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Experiment 10: ACID-CATALYZED DEHYDRATION OF AN ALCOHOL WITH REARRANGEMENT
Experiment 10:
ACID-CATALYZED DEHYDRATION OF AN ALCOHOL
WITH REARRANGEMENT
Objectives
To synthesize isomeric alkenes from a 2o alcohol.
To determine major product using GC analysis.
To understand product distribution using molecular modeling.
To learn how to use IR Spectroscopy to differentiate between reactants and products.
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.
H2SO4
heat+ +
OH
CH3 CH3 CH2CH3
• more substituted = lower energy = more stable = more predominate!
RATE LIMITING STEP C
H
C
OH OSO3H
....
H+ C
H
C
O
H
H+
+ HSO4-
Fast Reaction
C
H
C
O
H
H+
C
H
C+
+ H2O
Rate limiting step
C
H
C+
HSO4-
C C + 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
OH
CH3
OH SO3H
O
H
H
CH3 CH3
+ HSO4-
+ H2O
secondary carbocation
CH3
H H
H H H
H
H
H2O
CH3
H
H
a
a
a) -H3O+
b
b
b) -H3O+
CH3
c) Hydride ShiftH
H
H2O
d
de
e
d) -H3O+ e) -H3O+
CH3
CH2
C
H H H
H
H
+ HSO4-
H
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.
Products may form from the 2o carbocation, but it is more likely that the 2o C+ will rearrange to a 3o C+.
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.
2o
3o
Carbocation rearrangemen
t
1-methyl-1-cyclohexene
3-methyl-1-cyclohexene
methylenecyclohexane
1-methyl-1-cyclohexene
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 starting material 1 mol of starting material 1 mol product # g
# g 1 mol starting material
1 mol productAmount you
started with
Molecular weight of starting material
Stoichiometric ratio Molecular weight 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.
Determine steric energy of intermediates and products using HyperChem.
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.
Compare experimental product distribution to that predicted by HyperChem.
EXPERIMENTAL PROCEDURE:
(Simple distillation)
Heating Mantle
to voltage regulator
water out
water in
iron ring
• Place 2-methylcyclohexanol, sulfuric acid and boiling chips in 50 mL round bottom flask.
• Clamp flask to ring stand.
• Weigh 10 mL flask. Clamp to other ring stand.
• Attach clear hoses to condenser. Run water in at the bottom, out at the top!
• Build rest of distillation apparatus, using blue Keck clips to secure top and bottom joints around condenser.
EXPERIMENTAL PROCEDURE:
(Simple distillation)
Heating Mantle
to voltage regulator
water out
water in
iron ring
• Begin water flow, and apply heat (VR@30) to boil solution.
• Record temperature when distillate begins to collect in 10 mL flask (Ti).
• Collect ~ 5 mL distillate.
• Record temperature right before you drop the heating mantle (Tf).
• Allow the solution to cool.
• Reweigh 10 mL flask to obtain actual product yield.
• Prepare GC sample!
Table 10.1
Theoretical Yield (g)
must calculate the amount of product that can be formed based on the amount of 2-methylcyclohexanol used!
Actual Yield (g) This mass will be obtained by weighing the 10 mL 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)
Product Appearance physical state and color of distillate.
EXPERIMENTAL PROCEDURE:
(HyperChem)
To build models of the alkenes…
Construct a cyclohexane ring.
Add a methyl substituent.
Click and drag between carbons to insert a double bond.
Under BUILD, select “Add hydrogens and model build”.
Under SETUP, select PM3, then OK.
Under COMPUTE, select GEOMETRY OPTIMIZATION, then OK.
You do not need to keep a log file, you only need the final steric energy that appears at the bottom left side of the screen.
EXPERIMENTAL PROCEDURE:
(HyperChem)
To build models of the carbocations…
Construct a cyclohexane ring.
Add a methyl substituent.
Under BUILD, select “Add hydrogens and model build”.
Right click on the hydrogen that you wish to remove to form the carbocation.
Under SETUP, select PM3.
Select OPTIONS, set total charge = 1, and spin multiplicity = 1.
Under COMPUTE, select GEOMETRY OPTIMIZATION, then OK.
You do not need to keep a log file, you only need the final steric energy that appears at the bottom left side of the screen.
3o
2o
Table 10.2
Structure Steric Energy (kcal/mol)
2o carbocation Record values to 2 decimal places! All steric energies are NEGATIVE!
3o carbocation
1-methyl-1-cyclohexene
methylenecyclohexane
3-methyl-1-cyclohexene
• more substituted = lower energy = more stable = more predominate!
Table 10.3
Compound GC Retention time (min) Area
PercentAdjusted
Area PercentStandard
Sample
methanol 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 X 100Sum area% all alkenes
3-methyl-1-cyclohexene
Area % THIS alkene X 100Sum area% all alkenes
methylenecyclohexane Area % THIS alkene X 100Sum area% all alkenes
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
OH
CH3
CH3
THINGS TO CONSIDER…
•What kinds of bonds do I have?
• If they appeared in the IR spectrum, where would they be?
• Now, look at the spectrum. Are they there?
EXPERIMENTAL PROCEDURE:
IR Analysis
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
Full IR Absorption Correlation Table in Appendix J
Table 10.4
Functional Group
BaseValues(cm-1)
2-methyl-cyclohexano
l
1-methyl-1-cyclohexen
e
3-methyl-1-cyclohexen
e
Methylene-cyclohexane
Frequency (cm-1)
Frequency (cm-1)
Frequency (cm-1)
Frequency (cm-1)
OH stretch3200-3500
C-O stretch1000-1200
sp3 CH stretch2850-3000
sp2 CH stretch3000-3100
C=C stretch1600-1680
IR spectra are on page 87 in lab manual!
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.
Typically, this means to 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 your product in the container labeled “LIQUID ORGANIC WASTE”.
Add a small amount of water to the reaction flask residue and transfer to the container labeled “ACIDIC AQUEOUS WASTE”.
Rinse all distillation glassware with wash acetone into a waste beaker INSIDE YOUR HOOD! You should NOT remove this glassware from hood at any time unless it is for weighing purposes!
LABORATORY NOTEBOOK(Pre-lab)
•OBJECTIVE (Must clearly state…)•What compound you will make and how• How you will purify the compound• How you will determine the purity of your compound
• CHEMICAL EQUATION • Include the general chemical equation from top of page 81.
•TABLE OF PHYSICAL DATA (Complete the following table using MSDS sheets from a site on WWW Links ONLY. Wikipedia is unacceptable)
•REFERENCE TO PROCEDURE (Must include…)•full title, including edition and authors•page numbers where actual procedure can be found
Compound MW (g/mol)
bp (Co) d (g/mL) HAZARDS
Sulfuric acidMethanol2-methylcyclohexanol1-methylcyclohexene3-methylcyclohexenemethylenecyclohexane
LABORATORY NOTEBOOK(In-lab)
• DATA/CALCULATIONS • Initial weight of 2-methylcyclohexanol used• Distillation range • Initial weight of 10 mL round bottom flask• Final weight of 10 mL round bottom flask• Final product weight • physical state and color of product• GC vial slot # • theoretical yield calculation (not just the final value!)• % yield calculation (not just the value)• Give an example of an adjusted area % calculation
• EXPERIMENTAL PROCEDURE• In paragraph form, briefly describe the procedure that you actually
followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE.
• Include any volumes or weights of chemicals used during the experiment.• Include any mistakes, accidents, or observations if necessary.
