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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. Refractometry:
    Appendix & Mohrig, Technique
    13 pp. 161-164.
  4. Infrared Spectroscopy:
    Appendix & Mohrig, Technique 18.1-18.4, pp. 228-236 & 18.6-18.9, pp. 243-267.
  5. 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

nD20

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 1.4628
average

al, eth

phosphoric acid (15M)

H3PO4

N/A        

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

N/A

w

         
Theoretical Yield

           

1-methyl-cyclohexene

  96.17       -121 110 0.8102
1.4503
eth, bz

3-methyl-cyclohexene

96.17

-115.5

104

0.7990

1.4414

eth, bz, peth, chl

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

* CRC Online (87th edition)

Printable version of TPC!

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, infrared spectroscopy, and refractometry.

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.
    • Refractive Index. Estimate the mole percentage of the two constituents graphically.


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.

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.

Determine the refractive index of the product. The composition of the product can be estimated by assuming a linear relation between refractive index and molar concentration. Estimate the composition of the product in this way.

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. Calculate the percentage of each expected compound in your product using your refractive index data.
  5. Find the percentage of each expected compound in your product using your GC data. How do the nD and GC % compositions vary? Which is the more reliable method?
  6. Using the % composition, write a chemical equation for your reaction.
  7. Are your results consistent with Zaitsev's Rule?
  8. Discuss any errors in your experiment and state what you would do differently if you were to do this lab again.