Question

= Predict the splitting pattern for each kind of hydrogen in the following molecules: (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{3}\) (c) trans-But-2-ene

Short answer

(a) Decet for CH, doublet for CH3. (b) Quartet for CH2, triplet for CH3, singlet for CO2CH3. (c) Quartet for CH2, triplet for CH3.

Step-by-step solution

Identify the Types of Protons

Determine the different types of hydrogen environments in each molecule by analyzing its structure. This involves identifying which hydrogen atoms are attached to different carbon environments.

Use the 'N+1' Rule

For each distinct hydrogen environment, apply the 'N+1' rule for predicting splitting patterns, where 'N' is the number of equivalent neighboring protons. This rule helps identify the splitting pattern of each type of hydrogen atom.

Analyze (CH3)3CH

In \((CH_3)_3CH\), there are two distinct proton environments: the single CH proton and the CH3 protons. The CH proton is surrounded by nine equivalent CH3 protons, leading to splitting: \(9+1=10\) (a decet), while CH3 protons see one CH proton leading to \(1+1=2\) (a doublet).

Analyze CH3CH2CO2CH3

In \((CH_3)CH_2CO_2CH_3\), three types of protons exist: \(CH_3(1)\) in \((CO_2CH_3)\) sees zero equivalents (singlet), \(CH_2(2)\) interacts with CH3(3) neighboring protons, leading to \(3+1=4\) (a quartet), and \(CH_3(3)\) with two CH2(2) leads to \(2+1=3\) (a triplet).

Analyze Trans-but-2-ene

In trans-but-2-ene, at each end of the double bond, each CH3 sees the opposite CH2, consisting of two protons, resulting in a \(2+1=3\) splitting pattern (triplet), while each CH2 sees three protons from the adjacent CH3, resulting in a \(3+1=4\) splitting pattern (quartet).

Key concepts

Proton Environments

In NMR spectroscopy, understanding proton environments is crucial to interpreting spectra. A *proton environment* refers to the specific surroundings and chemical context of hydrogen nuclei within a molecule. These environments determine how each proton will appear in an NMR spectrum. Protons that are in identical environments will resonate at the same frequency and contribute to the same signal. To identify these environments, look at the structure of the molecule. Here, you assess which protons are bound to similar atoms and are in similar electronic conditions. For example, in the molecule (CH3)3CH, there are two distinct proton environments: the isolated CH proton and the CH3 protons bound to it. These groups experience different local magnetic fields due to their chemical surroundings, and therefore, appear at different frequencies in the NMR spectrum.

N+1 Rule

The N+1 rule is a simple guideline used to predict the splitting patterns in the NMR spectrum. This rule states that a signal will be split into \(N + 1\) peaks, where \(N\) is the number of equivalent neighboring protons. These neighbors must be located on *adjacent atoms* due to the limited range of magnetic field influence. To apply this rule, identify the number of equivalent protons adjacent to the proton being considered. In (CH3)3CH, the solitary CH proton has nine neighboring CH3 protons, resulting in the signal splitting into ten peaks (a decet). Conversely, each CH3 group is adjacent to one CH proton, leading to a doublet in their signal. This rule helps appreciate the complexity of interactions between protons, aiding the prediction of the spectra one might observe.

Splitting Pattern

Splitting patterns in NMR result from interactions between non-equivalent protons in a molecule. These interactions cause signals to split into multiple peaks, reflecting the magnetic influence of neighboring protons. Using the N+1 rule, we can predict these patterns to better understand the molecule's structural information. Different patterns like singlets, doublets, triplets, quartets, and higher order multiplets help chemists determine how protons are arranged in their chemical environments. For instance, the quartet pattern observed in \(CH_3CH_2CO_2CH_3\)'s \(CH_2\) group arises from the interaction of its two protons with three from a neighbor \(CH_3\) group. Identifying these patterns is essential to determine the connectivity and type of substituents in a molecular structure, providing critical structural insight.

Chemical Shift

Chemical shift in NMR is the relative position of a signal within the spectrum. The shift is measured in parts per million (ppm) and provides valuable information about the chemical environment of the protons. Chemical shifts arise due to the electron clouds around the nuclei: these electrons create localized magnetic fields that either shield or deshield the protons from the external magnetic field. A **downfield** shift (higher ppm) indicates deshielding, often due to electronegative atoms, while an **upfield** shift (lower ppm) suggests shielding. For example, in trans-but-2-ene, the different shifts of the protons signify variations in proximity to the electron-withdrawing double bond. Recognizing chemical shift values helps distinguish among hydrogen types and deduce molecular structure. As a rule of thumb, more shielded protons resonate at lower frequencies (upfield) and deshielded at higher (downfield), offering clues to the molecular layout.