Experiment 11:

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Experiment 11

• CONVERSION OF AN ALCOHOL TO AN ALKYL BROMIDE WITH
  REARRANGEMENT

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Objectives

• To perform the dehydration of 2-methylcyclohexanol
  to form isomeric alkyl halides under SN1
  conditions.
• To purify the product using liquid extraction.
• To analyze the product using a qualitative AgNO3
  test in order to identify determine product
  substitution of products.
• To characterize the reactant and major product
  using IR spectroscopy.

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CHEMICAL EQUATION
2-methylcyclohexanol
1-bromo-1-methylcyclohexanol (3o alkyl halide)
1-bromo-2-methylcyclohexanol (2o alkyl halide)
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SN1 MECHANISM
3. Water is eliminated-forms 2o carbocation.
2. Forms new O-H bond, oxygen bears a positive
charge (oxonium ion).
1. hydroxyl oxygen attacks and removes a proton
from Hydrobromic acid.
4. Products may form from the 2o carbocation, but
it is more likely that the 2o C will rearrange
to a 3o C.
5. Reaction of the carbocation with the bromide
ion yields the alkyl halide.
5. Reaction of the carbocation with the bromide
ion yields the alkyl halide.
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OVERVIEW

• Heat alcohol and acid under reflux to synthesize
  product(s).
• Purify product by extraction to remove unreacted
  starting materials.
• Obtain final product mass and calculate yield.
• Analyze product with AgNO3 test to determine
  which product forms.
• Characterize reactant and major product with
  provided IR spectra.

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EXPERIMENTAL PROCEDURE(Synthesis)
Leave open at top!

• Add 2-methylcyclohexanol, 48 HBr and 3 boiling
  chips to flask.
• Clamp flask to ring stand.
• Attach clear hoses to condenser and place
  condenser on top of flask.
• Begin water flow through condenser and apply
  heat.
• Reflux the solution for 30 minutes.
• Cool flask in beaker of tap water.

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EXPERIMENTAL PROCEDURE(Purification)

• Transfer solution to separatory funnel.
• Add concentrated sulfuric acid.
• Stir solution gently with glass rod. Do NOT
  agitate.
• Draw off bottom acidic layer into an Erlenmeyer
  flask.
• Transfer the top layer to a preweighed beaker
  using a glass Pasteur pipet.
• Reweigh beaker to obtain final mass.
• Perform silver nitrate test.

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Table 11.1 Experimental Results
Theoretical Yield (g) Must determine which reactant is your limiting reagent first, 48 HBr or 2-methylcyclohexanol. Calculate the amount of product that can be formed based on the limiting reagent.
Actual Yield (g) This mass will be obtained by weighing an empty small beaker, then reweighing after the product has been added. The difference in mass is the actual product yield.
Yield Actual yield (g) X 100 Theoretical yield (g)
Product Appearance Physical state and color of product.
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EXPERIMENTAL PROCEDURE (Silver Nitrate Test)

• When an alkyl halide is treated with a
  solution of alcoholic silver nitrate, a
  precipitate of silver halide forms.
• The rate limiting step is the formation of the
  carbocation.

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Table 11.2 Silver Nitrate Test Results
Test Solution Reaction Rate (min/sec)
1-bromobutane You only need 3 drops of each of these! Get ONLY what you need!
2-bromobutane
2-bromo-2-methylpropane
Product sample
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IR SPECTROSCOPY
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Table 11.3 IR Spectral Analysis Results
Functional Group Base Values (cm-1) 2-methylcyclohexanol Alkyl halide product
Functional Group Base Values (cm-1) Frequency (cm-1) Frequency (cm-1)
OH stretch 3200-3600 N/A
C-O stretch 1000-1200 N/A
sp3 CH stretch 2850-3000
C-Br stretch 500-700 N/A
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Infrared Spectroscopy (IR)(How to answer the
questions)

• How are you able to DIFFERENTIATE between
  reactant and product?
• 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

• Sulfuric acid is very corrosive and must be used
  with EXTREME caution!
• Silver nitrate will stain the skin! Use gloves
  at all times!
• Alkyl bromides are lachrymators! Keep covered at
  all times to avoid inhalation!

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WASTE MANAGEMENT

• Place the acidic solution from extraction in
  bottle labeled LIQUID WASTE.
• Place alkyl bromide product in the bottle labeled
  ALKYL BROMIDE WASTE.
• Pour the contents of the test tubes used in the
  silver nitrate test in the container labeled
  SILVER NITRATE WASTE.

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CLEANING

• Clean round bottom flask and separatory funnel
  with soap, water, brush, and wash acetone.
• Rinse condenser with wash acetone only.
• Clean all other glassware used with soap, water,
  brush, and final rinse with wash acetone.
• Do not return any glassware to plastic box or lab
  drawer unless it is COMPLETELY DRY!

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In-lab Question (The following question should
be answered in laboratory notebook.)

• There are two possible products from the reaction
  of
• 3-methyl-2-pentanol with aq. HBr.
• Draw the products and a complete mechanism for
  their formation, including all intermediates.
• Name both products based on the IUPAC system
• circle the major product.

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In-lab Question (The following question should
be answered in laboratory notebook.)

• Based on the reaction above
• Calculate the theoretical yield for the reaction
  based on 4.0 g of the alcohol and 20 mL of the
  48 HBr.
• Show calculations based on both reactants! The
  molecular weight of the alcohol is given, however
  the molecular weight of the product must be
  determined based on the structure.
• Be sure to give units.