Project Learning in Chemistry. Project. Mr YC Chan United Christian College. Project learning in TAS. Purpose Project schedule Grouping Topics Working schedule Project proposal/Teacher’s approval. Project. Assessment of Project learning.
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Project Learning in Chemistry
Mr YC Chan
United Christian College
Project learning in TAS
Assessment of Project learning
For TAS project work, teacher assessment can include:
Ho Yin Ming (7)
Wong Lai Ping (24)
Wong Yan Ting (26)
Yeung Kwai Fong (28)
Our objective is to extract the essential oil (limonene) from orange peels and analyze it by polarimetry.
Oh! Our topic is
Optical isomerism is characterised by compounds having
the same structure but different configurations,
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.
The four atoms or groups of atoms attached to the
asymmetric carbon atom are first arranged in order
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)
The molecule is then orientated so that the group of
lowest priority is directed away from us,
R and S system
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 calledlavorotator and has the prefix (-)-to its name.
Direction Of Rotation (+/-)
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
Limonene is one of the compounds that give lemons their
A polarimeter measures the optical activity of compounds,
that is, the effect a chiral compound has on plane-polarized
Structure of polarimeter:
A Real Polarimeter
Calculation of specific rotation :
[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)
Factors affecting magnitude of α :
˙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
3.Wavelength of light
˙Generally, the magnitude of rotation is larger at shorter
4.Solvent & Temperature
1. The white pulp was removed from the peels of two oranges.
3. The 35 ml water level was marked on the outside of the
Some substances that give an odor of limonene was
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.
Results & Calculation
Only few amount of limonene can be extracted in our experiment.Do you’ve some improvement?
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
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.
There are still some errors caused
in a polarimeter.
Hmmm… is only
this error in your experiment?
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.
This experiment can be improved by using polarimeter with digital readout of the observed rotation.This can avoid any human errors.
The calculated specific
rotation is affected.
(1)Why do we need to add 2g of sodium chloride into the
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
Ans: In order to prevent loss of the volatile limonene.