The progress of a chemical reaction may often be followed spectroscopically by r
ID: 1057364 • Letter: T
Question
The progress of a chemical reaction may often be followed spectroscopically by removing a small aliquot of the reaction mixture at periodic intervals and running its specturm. Describe how you could follow the progress of the bromination of trans stilbene (below) using IR and proton NMR spectroscopy. Specifically, fill in the following table to identify the peak (s) in the spectrum you would expect to appear, disappear, or change position as the organic reactant is converted into product. Give approximate positions of peaks in cm-1(IR) and ppm(NMR).Explanation / Answer
Alkene IR stretching frequencies =C-H stretch 3000-3100 cm-(from aromatic and olefin protons)
C=C stretch 1600 cm- (from olefin and aromatic protons)
Alkene NMR spectrum: 2 protons will give around delta 5 to 6 ppm, 10 protons will give around delta 7.0 to 7.5 ppm.
After reaction completion we will get dibromo compound.
Di bromo compound will give sp3 C-H stretch 3000 cm- and
=C-H stretch 3000-3100 cm-(from aromatic and olefin protons)
Di bromo compound proton NMR will show diappearence of olefine protons and appearence of sp3 C-H (benzylic) protons clearly indicates product formation.
The reaction proceeds through cyclic bromonium ion gives two peaks for one for aromatic protons another for c-H signal in bromoioinium intermediate. after that bromonium ion will open with bromine give dibromo product.
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