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A Facile Solvent-free Cannizzaro Reaction :

An Instructional Model for Introductory Organic Chemistry Laboratory.

Sonthi Phonchaiya1 and Bhinyo Panijpan2 Institute for Innovation and Development of Learning Process, Mahidol University, Rama 6 Rd., Bangkok, 10400, Thailand Tel. +662-2015104-8, Fax +662-3547345 E-mail: 1. [email protected]

2. [email protected] Shuleewan Rajviroongit Department of Chemistry, Mahidol University, Rama6 Rd., Bangkok 10400 Thailand Tel. + 662-2015134 Fax + 662-3547151 E-mail: [email protected] Tony Wright The School of Education, The University of Queensland, St Lucia, QLD 4072, Australia Tel. +617-33656465 (St Lucia), +617-33811507 (Ipswich) Email: [email protected] Joanne T. Blanchfield The School of Molecular and Microbial Science, The University of Queensland, QLD 4072, Australia Tel. +617-33653622 Fax +617-33654273 E-mail: [email protected]

A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory

2

Table of Contents

Supplements for Students 3 - 21 • Lab Instruction 3 • Pre – Lab Questions 15 • Results sheet 17 • Post - Lab exercise 21

Answer Key 22 – 28

• Pre – Lab Questions 22 • Results sheet 24 • Post - Lab exercise 28

Supplements for instructors 29 - 33

• Safety Information 29 • Lab Preparation 30 • Experimental Tips and Comments 31 • Additional Identification Activities 32 • Thermodynamics activity comments 33

CAS Registry Numbers 34 Product Information 34 IR spectra 35 1H NMR spectra 36 13C NMR spectra 37


3

An Environmentally Friendly Redox Reaction

Background In this experiment you will investigate a Cannizzaro reaction which is a self-redox reaction or a disproportionation reaction. The disproportionation reaction is fascinating because a single reactant gives rise to two products. Strong bases are used to catalyse the disproportionation reactions of some aldehydes to their corresponding alcohols and carboxylic acids.

This experiment provides you with an opportunity to design experimental procedures and characterize the oxidation and reduction products.

After performing the experiment you will be introduced to green chemistry concepts and

use them to evaluate the environmental impact of the reaction.

This experiment consists of four parts;

1. A solvent-free reaction 2. Separation and Identification of products 3. Thermodynamics 4. Green chemistry

Objectives

1. To perform a solvent-free self-redox reaction of 2-chlorobenzaldehyde.

2. To monitor the reaction progress by thin layer chromatography (TLC).

3. To design experimental procedures for separation and identification of products.

4. To learn green chemistry concepts.

SUPPLEMENTS FOR STUDENTS

SAFETY Always wear goggles, gloves and apron when conducting this experiment


4

PROCEDURE PART 1: The Solvent-Free Reaction

1. In a fume hood, 2-chlorobenzaldehyde (17.8 mmol, 2 mL) and potassium hydroxide (26.7

mmol, 1.5 g) are added to the mortar. 2. The mixture is ground with a pestle. If solid materials begin to coat the mortar or the pestle,

dislodge them with a spatula and put back into the mixture. 3. About 30-40 minutes of grinding are required for completion of the reaction at which point

the gummy mixture turns into a paste. 4. The reaction is monitored for completion using thin layer chromatography.

Thin Layer Chromatography (TLC) TLC is a primary tool for rapid qualitative analysis. It is convenient for monitoring the

progress of the reaction.

Preparing a TLC chamber o Place a piece of filter paper (size 4.5 x 9 cm) inside a 50-mL beaker. o Pour 5 mL of 30% ethyl acetate – hexane (eluent) into the beaker and cover it

with a Petri dish or watch glass. o The chamber is again covered for equilibration of the system, it becomes saturated

with solvent vapour in a few minutes and is ready for use.

Figure 2 The TLC chamber

Figure 1 Performing the reaction in a mortar

Petri dish or watch glass

beaker

filter paper

starting material + KOH grinding products


5

Marking the TLC plate

o A TLC plate (2.5 x 5 cm) will be provided for each of you in the laboratory. o Mark 3 well-separated points (A, B and C) on the TLC plate with a pencil about 1 cm from the bottom.

