Lab VIII: Dehydration of 2-Methylcyclohexanol

Pre-lab Work

1. Complete the TPC below (including a theoretical yield) and insert it in your notebook.
   

Reading Assignment:

1. Dehydration mechanism:
   Hornback; 2nd ed.; pp. 378-380
2. Mass Spectrometry:
   Appendix & Mohrig, Technique Technique 20.1-20.2, & 20.5, pp. 341-349, 353-358.
3. Infrared Spectroscopy:
   Appendix & Mohrig, Technique 18.1-18.4, pp. 228-236 & 18.6-18.9, pp. 243-267.
4. Fractional Distillation:
   Mohrig, See Figure 11.15 in Technique 11.4, p. 144.
   

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.
     
     

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 95^oC. 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.

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.

Prepare a gas chromatography/mass spectrum (GC/MS) sample of your product using the following method: Put 2-3 drops of your product into a small test tube. Add approximately 3 mL of methylene chloride. Shake well. Using a pipet, fill a GC/MS vial between 1.5-2 mL full. Cap your vial and see your instructor or lab assistant about signing up for the GC/MS and placing your vial in the appropriate position of the automatic sampling tray of the GC/MS instrument.

Run an IR spectrum of your neat product. Obtain an IR spectrum of the starting material. Analyze both IR spectra accompanying each one with an appropriate data table.


Special Waste Disposal
Put any phosphoric acid waste in a specially-labelled waste bottle.

Sodium Hydrogen Carbonate solution: Flush with water down the hood sink drain.

Post-lab
Print out your GC/MS information from this Dehydration Lab. You should print out 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 notebook. Draw out the mechanism by which the reaction occurs and copy/paste this into your notebook.

1. Do your results show that this reaction occurred?
   
2. Identify at least three product peaks (2 expected and 1 unexpected) using GC/MS.
   
3. Calculate the overall % yield for your reaction, assuming all that is isolated is alkene.
   
4. Find the percentage of each expected compound in your product using your GC data.
5. Using the % composition, write a balanced chemical equation for your reaction.
6. Write the mechanism for this reaction, showing phosphoric acid as a catalyst.
   
7. Are your results consistent with Zaitsev's Rule?
   
8. Discuss any unexpected results in your experiment. What you would do differently if you were to do this lab again?