Experiment 10:
ACID-CATALYZED
DEHYDRATION
OF AN ALCOHOL
WITH REARRANGEMENT
Objectives

To perform a dehydration of 2-methylcyclohexanol to
form isomeric alkenes under E1 conditions.

To purify the product using simple distillation.

To analyze the product using GC analysis in order to
identify and quantify products.

To characterize the reactant and products using IR
spectroscopy.
Before coming to lab…

Please review:

Simple distillation

E1 elimination reactions

GC Analysis
CHEMICAL EQUATION
• Three different ISOMERIC alkenes can be produced.
• Because this reaction is performed near equilibrium
conditions, the relative amount of each product reflects its
stability.
OH
CH3
CH3
CH3
+
+
CH2
H2SO4
heat
more substituted = lower energy = more stable = more predominate!
RATE LIMITING STEP
+
H OSO3H
..
C C
H OH
..
Fast Reaction
C C
H
O+ H
H
Rate limiting step
C C
HSO4-
C C
+
H
C C
+
H
C C
H O+ H
H
+ HSO4-
+ H2O
+ H2SO4
Fast Reaction
• The rate of elimination of water depends on the stability of the
carbocation formed.
• Formation of the carbocation is the most energetically
unfavorable, and therefore the slowest, step in dehydration
reactions.
E1 MECHANISM
2. …which forms a
new O-H bond, where
oxygen bears a
positive charge
(oxonium ion). Water
is eliminated-forms 2o
carbocation.
1. The hydroxyl
oxygen attacks and
removes a proton
from sulfuric acid…
H
OH
CH3
H
H
3. Products may
form from the 2o
carbocation, but it is
more likely that the
2o C+ will rearrange
to a 3o C+.
O
H
O
SO3H
H
H
a
2o
CH3
CH3
H
H + H2O + HSO4
secondary carbocation
+ HSO4-
H
b
H
H
H
CH3
d
c) Hydride Shift
H 2O
H
a
H
Carbocation
rearrangement
H
H
H e
d
3o
C
H
b
a) -H3O+
1-methyl-1cyclohexene
CH3
b) -H3O+
d) -H3O+
CH3
3-methyl-1cyclohexene
1-methyl-1cyclohexene
4. At the carbocation
stage, water will remove
a proton from the carbon
ADJACENT to the
carbocation. The
electrons form the pi
bond of the alkene.
H
H
e
H2O
e) -H3O+
CH2
CH3
methylenecyclohexane
THEORETICAL YIELD

The only reactant is 2-methylcyclohexanol. The H2SO4 is
simply a catalyst, since it is regenerated in the end.

Theoretical yield is calculated assuming that the major
product formed is one that results from the most stable
carbocation intermediate.

Theoretical yield (g) =
# g reactant
1 mol of reactant
#g
Amount
you started
with
Molecular
weight of
reactant
1 mol product
1 mol reactant
Stoichiometric
ratio
#g
1 mol product
Molecular
weight of
product
Always end up
in units of
grams of
product!!!
OVERVIEW







Set up and perform simple distillation to collect
products.
Obtain final product mass and calculate percent yield.
Prepare and submit GC sample for analysis.
Pick up GC results and record standard retention
times.
Identify components in sample chromatogram by
comparing to standard chromatogram.
Quantify alkenes by calculating adjusted area
percent.
Characterize reactant and products using provided IR
spectra.
EXPERIMENTAL PROCEDURE:
(Simple distillation)
Clamp flask
to ring
stand here!
Blue Keck
clips here!
Clamp flask
to ring
stand here!
water out
• Place 2-methylcyclohexanol,
sulfuric acid and boiling chips in 50
mL round bottom flask.
• Clamp flask to ring stand.
water in
• Weigh 10 mL flask. Clamp to
other ring stand.
50
mL
Heating Mantle
10
mL
• Attach clear hoses to condenser.
Run water in at the bottom, out at
the top!
iron ring
to
voltage
regulator
• Build rest of distillation
apparatus, using blue Keck clips to
secure top and bottom joints
around condenser.
EXPERIMENTAL PROCEDURE:
(Simple distillation)
• Begin water flow, and apply heat
(VR@30) to boil solution.
• Record temperature when
distillate begins to collect in 10 mL
flask (Ti).
water out
water in
• Collect ~ 5 mL distillate.
Keck
clips!
• Record temperature right before
you drop the heating mantle (Tf).
Heating Mantle
• Allow the solution to cool.
iron ring
to
voltage
regulator
• Reweigh 10 mL flask to obtain
actual product yield.
• Prepare GC sample and
submit! Don’t forget!!!
Table 10.1:
Experimental Results
 must calculate the amount of product that can be
Theoretical Yield (g)
formed based on the amount of 2methylcyclohexanol used!
 This mass will be obtained by weighing the 10 mL
Actual Yield (g)
round bottom flask before and after the distillation.
The difference in the mass is the actual product
yield.