Figure 3 The TLC with 3 spotting points

Monitoring the reaction progress Preparing a sample for monitoring

o A very tiny amount of the reaction mixture is transferred to a small vial by spatula. o Dissolve that mixture with a small amount of dichloromethane. (It may not all

dissolve, one product is insoluble.)

Figure 4 Preparing a sample for TLC

a tiny bit of products


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Spotting the TLC

o Dip a capillary tube into the reaction mixture. o At position A and B, gently place the tip of the capillary tube onto the surface of

the TLC plate and immediately withdraw it. A small amount of mixture in the capillary tube will be adsorbed on the TLC plate (repeat this step if too little of the sample is adsorbed).

o Dip a new capillary tube into a sample of 2-chlorobenzaldehyde (provided by the instructor or teaching assistants) and then spot onto the TLC plate at position B and C.

o Leave the TLC plate to dry for a few seconds.

Developing the TLC plate

o Carefully place the TLC plate in the TLC chamber, the level of the eluent in the chamber should be lower than the spots on TLC.

o Close the TLC chamber and leave the eluent to progress to the top of the TLC plate.

Figure 6 Developing the TLC plate

Figure 5 Spotting the TLC plate

reaction mixture

reaction mixture + 2-chlorobenzaldehyde 2-chlorobenzaldehyde


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Visualizing the substance spots on the TLC plate

o Remove the TLC plate from the chamber and leave it to dry for one minute. o Place the TLC plate under the UV lamp (wavelength 366 nm). o The appearance of new spots and the disappearance of 2-chlorobenzaldehyde at

position A, compared to position C, indicates the completion of the reaction (Figure 7a). If the reaction is not complete, additional grinding is required. The position A will show two spots corresponding to 2-chlorobenzaldehyde and one of the products (Figure 7b). The other product fails to show on the TLC due to its insolubility in the dichloromethane solvent.

a) the reaction is complete b) the reaction is not complete

Figure 7 The developed TLC plate under a UV lamp

2-chlorobenzaldehydea)

The spot for the reactant is “absent”.

b)

A B C A B C

Reduction product


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PART 2 Separation and Identification of Products You will design your own procedure to separate, isolate and identify your products. Discuss your experimental plan with your instructor or teaching assistants before starting.

Separation

1. Vacuum filtration (Appendix A) and acidification (Appendix B) steps are recommended. 2. Solubility properties of three substances are given in Table 1. 3. Outline your procedure for separation and isolation of products and explain how it may

work.

Table 1 The solubility of potassium 2-chlorobenzoate, 2-chlorobenzoic acid and 2-chlorobenzyl alcohol

Identification

1. Authentic samples of 2-chlorobenzoic acid, 2-chlorobenzyl alcohol and their melting points are provided.

2. You are required to consider physical properties of the substances such as adsorption to the TLC matrix (Rf value), IR, NMR spectra and melting points.

3. Write your method for identification of products and explain how it works. PART 3: Thermodynamics: Energy Involved in the Reactions

1. Use the molecular modelling software (Spartan 06 programme) to provide the internal

energy (heat of formation) and entropy change for each substance in the reaction (T = 298.15 K). Calculate the energy of the reaction and explain why the reaction should occur.

Substances Solubility (in water)

potassium 2-chlorobenzoate soluble 2-chlorobenzoic acid partially soluble

2-chlorobenzyl alcohol partially soluble

H

O

H2OOH

O

OH+ +2

-

Cl Cl Cl

(l) (s) (s)(l)


9

2. Search thermodynamic data from NIST webBook (http://webbook.nist.gov/chemistry) for

the given reaction.

