Questions of the Day: by L3Yaq6O

VIEWS: 55 PAGES: 14

									                     Questions of the Day:
                               Br                  Br
        CH 2 CH 3              CHCH 3              CCH 3
                    NBS                 NBS
                                                      Br
                    CCl 4               CCl 4


  The reactions in our Exp. 3 (NBS-Bromination) are
                   radical reactions!


     a) What is the structure of the radical formed
     when Br. radical reacts with ethyl benzene?

b) Why is there no radical formation in the beta position
                   of ethyl benzene?
               Today:
 • Exp.3: NBS Bromination: NMR
• Introduction to Diels-Alder (Exp.4)
             • MiniQuiz
                             Br                  Br
         CH 2 CH 3           CHCH 3              CCH 3
                     NBS              NBS
                                                  Br
                     CCl 4            CCl 4




1. What is the function of dibenzoyl peroxide?
2. What is the function of N-Bromosuccinimide (NBS)?
   For which compounds can it be used? (compare with
   your lecture text!)

3. Why do we use NBS and not Br2?
  Propagation is complex here …




How can you explain that the reaction mixture
  goes through a series of color changes?
 Synthesis of Bromoethylbenzene (Manual Exp.III)




1. Why is CCl4 a good solvent for our reaction
   mixture?
2. What are criteria for a "good solvent" in general?
3. What is the purpose of adding cyclododecane?
                   1H-NMR



1. Draw a rough 1H-NMR of a sample that contains
   (only)
  1. ethyl benzene in CCl4 and a drop of TMS standard.
  2. a-monobromoethyl benzene in CCl4 and a drop of
     TMS standard.
  3. a-dibromoethyl benzene in CCl4 and a drop of
     TMS standard.
  4. Cyclododecane in CCl4 and a drop of TMS
     standard.
        http://www.aist.go.jp/RIODB




ethylbenzene




α-bromoethylbenzene
                         singlet


          singlet
                    doublet

quartet
                    1H-NMR




Why is cyclododecane a good standard in our
  experiment?
                      Calculation of the Yield
%-yield of alpha –bromoethylbenzene …


For actual yield:
Compare the integrations of the cyclododecane peak with those of
   the a and b protons of the mono-brominated product.
             np =     nc x Aa/Ac x 24/1                  = nc x Ab/Ac x 24/3
                                          np = moles of mono-bromo product
  Br
  CHCH 3                                  nc = moles of cyclododecane
   a b                    (24)
  (1) (3)
                                          Aa = area of alpha protons
                        (number of equivalent protons)
                                          Ab= area of beta protons
                                          Ac= area of cyclododecane protons
       Use both the alpha and beta signals to calculate your yield and
       also report the average value.
      Diels – Alder Reactions (Exp.4)
                            O                          H
                                                       H
                  +          O                             O

                                              I        O
                            O
        diene         dienophile                   O
Cyclopentadiene        Maleic
   STINKS!!                                 Diels Alder Adduct
                      anhydride


Diels-Alder reactions are another example of syntheses that
introduce additional C's to a reactant.

A Diels-Alder reaction always forms a new cyclohexene ring.

Diels-Alder reactions are cycloaddition reactions. The addition is
stereospecific.
          Diels – Alder Reactions (Exp.4)
               Cyclopentadiene dimerizes easily.
           Thus we have to “crack” it first (STENCH!) :

                                            room temp.                          H
Step 1: Cracking                                                                H
                                   +
(A retro Diels                              high temp.                  I
Alder)
                                                            dicyclopenatdiene

Step 2: Diels-                       O                          H
                                                                H
Alder                      +           O                            O

                                                I               O
                                     O
                   diene       dienophile               O
                                                            H                               H
                                                            H           H2O
                                                                                             H
                    Step 3: Hydrolysis                          O
                                                            O                              CO 2H
                                                                                    II
                                                                                         CO 2H
                                                    O
        Next time:


   •Diels-Alder (cont.)
  •Esterification (Exp.5)
•MiniQuiz on today’s class
                  MiniQuiz 3 Questions:
                             Br               Br
     CH 2 CH 3               CHCH 3           CCH 3
                 NBS                  NBS
                                               Br
                 CCl 4                CCl 4




 a) What is the structure of the radical formed
          .
 when Br radical reacts with ethyl benzene?

b) Why is there no radical formation in the beta
          position of ethyl benzene?

								
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