A small pilot plant 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.

Verified step by step guidance

1. Examine the NMR spectrum and identify the number of distinct proton environments. There are two peaks, indicating two different types of protons.
2. Analyze the integration values of the peaks. The peak at around 4 ppm integrates to 4 protons, and the peak at around 2 ppm integrates to 2 protons.
3. Consider the chemical shifts. The peak at 4 ppm suggests protons near electronegative atoms (likely bromine), while the peak at 2 ppm suggests protons in a less deshielded environment.
4. Determine the splitting patterns. The peak at 4 ppm is a multiplet, indicating coupling with adjacent protons. The peak at 2 ppm is also a multiplet, suggesting similar coupling.
5. Propose a structure that fits the NMR data. Given the integration and chemical shifts, the structure is likely 1,2-dibromoethane, where the 4 protons are on the CH2 groups adjacent to bromine atoms, and the 2 protons are on the CH2 group in the middle.

Key Concepts

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It works by applying a magnetic field to nuclei of certain isotopes, such as hydrogen, causing them to resonate at specific frequencies. The resulting spectrum provides information about the number of hydrogen atoms, their environment, and how they are connected, which is crucial for identifying the structure of the compound formed in the reaction.

Chemical Shifts

Chemical shifts in an NMR spectrum indicate the electronic environment surrounding hydrogen atoms in a molecule. Measured in parts per million (ppm), these shifts help differentiate between protons in various chemical environments, such as those adjacent to electronegative atoms or in aliphatic versus aromatic systems. Understanding chemical shifts is essential for interpreting the NMR data and assigning peaks to specific protons in the structure of the product.

Integration of Peaks

The integration of peaks in an NMR spectrum reflects the relative number of hydrogen atoms contributing to each signal. The area under each peak corresponds to the number of protons in that environment, allowing chemists to deduce the ratio of different types of protons in the molecule. In this case, the spectrum shows peaks for 4 H and 2 H, which can help identify the structure of the impurities formed during the reaction by correlating these values with the expected proton counts in potential structures.