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Lab XII

Lab VIII: Dehydration of 2-Methylcyclohexanol

Pre-lab Work

Reading Assignment:

  1. Dehydration mechanism:
    Hornback; 2nd ed.; pp. 378-380
  2. Mass Spectrometry:
    Appendix & Mohrig,
    Technique 20.1-20.2, & 20.5, pp. 341-349, 353-358 or 3rd edition Techniques 23.1-23.2, &23.5, pp. 406-413, 417-422.
  3. Infrared Spectroscopy:
    Appendix & Mohrig, Technique 18.1-18.4, pp. 228-236 & 18.6-18.9, pp. 243-267
    or 3rd edition Techniques 201.-20.4 pp. 277-285.
  4. Fractional Distillation:
    Appendix and Mohrig, 2nd edition, See Figure 11.15 in Technique 11.4, p. 144 or 3rd edition, see Figure 13.17 in Technique 13.4, p. 160.

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

1. What are the reaction(s) that you will be doing in lab?
2. Why is it important that the temperature on your thermometer never go over 95 degrees Celsius?
3. How will you know when approximately 5 mL of distillate has been collected?
4. What is the theoretical yield for today's reaction?  (Since you will not be separating your two alkene products, please determine one theoretical yield, in grams, for both of them together, theoretically.)  Please show ALL work. 
5. What differences should we see between the FTIR of 2-methylcyclohexanol and the FTIR of your products if the reaction is successful?  (Please list frequencies and bond types.)
6. Does this reaction have all 4 components of a nucleophilic substitution?  If not, what is missing?
7. What's Zaitsev's Rule?
8. Complete the TPC below (including a theoretical yield) and insert it in your notebook.


Table of Physical Constants (TPC)*

Compound

Formula

MW

g/mol

g OR mL used

mol
used

mp
oC

bp
oC

Density g/mL

Solubility

2-methyl-cyclohexanol

(50/50 of cis-(d,l) and trans-(d,l))

C7H14O

114.19        

 

 

7 and -4

165 and 167-168 0.9304 average

al, eth

phosphoric acid (15M)

H3PO4

N/A        

N/A N/A N/A N/A

w

          Theoretical Yield

         

1-methyl-cyclohexene

  96.17       -121 110 0.8102 eth, bz

3-methyl-cyclohexene

96.17

-115.5

104

0.7990

eth, bz, peth, chl

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

* CRC Online (87th edition)

Introduction

Dehydration (elimination of water) of
2-methylcyclohexanol can give either or both of two major products: 1-methylcyclohexene and/or 3-methylcyclohexene. The course of the reaction can be determined by analysis of the product using gas chromatography (GC), mass spectrometry and infrared spectroscopy.

Experimental Work
Objectives:

  1. To determine which alkene (1-methylcyclohexene or 3-methylcyclohexene) is most likely to form in the dehydration of 2-methylcyclohexanol by doing the following:
    • Dehydration of 2-Methylcyclohexanol. See Experimental Procedure below.
    • Gas Chromatography and Mass Spectrometry. Identify the peaks and determine the % composition of the alkene products.
    • Infrared spectrum of starting material and product. Compare your spectra to note any changes in functional groups. Use the IR literature table to obtain literature values.

Safety Notes

The dehydration products formed in this reaction are flammable and volatile. Work in the hood and keep your product covered to avoid loss through evaporation.

Handle phosphoric acid with care--it is corrosive if spilled on your skin. In case of a spill, wash with water for several minutes.

Experimental Procedure
In a 50 mL standard taper flask obtained from your instructor, place 5.0 mL of 2-methylcyclohexanol [50/50 of cis-(d,l) and trans-(d,l)], 1.5 mL of 15M phosphoric acid, and a boiling chip. Attach the flask to a fractional distillation apparatus (
See Figure 11.15 in Technique 11.4, p. 144). Set Control at 60-65% of 120V. The flask should be heated at such a rate that the temperature of the distillate is not allowed to rise above 95oC. Higher temperatures result in the distillation of too much unreacted alcohol. Continue heating until about 5 mL of product has been collected in a standard taper flask set in an ice bath. What compounds should be present in your distilled 5 mL of product?

Transfer the distillate to a 60 ml separatory funnel. Wash the product with 3 mL of saturated sodium hydrogen carbonate solution, allow the layers to separate, and remove the aqueous layer. Wash the organic layer with about 3 mL of water, remove the aqueous layer, and dry the organic layer with phase paper and anhydrous magnesium sulfate, filtering the solution into a pre-weighed vial. Weigh the vial and contents, and calculate the % yield of product.
What's the purpose of each extraction and the phase paper and magnesium sulfate?

Prepare a dilute (approximately 10%) 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 your instructor for them to run your sample overnight.

Obtain and FTIR spectrum of your neat product. Obtain an FTIR spectrum of the starting material (2-methylcyclohexanol) from your instructor. Analyze both IR spectra accompanying each one with an appropriate data table listing functional group and specific bond causing the absorbtion.

Store your left-over product in a clean 20 mL screw top vial in your tote.

Waste Disposal and Clean-up

  1. Dispose of the phosphoric acid aqueous waste from your reaction in the phosphoric acid waste bottle.
  2. Dispose of the aqueous and bicarbonate washes in the sodium hydrogen carbonate waste bottle.
  3. Dispose of your excess GC-MS solution in the flammables 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.)

Obtain your GC/MS information from this Dehydration Lab. You should obtain the entire gas chromatograph and the mass spectra of 3 PRODUCTS from this lab. (Note that you might also have some left-over reactant!)

Copy and paste the spectra into your e-lab.
Draw out the mechanism by which the reaction occurs using either ChemDraw or a handwritten mechanism that you have taken a picture of with your laptop/smartphone.

  1. Do your results show that this reaction occurred?
  2. Tell me as much as possible about the structures of the three major product peaks in the GC of your product (2 expected and 1 unexpected).
  3. Calculate the overall % yield for your reaction, assuming all that is isolated is alkene. Show ALL your calculations please.
  4. Find the percentage of each expected compound in your product using your GC data. Please ignore all other peaks in your GC except your two expected products.  Remember; please use only the ‘Area’ column in the Percent Report to determine the % composition of just your two major products.  From this data, please state which proton was easiest to abstract from 2-methylcyclohexanol. 
  5. Write the mechanism for the reaction that produced your major product. Please show how phosphoric acid was used as a catalyst.
  6. Are your results consistent with Zaitsev's Rule?
  7. Why did we want to obtain the FTIRs of 2-methylcyclohexanol and your organic product?  Did they help in your analysis of your products?  Why?
  8. Discuss any unexpected results in your experiment. What you would do differently if you were to do this lab again?