Label each set of chemically equivalent protons, using a for the set that will be at the lowest frequency in the ¹H NMR spectrum, b for the next lowest, and so on. Indicate the multiplicity of each signal.
a.

Verified step by step guidance

1. Step 1: Identify the chemically equivalent protons in the molecule. Chemically equivalent protons are those in identical chemical environments. In the given structure, there are three sets of protons: (1) the protons on the CH3 group attached to the central CH carbon, (2) the protons on the CH2 group attached to the chlorine atom, and (3) the proton on the central CH carbon.
2. Step 2: Assign labels based on the expected chemical shift in the 1H NMR spectrum. The protons on the CH3 group will appear at the lowest frequency (most upfield) due to their electron-donating environment. Label this set as 'a'. The protons on the CH2 group will appear at a higher frequency (downfield) due to the electron-withdrawing effect of the chlorine atom. Label this set as 'b'. The proton on the central CH carbon will appear at the highest frequency (most downfield) due to its unique environment. Label this set as 'c'.
3. Step 3: Determine the multiplicity of each signal. The multiplicity is determined by the number of neighboring protons (n) using the n+1 rule. For the CH3 group (a), it is adjacent to one CH proton, so the signal will be a doublet. For the CH2 group (b), it is adjacent to one CH proton, so the signal will also be a doublet. For the CH proton (c), it is adjacent to six protons (three from CH3 and two from CH2), so the signal will be a septet.
4. Step 4: Summarize the chemical shift and multiplicity for each set of protons. The CH3 group (a) will have a doublet at the lowest frequency. The CH2 group (b) will have a doublet at a higher frequency. The CH proton (c) will have a septet at the highest frequency.
5. Step 5: Review the structure and confirm the assignments. Ensure that the labels and multiplicities are consistent with the molecular structure and the principles of NMR spectroscopy.

Key Concepts

Chemical Equivalence

Chemical equivalence refers to protons in a molecule that are in identical environments, leading to them having the same chemical shift in NMR spectroscopy. In the provided structure, protons on the same carbon or those that are symmetrically placed will resonate at the same frequency, allowing them to be labeled as equivalent.

NMR Multiplicity

Multiplicity in NMR spectroscopy indicates the number of peaks observed for a given signal, which is determined by the number of neighboring protons (n) according to the n+1 rule. For example, a proton with one neighboring proton will appear as a doublet, while one with two neighboring protons will appear as a triplet, providing insight into the molecular structure.

Chemical Shift

Chemical shift is the position of a signal in an NMR spectrum, measured in parts per million (ppm), and is influenced by the electronic environment surrounding the protons. Factors such as electronegative atoms (like chlorine in the structure) can deshield protons, causing them to resonate at lower frequencies, which is crucial for determining the order of signals in the spectrum.