% yield
Actual yield (g) X 100
Theoretical yield (g)
 physical state and color of distillate.
Product Appearance
Table 10.2:
GC Analysis Results
Compound
GC Retention time (min)
Standard
methanol
Sample
Area
Percent
Adjusted
Area Percent
 Never calculate adjusted
area % based on the solvent!
2-methylcyclohexanol
 No need to calculate
adjusted area % on the
reactant, either!
1-methyl-1-cyclohexene
Area % THIS alkene
Sum area% all alkenes
X 100
3-methyl-1-cyclohexene
Area % THIS alkene
Sum area% all alkenes
X 100
methylenecyclohexane
Area % THIS alkene
Sum area% all alkenes
X 100
Infrared Spectroscopy (IR)


Q: What is it?

Vibrational energy of bonds

Certain types of polar bonds absorb IR radiation
and vibrate (excited state)
Q: Why is it useful?

Certain functional groups absorb at characteristic
frequencies.

By looking at what frequencies are absorbed, we
can identify the presence or absence of certain
types of bonds!
Infrared Spectroscopy (IR)

Q: How does it work?



This molecule is represented with a potential
energy diagram.
Each horizontal line represents a vibrational state
of a C=O bond.
If we add IR light energy at the correct wavelength,
we get excitation to the next vibronic energy level.
Infrared Spectroscopy (IR)

Q: What is an IR spectrum?
% transmittance of IR
radiation
Frequency of vibration
(in wavenumbers)
EXPERIMENTAL PROCEDURE:
IR Analysis
THINGS TO CONSIDER…
OH
CH3
CH3
•What kinds of bonds do I
have?
• Ex. C-O, C=C, CH3, etc.
• If they appeared in the IR
spectrum, where would they
be?
• Use a correlation table
to determine the
approximate frequency
for that type of bond.
• Now, look at the spectrum.
Are they there?
EXPERIMENTAL PROCEDURE:
IR Analysis
Full IR Absorption Correlation Table in Appendix J
Base values for Absorptions of Bonds (cm-1)
OH
~3400
C-O
~1100
C-H (sp2)
~3100-3000
C-H (sp3)
~3000-2850
C=C
~1630
Table 10.3:
IR Spectral Analysis Results
IR spectra are on page 87 in lab manual!
Functional
Group
2-methylcyclohexanol
1-methyl-1cyclohexene
3-methyl-1cyclohexene
Methylenecyclohexane
Frequency (cm-1)
Frequency (cm-1)
Frequency (cm-1)
Frequency (cm-1)
3200-3500
N/A
N/A
N/A
1000-1200
N/A
N/A
N/A
Base
Values
(cm-1)
OH stretch
C-O stretch
2850-3000
sp3 CH stretch
3000-3100
N/A
1600-1680
N/A
sp2 CH stretch
C=C stretch
Infrared Spectroscopy (IR)
(How to answer the questions…)



Your goal is to explain clearly how you were able to use
IR spectroscopy to DIFFERENTIATE between reactant
and product.
Always discuss the appearance of certain types of
absorptions, or the disappearance of others, which
indicate that functional groups have changed.
Always answer like this: (fill in the blanks)

In the IR spectrum of the product, the appearance of the
_____ (type of bond) absorption at _____ (actual
frequency) indicates the conversion of the reactant to the
product. The typical frequency for this type of absorption
is _____ (base value frequency).
SAFETY CONCERNS
• The alcohol and resulting alkenes are extremely
flammable. Be very cautious when applying heat.
• Concentrated sulfuric acid is VERY CORROSIVE
and will burn skin on contact. Please use gloves
and goggles at all times when in laboratory.
WASTE MANAGEMENT

Place all liquid waste in the container labeled “LIQUID
WASTE”.

Be careful when disposing of acidic waste remaining in
50 mL round bottom flask! It is extremely corrosive. Use
a small amount of water to rinse it into the waste
container before cleaning it thoroughly using directions
on next slide…
CLEANING

After disposing of the liquid waste, clean the 50 mL
round bottom flask with soap, water, brush, and a
final rinse with wash acetone.

All other ground glass joint ware can simply be rinsed
with wash acetone into a waste container.

Be sure all ground glass joint ware is completely dry
before returning to plastic container in fume hood.

Be sure all other glassware used is completely dry
before returning to lab drawer.
IN LAB QUESTIONS
(The following questions should be answered in laboratory notebook.)

Predict the products and draw a complete mechanism
for their formation from acid-catalyzed dehydration
of 2-methyl-2-pentanol. Be sure to show all steps and
intermediates. Circle the major product.
HO
+
MW: 102.18 g/mol
H
O
SO3H
?
IN LAB QUESTIONS
(The following questions should be answered in laboratory notebook.)

Calculate the theoretical yield for the reaction above
based on 4.0 g of the starting alcohol and a catalytic
amount of sulfuric acid. The molecular weight of the
alcohol is given, but the molecular weight of the
product must be determined based on the structure.
Be sure to include units.