Then answer questions: 2.1 Is the reaction exothermic or endothermic? 2.2 Does the reaction proceed spontaneously? 2.3 Write the enthalpy diagram (the Hess’s diagram*) of the reactions using the given

equations. 14 C(s) + 7 H2(g) + (3/2) O2(g) C7H8O2(l) + C7H6O2(s) 14 C(s) + 7 H2(g) + (3/2) O2(g) 2 C7H6O(l) + H2O(l) 14 C(s) + 7 H2(g) + (3/2) O2(g) C7H8O(l) + C7H6O(l) + (1/2) O2(g)

14 C(s) + 7 H2(g) + (3/2) O2(g) C7H6O2(s) + C7H6O(l) + H2(g) 2 C7H6O(l) + H2O(l) C7H8O2(l) + C7H6O2(s) C7H8O(l) + C7H6O(l) + (1/2) O2(g) C7H8O2(l) + C7H6O2(s) C7H6O2(s) + C7H6O(l) + H2(g) C7H8O2(l) + C7H6O2(s)

* Hess’s law is implied in the 1st law of thermodynamics.

For a review of thermodynamics, please read

1. Atkins, P.; Paula, J. Physical Chemistry for the Life Sciences, W.H. Freeman and Company: 2006.

2. Chang, R. Chemistry, 9 ed.; Mc Graw Hill: 2007.

H

O

H2OOH

O

OH+ +2

-

(l) (l)(s) (l)


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PART 4: Green Chemistry

Reactions with a green chemistry consideration commonly emphasize the following

criteria: • Minimizing waste • Limiting the use of toxic compounds • Working safely • Minimizing cost By analysis of the chemicals and processes involved in this laboratory, you should be able

to evaluate its “greenness” compared to those in the conventional method. Data on chemical hazards and their commercial prices are summarized in Table 2.

Evaluate your approach in terms of environmental impact, economy and human health and compare it with a conventional approach in which refluxing and an organic solvent are used for the separation experiment.

Your analysis should be based on the 12 principles of green chemistry, which are:

1. Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.

2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Designing Safer Chemicals: Chemical products should be designed to effect their desired function while minimizing their toxicity.

5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.

6. Design for Energy Efficiency: Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

7. Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

8. Reduce Derivatives: Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical


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processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

9. Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10. Design for Degradation: Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.

11. Real-time Analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

12. Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.

Identify which principles are relevant.

Reference: Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998, p.30.

If you are interested in finding out more about green chemistry you can go to

http://www.chemsoc.org/pdf/gcn/Principles.ppt#257,1,Slide 1


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Table 2: Chemicals Hazard information and their commercial prices*.

compound

Flamm

ability

Toxic

ity

Body

contac

t

React

ivity

Chron

ic Effe

ct

Price(USD)/amount

2-chlorobenzaldehyde (98%) 1 0 3 0 0 22.20 / 250g 4-chlorobenzaldehyde (98%) 1 2 1 1 0 32.4 / 250g hydrochloric acid 0 2 3 1 0 33.2 / 2.5 L magnesium sulfate anh. 0 2 2 0 0 149 / 2.5 kg methanol 3 3 2 1 2 28.20 / 2.5 L methylene chloride 0 2 2 1 2 30.20 / 2.5 L potassium hydroxide 0 2 4 0 0 29.80 / 1 kg * from Sigma Aldrich Conventional experimental procedure:

Add 15 mL of chilled distilled water and transfer this solution to a separatory funnel.

Extract with three 10-mL portions of methylene chloride. Separate the organic and aqueous layers and treat each solution independently. Acidify the aqueous layer slowly with ca. 15 mL of 3 M HCl (use pH paper). This should result in the production of a white precipitate. Isolate your product using suction filtration to obtain 4-chlorobenzoic acid. Dry the combined organic extracts with MgSO4, filter, and then remove the solvent using a water bath to obtain 4-benzylalcohol. Record a melting point for each solid to confirm that you have produced the desired products.

Add 2 mL (17.80 mmol) of 4-chlorobenzaldehyde and 8 mL of methanol to a 100-mL round-bottomed flask containing a magnetic bar. With gentle stirring, add 8 mL of an 11 M aqueous solution of potassium hydroxide. Then equip the flask with a reflux condenser as shown in the picture.

