ProjectProject Learning in Chemistry

Mr YC Chan

United Christian College

Project learning in TAS

1. Purpose

2. Project schedule

3. Grouping

4. Topics

5. Working schedule

6. Project proposal/Teacher’s approval

Project

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

• Progress

• Product

• Presentation

Area A

Area B

S.6AS.6A

Ho Yin Ming (7)Ho Yin Ming (7)Wong Lai Ping (24)Wong Lai Ping (24)Wong Yan Ting (26)Wong Yan Ting (26)Yeung Kwai Fong (28)Yeung Kwai Fong (28)

  

Oh! Our topic is Isolation of (R)-(+)-Limonene(R)-(+)-Limonene from orange peels.

Our objective is to extract the essential oil (limonene) from orange peels and analyze it by polarimetry.

andbecause of their molecular asymmetry these compounds rotate the plane of polarization of plane-polarised light.

Optical isomerism is characterised by compounds having the same structure but different configurations,

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 (S)-(-)-Limonene

boiling point (℃)

density (g mL)

[α ]

molecular weight

125.6 -122.1

136.2 136.2

175.5-176 175.5-176.5

0.8402 0.8407-1

while the order three groups appear in ascending or descending order of priority

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 oflowest priority is directed away from us,

If the other three groups appear in a clockwise order of decreasing priority, the configuration is designated bythe 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 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.

Limonene is one of the compounds that give lemons their tangy odor.

A polarimeter measures the optical activity of compounds, that is, the effect a chiral compound has on plane-polarized light.

Structure of polarimeter:Structure of polarimeter:

Polarimeter tube

Power supply Sodium lamp

analyzer

polarizer

[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.

Heat

water out

water in

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.

[α]λ = ――

= _______________________

α = 0.543 。

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.

Since the minimum angle shows by the polarimeter is 1℃, thus, we could not get the value ofαin our experiment.

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:

Only few amount of Only few amount of limonenelimonene can be extracted in our can be extracted in our experiment.Do you’ve some experiment.Do you’ve some improvement?improvement?

limonene

Rotate

This error can be improved by This error can be improved by repeating the repeating the extraction processextraction process of limonene for more times.This of limonene for more times.This is to ensure that all limonene in the separatory is to ensure that all limonene in the separatory funnel is separated outfunnel 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 rotaryevaporator.It can remove all the unknown substance rapidly in a vacuum,except limonene. Rotary EvaporatorRotary Evaporator

Hmmm… Hmmm… is only is only this error this error in your in your experimeexperiment?nt? The calculated specific The calculated specific

rotation is affectedrotation is affected..

There are still some errors caused There are still some errors caused in a polarimeter.in a polarimeter.

The zero readings of polarimeter The zero readings of polarimeter don’t meet their positiondon’t meet their position. .

The length of the polarimeter The length of the polarimeter tube is measured by the ruler.tube is measured by the ruler.The measuring error is causedThe 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.

(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.