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#1 2011-11-03 17:09:48

orgopete
Administrator

Deduce C5H11Br product from addition of HBr

HBr addition to an alkene is a well-known organic reaction. When peroxides are present, one regioisomer is observed, while in the absence of peroxides the other regioisomer is observed.

Two separate HBr additions are carried out using the same starting alkene whose molecular formula is C5H10. One was carried out in the presence of peroxides and the other in the absence of peroxides. Use the NMR spectra provided to decide which addition product was obtained from each reaction.
(a) Draw the skeletal structure of the addition product whose molecular formula is C5H11Br that is consistent with the following NMR spectrum.

See spectrum and annotation below:

     http://www.curvedarrow.com/chem/ZZ1EAF3AFA.jpg

( b) Draw the skeletal structure of the addition product whose molecular formula is C5H11Br that is consistent with the following NMR spectrum.

See spectrum and annotation below:

     http://www.curvedarrow.com/chem/ZZ0342773C.jpg
Okay, to begin, let us assign some numbers to the signals. Since hydrogens come in 1, 2, or 3H per carbon, the total must equal 11H.
3:3:3:2, 6:3:2 or 9:2 = 11H
6:3:1:1 = 11H

(a) If there were five signals, then we could have something like 3:2:2:2:2 = 11H. With three or four signals, it has to be something like what I have above. For (a) above, it has to be 3:3:3:2 or 6:3:1:1. Even though no integral is present to make this easy, we can see from the similarity of the signals and their sizes, it isn't the 3:3:3:2 case. Therefore it is 6:3:1:1. Since the six hydrogen peak is a doublet, the 2xCH3 peaks must be attached to a CH, and there is a CH signal in the spectrum as well. The other CH3 group must also be attached to a CH as it too is a doublet. Therefore we must assemble a (CH3)2CH to a CH3CH + Br = (CH3)2CHCHBrCH3.

(b) This must be a 6:3:2 = 11H. A 6H singlet must be 2xCH3 groups attached to a C with no adjacent hydrogens. The other two signals must be a CH2 and a CH3. This can only be put together one way. Br + C(CH3)2 + CH2 + CH3 = BrC(CH3)2CH2CH3.

The reactions producing these compounds is shown below. Each leads to the more stable tertiary intermediate.
     
     http://www.curvedarrow.com/chem/ZZ4B2A9B16.jpg

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