Project Learning inProject
Chemistry
Mr YC Chan
United Christian College
Project learning in TAS
1. Purpose
2. Project schedule
Project
3. Grouping
4. Topics
5. Working schedule
6. Project proposal/Teacher’s approval
Assessment of Project learning
•Students can be assessed once for
ability area A and once for area B.
•A report may be submitted for the
whole group of students.
•Area B for individual students is
assessed by their presentation.
Assessment of Project learning
For TAS project work, teacher assessment can
include:
• Plan
Area A
• Progress
• Product
Area B
• Presentation
S.6A
Ho Yin Ming (7)
Wong Lai Ping (24)
Wong Yan Ting (26)
Yeung Kwai Fong (28)
Oh! Our topic is
Isolation of
(R)-(+)-Limonene
from orange
peels.
Our objective is to extract
the essential oil (limonene)
from orange peels and analyze
it by polarimetry.
Optical isomerism is characterised by compounds having
the same structure but different configurations, and
because of their molecular asymmetry these compounds
rotate the plane of polarization of plane-polarised light.
Optical isomers have similar physical and chemical
properties; the most marked difference between them
is their action on plane-polarised light.
For example,
(R)-(+)-Limonene
boiling point (℃)
(S)-(-)-Limonene
175.5-176
175.5-176.5
0.8402
0.8407
[α]
125.6
-122.1
molecular weight
136.2
136.2
-1
density (g˙mL)
The four atoms or groups of atoms attached to the
asymmetric carbon atom are first arranged in order
of priority.
The molecule is then orientated so that the group of
lowest priority is directed away from us, while the
order three groups appear in ascending or descending
order of priority
If the other three groups appear in a clockwise order
of decreasing priority, the configuration is designated by
the letter R (right).
If such movement is anticlockwise, the letter S (left)
is used.
Rotation of the plane of polarization to the right or in a
clockwise direction is given a + sign.
A substance having this property is called
dextrorotatory and has the prefix (+)- to its name.
Rotation of the plane of polarization to the left or in a
anti-clockwise direction is given a – sign.
A substance which rotates the plane of polarization to the
left is called lavorotator and has the prefix (-)-to its name.
Limonene is one of the compounds that give lemons their
tangy odor.
Limonene is an asymmetric object, which cannot be
divided by a plane into two identical halves.Therefore,
limonene is optically active.
Orange peels provide limonene that is virtually 100%
(R)-(+)-limonene,whereas pine needles provide essentially
100% (S)-(-)-limonene.
A polarimeter measures the optical activity of compounds,
that is, the effect a chiral compound has on plane-polarized
light.
Structure of polarimeter:
analyzer
Polarimeter
tube
Power supply
polarizer
Sodium lamp
[α] = ――
λ
α= the observed angle of rotation
l = the length of the light path through the sample (decimeters)
c = the concentration of the sample (g．mL-1 of solution)
1.Concentration
˙The greater the concentration of molecules in the path
of the beam, the greater the rotation.
2.Length of sample tube
˙The rotation of the beam is proportional to the length of
the path.
3.Wavelength of light
˙Generally, the magnitude of rotation is larger at shorter
wavelengths.
4.Solvent & Temperature
1. The white pulp was removed from the peels of two oranges.
2. The peels were ground in a blender to make a slurry. Then
the slurry was poured into a 500 ml round-bottomed flask.
4 drops of an antifoaming agent was added to the flask.
3. The 35 ml water level was marked on the outside of the
round-bottomed flask.
4. The steam distillation apparatus were set up without the
separatory funnel. The 50 ml round-bottomed flask was
used as receiver.
5. The orange peel mixture was boiled at a moderately rapid
rate by a heater. The 35 ml distillate was collected.
water out
water in
Heat
6. The distillate was poured into a 125-ml separatory funnel
and 2 g of sodium chloride was added in it.
7. The separatory funnel was stoppered, shaken and vented
for four or five times.
8. 15 ml chloroform was used to rinse the apparatus and
let the chloroform drain into the separatory funnel.
9. The limonene was extracted from the aqueous layer by
inverting the separatory funnel and shaking it back and
forth gently.
10. The layers were allowed to separate before draining the
bottom organic layer into a dry 50 ml Erlenmeyer flask.
11. The extraction of aqueous layer was repeated by using
another 10 ml of chloroform. Two chloroform extracts
were combined and dried with anhydrous calcium
sulfate for at least 10 min.
12. A dry watch glass was weighted. The chloroform was
filtered through filter paper into the watch glass.
13. The chloroform was evaporated in a steam bath.
14. The watch glass was cooled and was wiped the outside dry
before weighting it .
15. The watch glass was heated several additional minutes, and
then it was weighted again. The final mass of limonene was
obtained.
16. 5 ml of 95% ethanol was obtained to dissolve the obtained
limonene.
17. After filtration,the limonene and 5-mL ethanol was poured
directly into the polarimeter tube.
Some substances that give an odor of limonene was
extracted.
As the limonene extracted out is in very small amount,
the angle of rotation (α) cannot be observed.
By using the theoretical value of the specific rotation
of limonene and the data used in the experiment, the
observed angle of rotation (α) can be calculate as
following:
[α] = ――
λ
=
_______________________
α = 0.543。
Since the minimum angle shows by the polarimeter is 1℃,
thus, we could not get the value ofαin our experiment.
Only few amount of limonene can
be extracted in our experiment.Do
you’ve some improvement?
limonene
This error can be improved by repeating the
extraction process of limonene for more times.This
is to ensure that all limonene in the separatory
funnel is separated out
Rotate
After our discussion, we also think that
chloroform could be added to the orange
slurry before the process of steam
distillation.So more limonene can be
extracted.But,at the same time,many unknown
substances will formed besides limonene.
This problem can be improved by using a rotary
evaporator.It can remove all the unknown
substance rapidly in a vacuum,except limonene.
Rotary Evaporator
Hmmm… is
only
this error
in your
experiment?
There are still some errors caused
in a polarimeter.
The zero readings of polarimeter
don’t meet their position.
The length of the polarimeter
tube is measured by the ruler.
The measuring error is caused.
 The calculated specific
rotation is affected.
This experiment can be improved
by using polarimeter with digital
readout of the observed
rotation.This can avoid any human
errors.
(1)Why do we need to add 2g of sodium chloride into the
distillate?
Ans: The sodium chloride helps to minimize emulsions during
the extractions by making the organic layer less soluble
in the aqueous layer.
(2)Why do we dissolve the limonene in the ethanol at the end
of the experiment?
Ans: It is because ethanol have no optical activity. It does
not have specific rotation.
(3)Why we must peel the oranges just before grinding the
peels?
Ans: In order to prevent loss of the volatile limonene.