Heat the reaction mixture in an oil bath at a temperature of 65-75 oC for one hour. Allow the reaction mixture to cool.

aldehyde+ KOH

+ MeOH

aldehyde+ KOH

+ MeOH

Adapted from: 1. Mayo, D. W.; Pike, R. M.; Butcher, S. S. Microscale Organic Laboratory, 2nd ed.; John Wiley & Sons, New York: 1986, pp 502. 2. Pasto, D.; Johnson, C.; Miller, M. Experiments and Techniques in Organic Chemistry, Prentice Hall, New Jersey, 1992, pp 119.


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Appendix A Vacuum filtration Vacuum or suction filtration is a convenient rapid filtration technique often used to collect solid products from the reaction.

o The vacuum filtration apparatus is assembled as shown in figure 8.

Figure 8 Vacuum filtration apparatus

o Make sure the correct size of filter paper is used for the Buchner funnel.

(a) without filter paper (b) with filter paper Figure 9 Top view of the Buchner funnel

o Wet a filter paper with a few drops of water to ensure that the paper will firmly

stick to the funnel.


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o Turn on the water aspirator and carefully pour the mixture into the Buchner funnel. Solvent flows rapidly through the Buchner funnel into the flask whereas the solid remains in the funnel.

o You may need to remove the excess of KOH that might contaminate your solid products by rinsing them off with water.

o Leave the funnel under vacuum for a few minutes to get rid of most of the solvent. o When the solid dries, disconnect the hose, turn off the water aspirator and use a

spatula to transfer the solid products into a watch glass, beaker or vial. a) b) c)

Figure 10 a) Pouring the mixture into the Buchner funnel

b) Leaving the solid to dry under vacuum c) Transferring the solid onto the watch glass

Appendix B Acidification

The oxidation product, carboxylate salt, obtained from the reaction can be converted to the corresponding carboxylic acid by acidification with hydrochloric acid.

This acidification step involves heat production so we recommend you carry it out in the

ice-bath. o Carefully add 3M HCl dropwise to the solution in the flask kept cooled in an ice-bath

until turbidity is observed. o Stir the mixture thoroughly with a spatula or a stirring rod and check the pH of the

solution with the litmus paper. o Continue adding 3M HCl until the solution becomes acidic to the litmus paper. o Collect the precipitate by vacuum filtration.

Figure 11 Acidification of carboxylate salt by HCl


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An Environmentally Friendly Redox Reaction 1) Look for the website (http://www.geocities.com/self_redox/chemicals_main.htm), providing

the severity ratings for each of these properties for the following solvents/reagents: (Nil = 0, Low = 1, Moderate = 2, High = 3, Extreme = 4 )

2-chlorobenzaldehyde Potassium hydroxide 3M hydrochloric acid

2-chlorobenzoic acid 2-chlorobenzyl alcohol

Pre-Lab Questions

Flammability Toxicity Body Contact

Reactivity Chronic Effects










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2) Draw the general structure of the aldehyde, alcohol and carboxylic acid and comparatively rank their relative oxidation level by using number 1 to 3 (1 = highest)

Substance

General structure

Oxidation level

Aldehyde

Alcohol

Carboxylic Acid

3) Draw an equation for the reaction of a carboxylic acid and potassium hydroxide.

4) Identify the oxidation and reduction products of the following reaction.

Write “A” and “B” under the structures given

2H

O

+ KOHA + B

-

Cl

+ H2O

oxidation product reduction product

OH

-

Cl

-

OH

-

Cl

-O

O-K+

-

Cl

-O


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1. Draw a TLC plate to illustrate your results from the experimental part 1 and give a plausible explanation.

2. Use the solubility data from Table 1 in conjunction with the vacuum filtration and acidification steps to formulate your method of separation and isolation. Explain how it works.

Results sheet


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3. Design a way to identify your products. Use the authentic samples of the two desired products together with your knowledge of the properties of the substances.

4. Draw the structural formulas and name your final oxidation and reduction products.

An oxidation product A reduction product Structure

Structure

Name:

Name:


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5. Calculate energies of the reaction using the molecular modelling software and explain why it should occur.

6. Calculate energies of the given reaction using thermodynamic data from NIST webBook and answer the following questions.

