Compound 1 and PBr₃ react to form Compound 2 as shown below. Which position in Compound 2 corresponds to the hydrogen(s) that appear as a doublet at approximately 4.0 ppm in the ¹H NMR spectrum?

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11ecba2d_1288_e9b4_a3bf_415dcbd8fbf9_MD0008_00 Proton nuclear magnetic resonance spectroscopy (¹H NMR) detects hydrogen atoms (¹H isotope) in a molecule as an external magnetic field and radio frequency pulse are applied to the sample. The hydrogen atoms in different magnetic environments (resulting from molecular structure) are nonequivalent, and neighboring nonequivalent hydrogens within three bonds of each other cause spin-spin splitting of the peaks in the NMR spectrum. The splitting pattern can be determined by n+ 1 rule, where n is the number of nonequivalent hydrogen atoms on adjacent carbons. Position IV in Compound 2 contains two hydrogen atoms that are adjacent to a single H atom bonded to an sp² carbon. According to the n + 1 rule, n = 1 because there is only one coupled hydrogen to the hydrogen atoms at position IV. Therefore, because n + 1 = 2, the peak will be split in two, making the splitting pattern a doublet. (Choice A) Position I contains two hydrogen atoms that are split by three adjacent hydrogen atoms. Although a quartet might be expected, complex, secondary splitting causes overlapping peaks to appear as a multiplet at 2.1 ppm. (Choice B) Position II consists of a methyl group (three hydrogen atoms) that is adjacent to a 4° carbon (no bonds to hydrogen). Because there are no neighboring hydrogen atoms coupled to the methyl hydrogens, the peak is not split and appears as a singlet at 1.8 ppm. (Choice C) Position III contains an alkene hydrogen atom that is adjacent to an alkyl (-CH₂-) group. The two alkyl hydrogen atoms are coupled to the alkene hydrogen, causing the peak to be split into a triplet. Educational objective: ¹H NMR detects hydrogen atoms in a molecule as an external magnetic field and radio frequency pulse are applied to the sample. Hydrogen atoms in different magnetic environments within three bonds of each other cause spin-spin splitting of the peaks in the spectrum. The splitting pattern can be determined by the n + 1 rule (n = number of nonequivalent hydrogen atoms on adjacent carbons).

Compound 1 and PBr3 React to Form Compound 2 as Shown

Compound 1 and PBr₃ React to Form Compound 2 as Shown