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Magazine Chapter 13: Q.13-43P (page 703)
A small pilot was adding bromine across the double bond of but-2-ene to make 2,3-dibromobutane. A controller malfunction allowed the reaction temperature to rise beyond safe limits. A careful distillation of the product showed that several impurities had formed, including the one having the NMR spectra that appear below. Determine its structure and assign the peaks to the protons in your structure.
Short Answer
This structure has four different kinds of protons, thus will exhibit four signals in proton NMR.
The values of chemical shifts and splitting of peaks is,
a =4.3 (sextet,
) c =
2.3 (multiplet,
)
b =3.6 (triplet,
) d =
1.7 (doublet,
)
Step by step solution
Determination of structure:
There is no evidence present for vinyl hydrogens so the double bond cannot be present. Integration gives eight hydrogens, so the formula must be , and the four carbons must be in a straight chain because the starting material was but-2-ene. From the integration, four carbons must be present as one methyl, one methine, and two methylene groups. The methyl is split into doublet so it must be adjacent to the
. The two methylene groups must follow in succession with two bromine atoms filling remaining vacancies. Thus, the structure would look like as follows
Structure derived from given NMR spectrum
Values of chemical shifts and splitting of peaks in structure determined:
The structure determined from NMR spectrum has four different kinds of protons, thus will exhibit four signals in proton NMR.
The values of chemical shifts and splitting of peaks is,
a =4.3 (sextet,
) c =
2.3 (multiplet,
)
b =3.6 (triplet,
) d =
1.7 (doublet,
)
Chemical shift values depends on electronegativity of nearby atoms and also whether the electron density is more or less around that proton whose chemical shift value we are determining. In the given structure, bromine is an electronegative atom, thus, protons-a and b will have higher chemical shift value than protons-c and d. Due to more electronegative atom present, region of low electron density or deshielded environment is created due to which chemical shift increases.
Different kinds of protons gives different values of chemical shifts
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