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Lab V

Lab VI

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Lab VIII

Lab IX

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Lab XI

Lab XII

Lab VI: Nucleophilic Substitution Reactions: Competing Nucleophiles

Pre-lab Work

Reading Assignment:

  1. Description of Experiment--See Below.
  2. Reflux:
    Appendix & Mohrig, 2nd edition, Technique 7.1, pp. 56-58 or 3rd edition, pp. 59-60.
  3. Extraction:
    Appendix & Mohrig, 2nd edition, Technique 8.1-8.4, 8.7-8.8, pp. 75-84, 92-95 or 3rd edition Techniques 11.1-11.4, 11.6-12.1 pp. 114-125, 131-135.
  4. Gas Chromatography-Mass Spectrometry (GC/MS):
    Appendix & Mohrig, 2nd edition, Technique 16, pp. 190-205 or 3rd edition, Technique19, pp. 256-273.

Pre-lab Questions:
(Please turn in the answers to these questions when you arrive to lab.)

1. Fill in the grams and moles used of each reagent in the Table of Physical Constants shown below.
2. Fill in the theoretical yields of chlorohexane and bromohexane below and in the Table of Physical Constants
.
3. IF the only product of this reaction was 1-chlorohexane, how many moles of 1-chlorohexane could theoretically be made (i.e. what would be the theoretical yield of 1-chlorohexane? ANSWER: ________ moles. What is the theoretical yield in grams? ______g.
4. IF the only product of this reaction was 1-bromohexane, what would be the theoretical yield of 1-bromohexane (a) in moles ___________(b) in grams __________ ?

5. Please define the following terms in your own words (i.e.-words and/or pictures that you understand.)
a.) reflux                b.) aqueous-organic extraction         c.) powder funnel                 d.) separatory funnel                          e.) nucleophile                         
6. Will 1-hexanol be a solid or liquid at room temperature?
7. Why is it important to transfer ALL of the ammonium salts into your reaction flask?
8. What are your purpose and hypothesis statements for this lab?  (Make sure to include what data you will need to answer your purpose statement.)

Table of Physical Constants (TPC)* for Competing Nucleophiles Lab: 

Compound

Formula
MW
Grams
OR
mL
used
moles
used
Melting Point

oC

Boiling Point

oC

Density

(g/mL)

Solubility

1-hexanol

C6H14O

102

 

  -47 158 0.8136 sl H2O; s EtOH, ace, chl; msc eth, bz

ammonium
chloride

NH4Cl

53.49

    N/A N/A N/A  

ammonium
bromide

NH4Br

97.95

    N/A N/A N/A  

concentrated
sulfuric acid

H2SO4

(18M)

N/A

 

N/A N/A N/A  
 

 

Theoretical Yield

       

1-chlorohexane

C6H13Cl

120

-94 135 0.8738 i H2O; s EtOH, eth, ace, bz; vs chl; sl ctc

1-bromohexane

C6H13Br

165

-84 155 1.1744 i H2O; msc EtOH, eth; s ace; vs chl

Data is from the CRC Handbook of Chemistry and Physics, 91st edition.


Introduction
This experiment is designed to demonstrate principles of substitution reactions with nucleophiles.

General Reaction

lab 5gen



Experimental Work
Objectives:

  1. To perform a Nucleophilic Substitution Reaction: Competing Nucleophiles (see below).
  2. To determine the ratio of 1-chlorohexane to 1-bromohexane, using GC/MS.
  3. To determine which ion is a better nucleophile, Cl- or Br-.

Safety Notes: Concentrated sulfuric acid can cause severe burns on contact with the skin or eyes. Wear gloves and goggles when handling this compound. Wash immediately with cold water for several minutes in case of a spill.

