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Lab V: Nucleophilic Substitution Reactions: Competing Nucleophiles

Pre-lab Work

Fill in as much of the Table of Physical Constant as possible, for this experiment: a blank one is shown below. For this experiment you can't calculate a theoretical yield, therefore leave that section of your TPC is listed as "N/A" for "Not Applicable.".

Reading Assignment:

  1. Description of Experiment--See Below.
  2. Reflux:
    Appendix & Mohrig, Technique 7.1, pp. 56-58.
  3. Extraction:
    Appendix & Mohrig, Technique 8.1-8.4, 8.7-8.9, 8.11, pp. 75-84, 92-96, 99.
  4. Gas Chromatography:
    Appendix & Mohrig, Technique 16, pp. 190-205.

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

  lab 5

Compound

Formula
MW
Grams
OR
mL
used
moles
used
Melting Point

oC

Boiling Point

oC

Density

(g/mL)

Refrac. Index

n20D

Solubility

1-Butanol

C4H10O

74.12

 

           

Ammonium
Chloride

NH4Cl

53.49

         

N/A

 

Ammonium
Bromide

NH4Br

97.95

         

N/A

 

Concentrated
Sulfuric Acid

H2SO4

(18M)

N/A

 

 

 

     

N/A

 
 

 

Theoretical Yield

         

1-Chlorobutane

C4H9Cl

92.49

N/A1

N/A1

         

1-Bromobutane

C4H9Br

136.9

N/A2
N/A2
         

Note: Will 1-butanol be a solid or liquid at room temperature?

*CRC Handbook of Chemistry and Physics, 52nd ed.

** CRC Online (87th edition)

Printable Version of TPC!


Pre Lab Questions

  1. IF the only product of this reaction was 1-chlorobutane, how many moles of 1-chlorobutane could theoretically be made (i.e. what would be the theoretical yield of 1-chlorobutane? ANSWER: ________ moles. What is the theoretical yield in grams? ______g.
  2. IF the only product of this reaction was 1-bromobutane, what would be the theoretical yield of 1-bromobutane (a) in moles ___________(b) in grams __________ ?


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 prove that the reaction occurred using IR spectroscopy.
  3. To determine the ratio of 1-chlorobutane to 1-bromobutane, using gas chromatography.
  4. To determine which ion is a better nucleophile, Cl- or Br-.


Experimental Procedure

Place approximately 10 g of ice in a 50 mL beaker and carefully add 10 mL concentrated sulfuric acid. Set this mixture aside to cool. Obtain 3.2 g ammonium chloride and 5.9 g ammonium bromide. Carefully transfer all the reagents, using a powder funnel, to a 50 mL pear-shaped flask. Exercising caution, add the sulfuric acid mixture to the ammonium salts a little at a time. Swirl the mixture. Carefully add 4.5 mL 1-butanol into the reaction mixture. Add a few boiling stones to the pear-shaped 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 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 40% 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 1 hour.

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 be shown how to use the Gas Chromatograph (GC).

Upon completion of the reflux period, remove the heating mantle. Allow the reaction to become cool enough to handle before you remove the reflux condenser. Cool the reaction mixture to room temperature.

Transfer the warm solution 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 water. Shake the mixture in the separatory funnel and allow the layers to separate. Separate the organic layer from the aqueous layer. (Warning: The organic layer may be on the top or bottom depending on how much unreacted 1-butanol is present. How will you know which layer is organic and which layer is aqueous?)

To the organic layer in the separatory funnel, add 10 mL of saturated sodium hydrogen carbonate solution. Shake the mixture and separate the layers, transferring the organic layer to a 25 mL Erlenmeyer flask. Add a few grains of sodium sulfate to dry the organic layer. If the drying agent clumps, add a few more grains until additional drying agent is free-flowing. Wait 10 minutes, then decant the clear organic layer into a clean, dry, pre-weighed, sample vial. This product should now be weighed, then analyzed by GC and FTIR.

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

You should obtain a GC of a standard mixture of 1-bromobutane and 1-chlorobutane and an IR spectrum of 1-butanol.


Special Waste Disposal
Place the aqueous phase of the reaction mixture in the sulfuric acid waste disposal container located in the hood.
Flush aqueous extracts down the hood sinks.


Post-lab Work

  1. Calculate the peak areas for each peak in the GC of your product. Use the following formula to calculate peak areas:

    [Peak Area = (height of peak) x (width of peak half way up the peak)]
    {as seen in Mohrig, Figure 16.13}

    Use these areas to calculate the % composition of your product (ignore all other impurities).
  2. Given your overall percent composition, determine the molar ratio between 1-chlorobutane and 1-bromobutane
  3. Write a balanced chemical reaction using your experimental molar composition of your 2 alkyl halide products.
  4. What information can you obtain from your IR data?
  5. Determine which nucleophile "won" and give possible reasons why.
  6. How many grams of product did you obtain?
  7. What is the purpose of the acid in this experiment?
  8. Why was it important to transfer all of the ammonium salts to the flask?
  9. Discuss any errors that impacted the overall outcome of your experiment and state what you would do differently if you were to do this lab again.