H

O

H2OOH

O

OH+ +2

-

Cl Cl Cl

(l) (s) (s)(l)

H

O

H2OOH

O

OH+ +2

-

(l) (l)(s) (l)


20

6.1 Is the reaction exothermic or endothermic?

6.2 Does it proceed spontaneously?

6.3 Write the enthalpy diagram.


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Green Chemistry Exercise


Post - lab exercise

Compare the conventional method mentioned above with the one used here in terms of environmental impact, economics and human health. Based on the 12 principles of green chemistry, is the procedure you carried out any greener? Explain your answer.


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1. Look for the website (http://www.geocities.com/self_redox/chemicals_main.htm), providing the severity ratings for each of these properties for the following solvents/reagents: (Nil = 0, Low = 1, Moderate = 2, High = 3, Extreme = 4 )

2-chlorobenzaldehyde Potassium hydroxide 3M hydrochloric acid

2-chlorobenzoic acid 2-chlorobenzyl alcohol

Pre-Lab Questions

ANSWER KEY

Flammability 1 Toxicity 0 Body Contact 3

Reactivity 0 Chronic Effects 0










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2. Draw the general structure of the aldehyde, alcohol and carboxylic acid and rank their relative oxidation level by using number 1 to 3 (1 = highest)

Substances

General structure

Oxidation level

Aldehyde

RCHO

2

Alcohol

ROH

3

Carboxylic Acid

RCO2H

1

3. Draw an equation for the reaction of a carboxylic acid and potassium hydroxide.

4. Identify the oxidation and reduction products of the following reaction.

Write “A” and “B” under the structures given.

2H

O

+ KOHA + B

-

Cl

+ H2O

oxidation product reduction product

OH

-

Cl

-

OH

-

Cl

-O

O-K+

-

Cl

-O

B A

R OH

- O

+ KOHR O- K +

- O

+ H2O


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1. Draw a TLC plate to illustrate your results from the experimental part 1 and give the plausible explanation.

2. Use the solubility data from Table 1 in conjunction with the vacuum filtration and acidification steps to formulate your method of separation and isolation. Explain how it works. The separation and isolation steps are summarized as below.

Results sheet

The reaction is complete because total disappearance of starting material is observed.

A B C

(see under UV lamp)

A = mixture B = mixture + starting material C = starting material

The reaction mixture

vacuum filtration

2-chlorobenzyl alcohol

filtrate precipitate

add water (~15 mL)

Water is added to dissolve potassium 2-chlorobenzoate.

2-chlorobenzyl alcohol is isolated as a solid because it is water partially insoluble. Rinsing of the precipitate is done to rid the product of any 2-chlorobenzoate.

acidify with 3M HCl (~10mL)

2-chlorobenzoic acid

Potassium 2-chlorobenzoate (water soluble) is acidified to form the precipitate which can be isolated by vacuum filtration. This is also needed to cleanse it of the alcohol product.

vacuum filtration

Cl

OH

O

Cl

O

O-

K

A new spot at position A corresponds to one product. (another product fails to show on the TLC due to its insolubility).

Cl

OH


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3. Design a method to identify your products. Use the authentic samples of the two products

together with your knowledge of the properties of the substances.

The two products will be identified by TLC*, using respective authentic samples for comparison. Firstly, the oxidation product will be compared with 2-chlorobenzoic acid. There will be 3 spots (A, B and C) on the TLC plate as shown in the picture. Positions A, B and C are the spots of “oxidation product”, “oxidation product + 2-chlorobenzoic acid”, and “2-chlorobenzoic acid” respectively.

The oxidation product gives the same Rf value (0.15) as 2-chlorobenzoic acid. The reduction product gives the same Rf value (0.46) as 2-chlorobenzyl alcohol.

*If students are familiar with the determination of mixed melting point, IR and NMR

spectroscopy, these techniques could also be used to identify the products.

