Today: • Exp.3: NBS Bromination: NMR • Introduction to Diels-Alder (Exp.4) • MiniQuiz
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? • What is the function of dibenzoyl peroxide? • What is the function of N-Bromosuccinimide (NBS)? For which compounds can it be used? (compare with your lecture text!) • Why do we use NBS and not Br2?
Synthesis of Bromoethylbenzene (Manual Exp.III) • Why is CCl4 a good solvent for our reaction mixture? • What are criteria for a "good solvent" in general? • What is the purpose of addingcyclododecane?
1H-NMR • Draw a rough 1H-NMR of a sample that contains (only) • ethyl benzene in CCl4 and a drop of TMS standard. • a-monobromoethyl benzene in CCl4 and a drop of TMS standard. • a-dibromoethyl benzene in CCl4 and a drop of TMS standard. • 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 … (check bottle for %ethylbenzene/cyclododecane !) 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 nc = moles of cyclododecane Aa = area of alpha 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) Cyclopentadiene STINKS!! Maleic anhydride Diels Alder Adduct 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!) : Step 1: Cracking (A retro Diels Alder) Step 2: Diels-Alder Step 3: Hydrolysis
Next time: • Diels-Alder (cont.) • Esterification (Exp.5) • MiniQuiz on today’s class