Experiment 17: by co2Sqr

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									    Experiment 15:


SUBSTITUENT EFFECTS ON
      THE RATE OF
ELECTROPHILIC AROMATIC
     SUBSTITUTION
Objectives:
   To explore how different substituent
    groups on an aromatic ring affect the rate
    and orientation of electrophilic aromatic
    substitution using a qualitative bromine
    test.

   To determine directing ability of acetamide
    group using TLC analysis.
          Before coming to lab…

   Review these techniques:

       TLC Analysis

       Acid-base Extraction
              CHEMICAL EQUATION
• The progress of the bromination of an aromatic
  ring can be followed easily by a color change.
• The more reactive the aromatic ring is, the
  faster the color will disappear.



     G                       G


              +                        +    HBr
                  Br2             Br
  colorless       red      colorless       colorless
                                  MECHANISM


           CH2CH2CH3                        CH2CH2CH3                        CH2CH2CH3



                  +   Br   Br                           Br                          +   HBr

   H                               H                                 H

           H                            H    Br                              Br


An electron pair from           …and leaving a                The carbocation intermediate
the aromatic ring               nonaromatic, carbocation     loses H+, and the neutral
attacks Br2, forming a          intermediate.                substitution product forms as
new C-Br bond…                                               two electrons from the C-H
                                                             bond move to regenerate the
                                                             aromatic ring. HBr forms as a
                                                             byproduct.
    REACTIVITY OF AROMATIC
             RINGS
   The substituent ALREADY ON the
    aromatic ring determines the position
    and rate of substitution of the second
    (INCOMING) electrophile.
   We use the reactivity of BENZENE
    (no substituent) as a reference point.
        REACTIVITY OF AROMATIC
                 RINGS
   If we use the reactivity of benzene (substituent =
    H) as a reference point, activating substituents
    are all electron donating groups, and their relative
    activation strengths are:
      H < Phenyl < CH3 < NHCOCH3 < OCH3 < OH < NH2

   Deactivating groups are electron withdrawing
    groups and their activities relative to hydrogen
    are:
           NO2 < COR < CHO < I < Br < Cl < F < H
SUBSTITUENT EFFECTS
         GENERALIZATIONS
   Substituents in which the atom bonded to
    the ring has an unshared pair of electrons,
    with the exception of alkyl and phenyl
    groups, are ortho-para directing. All
    other substituents are meta directing.
   All ortho/para directors are activators,
    with the exception of halogens. Halogens
    are ortho-para deactivators.
   All meta directors are deactivators.

   Alkyl and Phenyl groups are also ortho-
    para directing.
               OVERVIEW
   React bromine solution with six
    monosubstituted aromatic compounds.
   Follow rate of reaction based on color
    change.
   Neutralize and extract acetanilide
    product to purify compound for
    further analysis.
   To analyze the bromoacetanilide
    product to identify product
    substitution.
                    SYNTHESIS
   Place small amount of
    monosubstituted aromatic
    compound in small test tube.
   Place test tubes in water bath.
   Add Bromine solution to each.
   Record the approximate amount of
    time it takes for the solution to
    lose color.
   Place in hot water bath if
    necessary to complete reaction.
   After 1 hour, estimate the reaction
    order based on the relative amount
    of color lost.
                                  Table 15.1



Compound     ethyl     anisole   acetanilide   phenol     benzaldehyde nitrobenzene
            benzene
                      ***Aromatic ring is given for each compound…remember to
Structure             complete structure with appropriate substituent!***

  OPA,                ***Identify each substituent as an ortho/para activator (OPA),
  OPD,                ortho/para deactivator (OPD), or a meta deactivator (MD).***
 or MD
                      ***Simply record the order that the reactions occurred. Actual
Reaction              reaction times are not required. 1=fastest  6=slowest! ***
 Order
    PURIFICATION/ISOLATION
   Once pale yellow, remove acetanilide
    tube.
   Add deionized water to tube.
   Add 5 drops of NaOH. Test pH using
    glass rod.
   Repeat until the solution is basic.
   Add ethyl acetate. Place small cork in
    top of test tube, and shake to mix.
   Allow layers to separate.
                 ANALYSIS
• Prepare TLC plate and chamber.

• Apply provided standards to TLC
plate.
• Apply TOP layer from test tube to     A       B U   C D


TLC plate ( sample solution).
• Develop plate and visualize spots                         filter paper



using UV lamp.
• Calculate Rf value of all spots and
identify product in sample solution.
                                            A   B U C D
             Tables 15.2 and 15.3
                             Rf Values
    Identification               Standard                   Sample

  o-bromoacetanilide    ***Rf values are 2 decimal
                        places, never more, never less.
                        And unitless.***

 m-bromoacetanilide
  p-bromoacetanilide


MAJOR       ***Write in name of product formed from bromination of
            acetanilide.***
Product
 Name
 MAJOR      ***Draw structure of major product formed.***
 Product
Structure
                  HINTS…
   Be sure that your test tubes do NOT
    contain any acetone. It reacts very
    quickly with bromine and can give
    inaccurate test results.

    If after the one hour period of heating in
    the water bath the solutions have not
    completely lost their color, estimate by
    the relative amount of color lost.
         SAFETY CONCERNS

   CAUTION: In this experiment you
    are using bromine, which is poisonous
    and can cause severe burns!

   All aromatic solutions are prepared in
    glacial acetic acid which can cause
    severe burns!
          WASTE MANAGEMENT
   Place ALL liquid waste in container labeled
    “LIQUID ORGANIC WASTE”.
   Place used TLC plates and filter papers in
    yellow trash can.
   Place used TLC spotters broken glass box
    (NOT TRASHCAN!!!!!!!!!!!).
              CLEANING UP…
   TEST TUBES: clean all test tubes with
    soap/water/brush and rinse with wash acetone.
    Leave inverted in test tube rack to dry.

   TLC CHAMBER: remove filter paper and leave
    in drawer with cap off.

   BEAKER: if only used for water, simply dry out
    with a paper towel.

   GRADUATED CYLINDER: rinse any excess
    bromine solution into wash acetone container.
    Clean with soap/water/brush and rinse with
    wash acetone.
               LABORATORY NOTEBOOK
                     (Pre-lab)
• OBJECTIVE              (Must clearly state…)
         • What you will be doing
         • How you are going to do it
         • How you will determine if it worked

• CHEMICAL EQUATION
    • Include the chemical equation from the top of page 131 of the lab manual.

• TABLE OF PHYSICAL DATA                        (Complete the following table using a
site listed on WWW Links ONLY. Wikipedia is unacceptable!)
                    Compound      MW (g/mol)   bp(oC)   d (g/mL)                HAZARDS
                  Ethylbenzene
                     Anisole
                   Acetanilide
                     Phenol
                  Benzaldehyde
                  Nitrobenzene
                   Acetic acid
                  Ethyl acetate
                    Hexane


• REFERENCE TO PROCEDURE
                    bromine
                                                                   (Must include…)
    •full title including edition and authors
    •page numbers where actual procedure can be found
          LABORATORY NOTEBOOK
                 (In-lab)

• DATA/CALCULATIONS
  •   Reaction rates of aromatic compounds with bromine solution.
  •   Sketch TLC diagram, including all cm measurements
  •   TLC developing solvent system
  •   Show at least one TLC Rf calculation


• EXPERIMENTAL PROCEDURE
  •   In paragraph form, 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 applicable.
For next lab…

   Pre-lab notebook entry for
    Experiment 17 will be due at the
    beginning of lab!

   Final lab report for Experiment 15,
    along with completed lab notebook
    pages, will be due at the beginning of
    lab!

								
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