4. Draw the structural formulas and name your final oxidation and reduction products.

oxidation product reduction product Structure

Structure

Name: 2-chlorobezoic acid

Name:2-chlorobenzyl alcohol

Two compounds that provide the same Rf value are most likely to be the same substance (Though it is not generally true but in this experiment it is the case.). The reduction product will be identified in the same way with authentic 2-chlorobenzyl alcohol sample.

Cl

OH

O--

Cl

OH

--

-

A B C A B C Oxidation product Rf = 0.15 (30% EtOAc-Hexane)

Reduction product Rf = 0.46 (30% EtOAc-Hexane)


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5. Calculate energies of the reaction using the molecular modelling software and explain

why it should occur. The calculated energies are shown in the table.

substances Hf (kJmol-1) S (kJmol-1) Gf (kJmol-1) C7H5O2Cl -295 0.383 -408 C7H7O2Cl -154 0.369 -264 C7H5O2Cl -60 0.363 -168

H2O -248 0.189 -304

** G is calculated from equation G = H – TS ( T = 298.15 K) **

∆Greact = ∑n∆G (products) - ∑m∆G (reactants) = {∆Gsolid (C7H5O2Cl) + ∆Gsolid (C7H7O2Cl)} - {2 x ∆Gliquid (C7H5O2Cl) + ∆Gliquid (H2O )}

= {(-408)+(-264)} – {(2 x (-168)) + (-304)} = -32 kJmol-1 ∆Greact is negative, therefore; the reaction is spontaneous in the forward direction.

6. Calculate energies of the given reaction using thermodynamic data from NIST webBook

and answer these following questions.

H

O

H2OOH

O

OH+ +2

-

Cl Cl Cl

(l) (s) (s)(l)

H

O

H2OOH

O

OH+ +2

-

(l) (l)(s) (l)


27

6.1 Is the reaction exothermic or endothermic?

∆Hreact = ∑n∆fH (products) - ∑m∆fH (reactants) = {∆fHsolid (C7H6O2) + ∆fHliquid (C7H8O)} -

{2 x ∆fHliquid (C7H6O) + ∆fHliquid (H2O )}

= {(-385)+(-155)} – {(2 x (-87)) + (-286)} = -80 kJmol-1 the reaction is exothermic

6.2 Does the reaction proceed spontaneously?

∆Greact = {(-434)+(-219)} – {(2 x (-153)) + (-307)} = -40 kJmol-1 the reaction is spontaneous

6.3 Write the enthalpy diagram


28

Green Chemistry Exercise


Compare the conventional method with the one used here in terms of environmental impact, economy and human health. Based on the 12 principles of green chemistry, is the method you carried out any greener? Explain your answer.

The experiment I have just carried out is more environmentally friendly than the conventional method because of many reasons as described below.

This experiment Conventional experiment Environmental Impact There was no organic solvent required for the reaction and water was the only solvent used for the separation process.

Methanol was used for the reaction and methylene chloride was chosen in the separation step.

Economics The reaction was performed under ambient temperature. There was no heating nor electricity involved. Starting material (2-chlorobenzaldehyde) is cheaper (22.2USD/250g).

This method required heat and electricity for the reaction. Starting material (4-chlorobenzaldehyde) is more expensive (32.4USD/250g). This method required additional cost for methanol, methylene chloride and magnesium sulfate anhydrous.

Health and Safety 2-chlorobenzaldehyde was highly irritating, it is necessary to avoid body contact. Mortar and pestle was used as the reaction vessel.

4-chlorobenzaldehyde was less of an irritant; however, more precautions should be taken because the reaction required reflux with methanol which is a highly flammable solvent. Breakable glassware was used for reflux.

This experiment complies with some principles of green chemistry. They are:

- Minimize the waste. - Minimize the use of solvents, particularly hazardous ones. - Design the reaction for energy efficiency. - Minimize the number of steps in the process. - Minimize the risk of accidents.

