the liquid phase. If there is water present along with the nonvolatile impurity, then

the head temperature of the distillation will be the same as the one for pure water because the

water condensing on the thermometer bulb will remain uncontaminated by the nonvolatile

impurity; however, the pot temperature will be elevated because of the decreased vapor pressure

of the solution. The nonvolatile impurity reduces the vapor pressure at any temperature by a

constant amount in accord to Raoult’s Law. This law is only applicable to ideal solutions-which

are defined as those in which the interactions between like molecules are the same as those

between unlike molecules.

Fractional distillation is normally used to obtain each volatile component in pure form. In

fractional distillation at any given temperature, the vapor pressure is richer in the more volatile

component than its boiling liquid (where the vapor is in equilibrium). This phenomenon allows

us to use fractional distillation. The forces of attraction between the molecules of the two

components are weaker than the ones between the molecules of each individual component. The

vapor pressure of the solution is greater than that of the pure, more volatile component (for a

particular range of the compositions of two liquids). The head temperature should rise to the

normal boiling point of the more volatile compound, and it should remain there until the

component is mostly removed. The head temperature should then drop. When additional heat is

provided to the still pot, the less volatile component should begin to distill, and then the boiling

point should rise to the boiling point of the second component.

There are three types of distillation: steam, simple, and fractional. Simple distillation is useful for

isolating a pure liquid from other substances that are not volatile. The boiling points of each pure

substance must be greater than 40-50°C for this distillation to work. Steam distillation is used for

the separation and purification of volatile organic compounds that are immiscible in with water.

Unlike simple distillation, this technique can’t be used for substances that decompose on

prolonged contact with steam or hot water, that react with water, that have a vapor pressure of 5

torr, or are less than 100°C. If the volatile compounds can’t be separated by simple distillation,

then fractional distillation can be performed. This usually happens when the difference in boiling

points is too small to perform simple distillation, but the temperature ranges is still between 20-

30°C. Fractional distillation can also be performed when the mixture’s boiling point is lower

than the boiling temperature of either the pure compound.

Steam distillation is usually used for separating and purifying moderately volatile liquids and

solid organic compounds that are insoluble or nearly soluble in water from nonvolatile

compounds.

Data and Observations

Simple distillation of Cyclohexane: At 68°C the compounds started distilling, and the finishing

temperature was around 76°C.

Steam distillation Lemon Grass Oil Temperature: 98°C

Simple distillation Lemon Grass Oil Temperature: 35°C

Compound

Name and

Structure

Weight of

the

Starting

Product in

Grams

Weight of

the Final

Product in

Grams

Molecula

r Weight

of the

Product

in g/mol

Amount of

moles of

the

product

(final)

Melting

Point

Range of

Starting

Material

Melting

Point

Range of

the Final

Product

Cyclohexane 7.78 g 6.7964 g 84.16

g/mol

.0808

moles

NA NA

Citral Oil 1.95 g .8002 g 152.24

g/mol

.00526 NA NA

Results and Discussion

Simple Distillation of Cyclohexane

Simple distillation of cyclohexane showed that the observed boiling point was between 68°C and

76°C. The boiling point of cyclohexane in the catalogue is 80.74°C. The cyclohexane that was

used in the experiment might have been affected by the lower barometric pressure of that day,

since the temperature was lower than the expected. The temperature in on the thermometer gives

us the boiling point for pure cyclohexane, and it does not reflect the mixture in the still pot. The

percent recovery of cyclohexane in this experiment was 87.4%. The refractive index of

cyclohexane from the experiment was 1.424 which is the same as the refractive index of pure

cyclohexane. This shows that simple distillation was successful because it was the right form of

distillation to use, since the refractive index matched pretty perfectly.

Steam Distillation of Lemon Grass Oil

In the process of steam distillation the temperature of co-distillation of the two immiscible

liquids was 98°C. After the steam distillation took place the receiving flask contained more of

more volatile compound that the starting mixture did. The purpose of the extraction with diethyl

ether was to separate the organic and the inorganic layers because only organic compounds are

soluble in diethyl ether. Extraction is a technique based on the ability of a compound to dissolve

in certain liquid. Citral oil was the organic compound that was soluble in diethyl ether, and the

other inorganic impurities and water were not. Sodium sulfate was added to the organic layer to

absorb the excess water that was still left in the organic layer. The organic solution was then

decanted into a clean flask, and the simple distillation made the solvent evaporate from the

solution. The weight percentage of citral oil in lemon grass was calculated to be 41.04%.

Conclusion

Distillation is a good way to purify liquids. It is based on the differences of the boiling points in

the mixtures. It is very important to choose the right type of distillation based on the properties of

the mixture that we want to purify. In these two experiments simple and steam distillation were

used. The simple distillation was used to purify cyclohexane. The steam distillation was used to

separate citral oil from lemon grass oil-extraction was also used to get pure citral oil.

Works Cited:

Experimental Organic Chemistry 5th Edition

http://en.wikipedia.org/wiki/Citral

http://en.wikipedia.org/wiki/Cyclohexane