Experimental Procedure
Place approximately 10 g of icein a 50 mL beaker and carefully add 10 mL concentrated sulfuric acid.
Why ice and not water? Set this mixture aside to cool. Obtain 3.2 g ammonium chloride and 5.9 g ammonium bromide. Carefully transfer all the reagents to a 100 mL round bottom flask. Exercising caution, add the sulfuric acid mixture to the ammonium salts a little at a time. Swirl the mixture. Carefully add 6.5 mL 1-hexanol to the reaction mixture. Add a few boiling stones to the flask and set up your reflux apparatus. Turn the water on for the condenser. See the Appendix for the information on assembling a reflux apparatus. A demo of a reflux apparatus will also be set up in lab.

Turn the heating mantle on and adjust the voltage so that the mixture maintains a gentle boiling action (set Variac at approximately 30% of 140 V). Be careful to adjust the reflux ring so that it remains in the lower fourth of the condenser. Violent boiling will cause loss of product. Continue refluxing the mixture for approximately 45 minutes.

During the reflux time obtain a separatory funnel (60 mL), and set it up. You will be instructed on the use of the separatory funnel and you will also be shown how to prepare your product for GC/MS.

Upon completion of the reflux period, lower and turn off the heating mantle. Allow the reaction mixture to cool to a comfortable temperature before turning off the water and removing the reflux condenser.

Transfer the warm reaction mixture to the 60 mL separatory funnel, taking care to leave behind any precipitated salts/boiling stones. Allow the organic and aqueous layers to separate and then remove the aqueous layer. To the organic layer in the separatory funnel add 10 mL of distilled water. Shake the mixture in the separatory funnel and allow the layers to separate. Separate the organic layer from the aqueous layer. (The organic layer may be on the top or bottom depending on how much unreacted 1-hexanol is present. How will you know which layer is organic and which layer is aqueous?) Wash the organic layer a second time with an additional 10 mL of distilled water.

Wash the organic layer with about 10 mL of 5% sodium hydrogen carbonate solution, and remove the aqueous layer. Dry the organic layer with phase paper and anhydrous magnesium sulfate, filtering the solution into a small beaker. Keep the beaker covered to prevent evaporation of the product. What's the purpose of the phase paper and anyhydrous magnesium sulfate?

What was the purpose of the water and sodium hydrogen carbonate extractions?

Prepare a dilute solution of your sample in methylene chloride for GC/MS. Using a disposable glass pipet, measure 3-4 drops of your sample into a small test tube. Use another disposable glass pipet to add approximately 2-3 mL methylene chloride to the test tube. Transfer this solution to a GC/MS vial, and give your sample to the student lab assistant or instructor. Please use the sign-up sheet to give your gc/ms sample a file name that you will be able to recognize when you need to analyze your GC/MS data in a couple days. Save the remainder of your product in a small screw-top vial in your tote.

Obtain an FTIR of your pure product.

Waste Disposal and Clean-up

  1. Dispose of the sulfuric acid aqueous waste from your reaction in the sulfuric acid waste bottle.
  2. Dispose of the aqueous and bicarbonate washes in the sodium hydrogen carbonate waste bottle.
  3. Dispose of your excess product in the flammable waste container.
  4. Rinse your glassware with acetone and dispose of the acetone rinse in the flammables waste bottle.
  5. Wash your glassware in hot soapy water and return to the appropriate storage area.

Post-lab Questions
(Please turn in the answers to these questions when you arrive at your NEXT lab.)

1. What is the mole ratio of 1-bromohexane to 1-chlorohexane in your product?  Please show your calculations and mention the retention times for the two peaks you identified as 1-bromohexane and 1-chlorohexane.
2.  Which halide, Cl- or Br-, was the better nucleophile in your reaction?  Please justify with lab data.
3. Using your FTIR and GC/MS data, please hypothesize what the third most abundant compound could be in your product mixture.  Please justify your answer.
4. What was the purpose of the acid in this experiment?
5. Why was it important to transfer all of the ammonium salts to the reaction flask?
6. Were the results as you expected from your purpose/hypothesis statements? Why or why not?