Post - lab exercise


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Safety Information TOXICOLOGY 2-chlorobenzaldehyde is a skin, eye and respiratory irritant. It may cause burns and is destructive of mucous membranes. 2-Chlorobenzoic acid may produce discomfort of the eyes and skin. Ingestion may cause health damage. 2-Chlorobenzyl alcohol is an irritant and may be harmful if swallowed in large quantity. The dust may be discomforting to the upper respiratory tract. Potassium hydroxide is corrosive and may cause serious burns. It is harmful by ingestion, inhalation and in contact with skin. If the solid or solution comes into contact with the eyes, serious eye damage may result. Hydrochloric acid is extremely corrosive. Inhalation of vapour can cause serious injury. Ingestion may be fatal. Liquid can cause severe damage to skin and eyes. Hexane is an irritant and it is harmful by inhalation. It may cause impaired fertility. Prolonged exposure may cause serious health damage. Ethyl acetate is an irritant and it is harmful if swallowed in quantity. Vapours may cause drowsiness. PROTECTION

1. Goggles, gloves and apron must be worn when performing the experiment. 2. The reaction should be conducted in the ventilation fume hood. 3. Acidification should be performed with care. 4. Hexane and ethyl acetate are highly flammable so flames should not be opened in

the laboratory.

More information is available on: 1. http://ptcl.chem.ox.ac.uk/MSDS/newcas.html 2. http://ptcl.chem.ox.ac.uk/MSDS/#CASnumbers 3. http://www.geocities.com/self_redox/chemicals_main.htm

SUPPLEMENTS FOR INSTRUCTORS


30

Lab Preparation Before the laboratory begins, the following items must be prepared accordingly. The reaction For a class 1. 2-chlorobenzaldehyde (60 g per 30 student groups) 2. potassium hydroxide (45 – 60 g) 3. 2 or 5 mL pipettes or auto pipettes For each group 1. mortar and pestle ( x 1 ) 2. spatula (x 1 or 2) The TLC (reaction progress and product identification) For a class 1. starting material and products*

(dissolve each sample with methylene chloride in vial) 2. 30% ethylacetate/hexane (about 60 mL)

3. UV lamp (λ = 366 nm) 4. capillary tubes

5. filter papers (for TLC chamber)

For each group 1. TLC plates cut into 2.5 x 5 cm (x 4, 3 required + 1 spare) 2. beaker 50 mL (x 1) + Petri dish diameter 5 cm (x 1) 3. pencil (x1) 4. vial (x 3-5) 5. bottle of methylene chloride 6. forceps or tweezers (x 1 if any)

* The authentic samples must be checked to ensure that each appears as only one spot under the UV lamp. If samples are contaminated, crystallization is required. Both products can be crystallized from a mixture of methylene chloride and hexane or from ethanol.


31

The vacuum filtration For a class

1. Water aspirators 2. Filter papers of matching size to Buchner funnel

For each group 1. Buchner funnel, Buchner flask and filter rubber (x 1)

2. Vials or other containers (to collect products) 3. Water bottle (x 1) The acidification For a class

1. 3 M Hydrochloric acid (about 300 - 500 mL) 2. Blue litmus paper or universal litmus paper

For each group 1. Flask or beaker for acidification 2. Pipette and bulb (dropper) 3. Ice and ice-bath

Experimental Tips

1. After grinding for 5 - 10 minutes, a sticky solid (as a gum) begins to form. With further grinding for another 5 minutes, the reaction mixture becomes moist and turns more sticky. Additional 20 – 30 minutes of grinding are required to turn the gummy mixture to a relatively drier paste. This is an indication for the completeness of the reaction.

2. When the reaction is complete, distilled water is added into the mortar to dissolve the carboxylate salt. The residue is separated by vacuum filtration. The solid alcohol product is collected whereas the carboxylate salt remains in the filtrate. The carboxylate salt is precipitated with HCl and then collected by vacuum filtration. Washing the crude products with water will ensure purity while sacrificing a little of the yield.

Experimental Comments

1. The reaction is not 100% complete sometimes. Students may be advised to stop the reaction when the TLC plate shows a low intensity of starting material.

2. During the acidification step, even though the filtrate is acidic to the litmus paper, students should be advised to add more drops of HCl until there is no more precipitate.


