Exp.3: NBS Bromination/ NMR Introduction to Diels-Alder (Exp.4)

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Exp.3: NBS Bromination/ NMR Introduction to Diels-Alder (Exp.4) Today: Exp.3: NBS Bromination/ NMR Introduction to Diels-Alder (Exp.4)

Synthesis of Bromoethylbenzene; H-NMR (Exp. 3) What type of a reaction is this? What is a “radical” in chemistry? What is the geometry of a carbon radical? What reaction conditions generally tend to lead to radical intermediates? What are the major steps found in every radical reaction?

Why does bromination in our reaction occur at the α-position only? What is the function of dibenzoyl peroxide? What is the function of N-Bromosuccinimide (NBS)?

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) List some properties of CCl4. Why is CCl4 a good solvent for our reaction mixture? What are criteria for a "good solvent" in general? What is the purpose of adding cyclododecane?

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

1H-NMR Why is cyclododecane a good standard in our experiment?

Calculation of the 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

MiniQuiz on this and last class Next time: Diels-Alder (cont.) Esterification (Exp.5) MiniQuiz on this and last class