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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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