|
Lab VIII:
Dehydration of 2-Methylcyclohexanol
Pre-lab
Work
- Complete
the TPC below (including a theoretical yield) and insert it in your notebook.
Reading
Assignment:
- Dehydration
mechanism:
Hornback; 2nd ed.; pp. 378-380
- Mass
Spectrometry:
Appendix
& Mohrig, Technique Technique 20.1-20.2,
& 20.5, pp. 341-349, 353-358.
- Refractometry:
Appendix
& Mohrig,
Technique 13 pp. 161-164.
- Infrared
Spectroscopy:
Appendix
& Mohrig,
Technique 18.1-18.4, pp. 228-236 & 18.6-18.9, pp. 243-267.
- 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:
- 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.
- Do your results
show that this reaction occurred?
- Identify at least
three product peaks (2 expected and 1 unexpected) using GC/MS.
- Calculate the
overall % yield for your reaction, assuming all that is isolated is
alkene.
- Calculate the
percentage of each expected compound in your product using your refractive
index data.
- 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?
- Using the % composition, write a chemical equation for your reaction.
- Are your results
consistent with Zaitsev's Rule?
- Discuss any errors
in your experiment and state what you would do differently if you were
to do this lab again.
|