Dehydration of Alcohols
Dehydration of Alcohols
Dehydration of Alcohols
ACID-CATALYZED
DEHYDRATION
OF AN ALCOHOL
WITH REARRANGEMENT
Objectives
Please review:
Simple distillation
E1 elimination reactions
GC Analysis
CHEMICAL EQUATION
Three different ISOMERIC alkenes can be
produced.
OH
CH3
H2SO4
heat
CH3
CH3
CH2
H OSO3H
Fast Reaction
C C
H
O+ H
H
HSO4-
C C
C C
+
H
C C
C C
H O+ H
H
+ HSO4-
+ H2O
+ H2SO4
Fast Reaction
E1 Mechanism reaction
1. The hydroxyl
oxygen attacks
and removes a
proton from
sulfuric acid
2. which forms a
new O-H bond,
where oxygen
bears a positive
charge (oxonium
ion). Water is
eliminated-forms
2o carbocation.
OH
CH3
H
H
Products may
form from the 2o
carbocation, but it is
more likely that the
2o C+ will rearrange
to a 3o C+.
H
H
H
H
CH3
CH3
H + H2O + HSO4
secondary carbocation
+ HSO4a
2o
SO3H
H
b
CH3
H2O
c) Hydride Shift
Carbocation
rearrangement
H e
d
3o
C
H
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
H 2O
e) -H3O+
CH2
CH3
methylenecyclohexane
THEORETICAL YIELD
Amount
you
started
with
1 mol of reactant
Molecular
weight of
reactant
1 mol reactant
1 mol product
1 mol product
Stoichiometric
ratio
Molecular
weight of
product
Always
end up in
units of
grams of
#g
product!!!
OVERVIEW
EXPERIMENTAL
PROCEDURE:
(Simple distillation)
Clamp
flask to
ring
stand
here!
Place 2methylcyclohexanol,
sulfuric acid and boiling
chips in 50 mL round
bottom flask.
Blue Keck
clips
here!
water out
water in
Clamp
flask to
ring
stand
here!
50
mL
Heating Mantle
10
mL
iron ring
to
voltage
regulator
EXPERIMENTAL
PROCEDURE:
(Simple distillation)
Begin water flow, and
apply heat (VR@30) to boil
solution.
water out
water in
Keck
clips!
Heating Mantle
iron ring
to
voltage
regulator
Collect ~ 5 mL distillate.
Reweigh 10 mL flask to
obtain actual product yield.
Prepare GC
sample and
Table 10.1:
Experimental Results
must calculate the amount of
product that can be formed based on
the amount of 2-methylcyclohexanol
used!
% yield
Product Appearance
Table 10.2:
GC Analysis Results
Compound
GC Retention time
(min)
Standar
d
Sample
Area
Percent
Adjusted
Area Percent
methanol
Never calculate
adjusted area % based
on the solvent!
2-methylcyclohexanol
No need to calculate
adjusted area % on the
reactant, either!
1-methyl-1cyclohexene
3-methyl-1cyclohexene
methylenecyclohexane
Infrared Spectroscopy
(IR)
Q: What is it?
Q: Why is it useful?
Infrared Spectroscopy
(IR)
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
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.
CH3
EXPERIMENTAL
PROCEDURE:
IR Analysis
Full IR Absorption Correlation Table in Appendix J
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-methylcyclohexano
l
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
sp CH
stretch
2
C=C
stretch
Infrared Spectroscopy
(IR)
(How to answer the questions)
SAFETY CONCERNS
The alcohol and resulting alkenes are
extremely flammable. Be very cautious
when applying heat.
WASTE MANAGEMENT
CLEANING
IN LAB QUESTIONS
HO
SO3H
IN LAB QUESTIONS