32

Additional Identification Activities

Students familiar with spectroscopy have an opportunity to predict and then compare the

IR and / or the 1H NMR spectra of the two products with those of the starting material. The differences in IR spectra of the alcohol, the carboxylic acid and the aldehyde are readily detected (Figure 12). The C=O stretching in the aldehyde (A) at 1694.9 cm-1 is easily recognized. The spectrum of the acid (B) clearly exhibits a shifted strong C=O stretching (1680.48 cm-1) of the carboxylic group and a broad OH band at 2820.43 cm-1. The disappearance of the C=O band, the absorption band of OH (3302.36 cm-1) obviously indicates the alcohol product. Authentic samples give the same respective spectra.

Figure 12. IR spectra of 2-chlorobenzaldehyde (A), 2-chlorobenzoic acid (B) and

2-chlorobenzyl alcohol (C).


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In a similar manner, the 1H NMR spectrum of A shows (Figure 13) a characteristic aldehyde proton, -CHO, at 10.43 ppm (1H) while there is none in carboxylic acid (B). The alcohol (C) clearly distinguishes from those two compounds by two peaks at 1.98 ppm (1H) and 4.71 ppm (2H), belonging to one hydroxyl proton (HO-CH2-) and two methylene protons (HO-CH2-) respectively. Authentic samples also give identical respective 1H NMR spectra. This comparison provides a simple and effective way of reinforcing the use of IR and 1H NMR in structural determination.

Figure 13. 1H NMR spectra (in CDCl3) of 2-chlorobenzaldehyde (A), 2-chlorobenzoic acid

(B) and 2-chlorobenzyl alcohol (C).

Thermodynamics Activity Comments

While performing an extra exercise on using thermochemical data, students may encounter two problems. Firstly the data for the carboxylate salt formed in the reaction using KOH are not available. Students can solve this problem by writing the related reaction using water rather than KOH (although this reaction does not occur at an appreciable rate). Secondly data are not available for the 2-chlororbenzyl alcohol. Two approaches can solve this problem; either the reaction can be modelled by the non-chlorinated compounds for which the data are available, or, if molecular modelling software is used in the course, students can calculate the required data.


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2-chlorobenzaldehyde 89-98-5 2-chlorobenzoic acid 118-91-2 2-chlorobenzyl alcohol 17849-38-6 potassium hydroxide 1310-58-3 hydrochloric acid 7647-01-0 hexane 110-54-3 ethyl acetate 141-78-6

2-chlorobenzoic acid Physical property : white solid mp. = 140-143 OC Infrared Spectroscopy (IR) : 2820 cm-1 (-OH stretching) 1680 cm-1 (-C=O stretching) Nuclear Magnetic Resonance spectroscopy (NMR ): 1H NMR (300MHz ,CDCl3 + TMS) δ 7.36 (m, 1H), 7.47-7.50 (m, 2H), 8.04( m, 1H) 13C NMR (300MHz ,CDCl3 + TMS) δ 126.7, 128.4, 131.5, 132.5, 133.6, 134.8, 170.8 2-chlorobenzyl alcohol Physical property : white solid mp. = 71-73 OC Infrared Spectroscopy (IR) : 3302 cm-1 (-OH stretching) Nuclear Magnetic Resonance spectroscopy (NMR ): 1H NMR (300MHz ,CDCl3 + TMS) δ 7.49 (m, 1H), 7.35 (m, 1H), 7.26 ( m, 2H), 4.71 (d, 2H), 1.98 (1H) 13C NMR (300MHz ,CDCl3 + TMS) δ 62.9, 127.0, 128.8, 128.9, 129.4, 132.7, 138.1

CAS Registry Numbers

Product Information


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Figure 13 Infrared spectra of 2-chlorobenzaldehyde, 2-chlorobenzoic acid and 2-chlorobenzyl alcohol

Infrared Spectra


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Figure 14 1H NMR spectra of 2-chlorobenzaldehyde, 2-chlorobenzoic acid and 2-chlorobenzyl alcohol

1H NMR Spectra


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Figure 15 13C NMR spectra of 2-chlorobenzoic acid and 2-chlorobenzyl alcohol

13C NMR Spectra

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