Experiment 10:

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Experiment 10

• ACID-CATALYZED
• DEHYDRATION
• OF AN ALCOHOL
• WITH REARRANGEMENT

2
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.

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Before coming to lab

• Please review
• Simple distillation
• E1 elimination reactions
• GC Analysis

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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.

more substituted lower energy more stable
more predominate!
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RATE LIMITING STEP

• 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.

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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
3. 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
Carbocation rearrangement
3o
1-methyl-1-cyclohexene
1-methyl-1-cyclohexene
methylenecyclohexane
3-methyl-1-cyclohexene
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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
  1 mol product g
• g 1 mol
  reactant 1 mol product

Always end up in units of grams of product!!!
Amount you started with
Molecular weight of reactant
Stoichiometric ratio
Molecular weight of product
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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.

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EXPERIMENTAL PROCEDURE(Simple distillation)
Clamp flask to ring stand here!

• 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.

Blue Keck clips here!
Clamp flask to ring stand here!
50 mL
10 mL
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EXPERIMENTAL PROCEDURE(Simple distillation)

• Begin water flow, and apply heat (VR_at_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 and submit! Dont forget!!!

Keck clips!
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Table 10.1 Experimental Results
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.
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Table 10.2 GC Analysis Results
Compound GC Retention time (min) GC Retention time (min) Area Percent Adjusted Area Percent
Compound Standard 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-1-cyclohexene Area THIS alkene X 100 Sum area all alkenes
3-methyl-1-cyclohexene Area THIS alkene X 100 Sum area all alkenes
methylenecyclohexane Area THIS alkene X 100 Sum area all alkenes
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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!

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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 CO bond.
• If we add IR light energy at the correct
  wavelength, we get excitation to the next
  vibronic energy level.

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Infrared Spectroscopy (IR)

• Q What is an IR spectrum?

transmittance of IR radiation
Frequency of vibration (in wavenumbers)
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EXPERIMENTAL PROCEDUREIR Analysis

• THINGS TO CONSIDER
• What kinds of bonds do I have?
• Ex. C-O, CC, 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?

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EXPERIMENTAL PROCEDUREIR Analysis
Full IR Absorption Correlation Table in Appendix J
Base values for Absorptions of Bonds (cm-1) Base values for Absorptions of Bonds (cm-1)
OH 3400
C-O 1100
C-H (sp2) 3100-3000
C-H (sp3) 3000-2850
CC 1630
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Table 10.3 IR Spectral Analysis Results
IR spectra are on page 87 in lab manual!
Functional Group Base Values (cm-1) 2-methyl- cyclohexanol 1-methyl-1- cyclohexene 3-methyl-1- cyclohexene Methylene- cyclohexane
Functional Group Base Values (cm-1) Frequency (cm-1) Frequency (cm-1) Frequency (cm-1) Frequency (cm-1)
OH stretch 3200-3500 N/A N/A N/A
C-O stretch 1000-1200 N/A N/A N/A
sp3 CH stretch 2850-3000
sp2 CH stretch 3000-3100 N/A
CC stretch 1600-1680 N/A
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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).

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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.

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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

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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.

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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.

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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.