Expt #3. Friedel Crafts Acylation of Naphthalene

Relevant textbook reading - Klein, Chapter 19.1-19.10

Overview - You will acylate naphthalene with propionyl chloride/AlCl₃ to form propionyl naphthalene. NMR and GC/MS will be used to determine the relative yields of the two possible propionyl naphthalene products.

Procedures

Running the reaction.

• Using the balance located in the dispensing fume hood, weigh out 11 mmol AlCl₃ into a dry 50-mL round bottom flask.
• Add a small stir bar and 10 mL dichloromethane (DCM).
• Cautiously add 1.0 mL propionyl chloride.
  
• Attach a reflux condenser, Claisen adapter, and addition funnel as shown in Fig 7.4a (p 63) in Mohrig.
• Prepare a solution of naphthalene (10 mmol) in 5 mL DCM and add this to the addition funnel.
• Add the naphthalene solution dropwise over a period of approx 5 min to the stirring solution.
• Stir for an additional 30 min.

Work-up procedures.

• Slowly add 7.5 mL cold water via the addition funnel to stop the reaction and quench any left over acyl chloride.
• Place the flask on an ice bath and slowly add 7.0 mL concentrated HCl. At this point all of the aluminum salts should have dissolved. If this is not the case add another 0.5 mL of HCl.
• Transfer the reaction mixture to a separatory funnel and drain the lower layer into a small Erlenmeyer flask. Avoid getting any of the aqueous layer when carrying out this step.
• Extract the aqueous layer with 5 mL additional DCM and combine the organic layer with the previous organic layer.
• Wash the combined organic layers with 5 mL 1% NaHCO₃(aq). Caution, CO₂ gas is liberated here so the separatory funnel will need to be vented early and often.
• Dry the organic solution over Na₂SO₄.
• Use the rotary evaporator to remove the DCM solvent in vacuo.
• Determine the yield.
  

Characterization of Product

• Obtain the ¹H and ¹³C NMR spectrum using CDCl₃ as the solvent.
• Dilute 1-2 drops of the NMR sample solution in 5 mL DCM and submit the solution for GC-MS analysis. (You will need to process your GC/MS data file using either WSearch (PC) or OpenChrom (Mac) as was done previously in Chem 350.)
  
• Measure the refractive index of the product. (The instructor or TA will assist you in this.) Mohrig technique 16.3-16.4.

Assigned Questions

1. Use both the proton and C-13 NMR spectra as well as the GC/MS to determine the relative yields of the two products (i.e. the product ratio). How well do the various methods agree with each other?
   
2. Use resonance theory to explain the observed product ratio. In order to do so properly, you will need to draw out all resonance structures of the sigma complex intermediate involved in the formation of both the 1-propionyl and 2-propionyl napthalene products.
3. DCM may seem a poor choice of solvent to use for a Friedel-Crafts reaction due to an obvious side reaction that could occur. What is this side reaction and propose a reason why it does not occur to a significant extent in our experiment.