Problem 28 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Predict the splitting patterns in the \(^{1} \mathrm{H}\) NMR spectra of the following molecules (Section \(10.7)\) (a) propanone \(\left(\mathrm{CH}_{3} \mathrm{COCH}_{3}\right)\) (b) 1 -bromopropane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\right)\) (c) 1,1 -dichloroethene \(\left(\mathrm{CCl}_{2}=\mathrm{CH}_{2}\right)\) (d) \(E-1,2\) -dichloroethene \((\mathrm{CHC})=\mathrm{CHC} \mathrm{C}\); \((\mathrm{Z} \text { and } E \text { isomers })\) (e) nitrobenzene; (1) 1,2 -dinitrobenzene (g) 1,3 -dinitrobenzene (h) 1,4 -dinitrobenzene.

Step-by-Step Solution

Answer

Propanone gives a singlet, 1-bromopropane gives a triplet and multiplets, 1,1-dichloroethene a singlet, E-1,2-dichloroethene singlets, nitrobenzene and dinitrobenzenes produce multiplets.

Step 1: Understanding Chemical Equivalence

Proton NMR spectra are affected by the chemical environments of protons. Equivalent protons appear as a single signal because they are in identical environments. Identify distinct sets of chemically equivalent protons in each molecule.

Step 2: Propanone Analysis

In propanone \[ CH_3COCH_3 \], the protons in each of the two \(CH_3\) groups are equivalent and experience the same environment. Therefore, there is only one signal in the NMR, which is a singlet, as there are no neighboring protons to cause splitting.

Step 3: 1-Bromopropane Analysis

In 1-bromopropane \[ CH_3CH_2CH_2Br \], the \(CH_3\) group is adjacent to a \(CH_2\) group, causing a triplet (splitting of \(n+1 = 3\)) due to two neighboring protons. The middle \(CH_2\) group is adjacent to both the \(CH_3\) and \(CH_2Br\) groups, leading to a multiplet (complex splitting). The \(CH_2Br\) group is adjacent to one \(CH_2\) group, causing a triplet.

Step 4: 1,1-Dichloroethene Analysis

In 1,1-dichloroethene \[ CCl_2=CH_2 \], the \(CH_2\) protons are equivalent and split each other, resulting in a singlet as there are no neighboring non-equivalent protons to further split these signals.

Step 5: E-1,2-Dichloroethene Analysis

In \(E-1,2\)-dichloroethene \[ CHCl=CHCl \], the \(CH\) protons are in different environments because of the trans configuration, causing them to appear as individual singlets due to the lack of additional adjacent protons.

Step 6: Nitrobenzene Analysis

In nitrobenzene, \(C_6H_5NO_2\), the protons on the benzene ring are in slightly different environments. They will present as a complex multiplet due to coupling, influenced by the electron-withdrawing effect of the \(NO_2\) group.

Step 7: 1,2-Dinitrobenzene Analysis

In 1,2-dinitrobenzene, the ortho position of the second \(NO_2\) group results in a complex splitting pattern due to the difference in chemical environments for each hydrogen on the benzene ring. Each set will appear as multiplets.

Step 8: 1,3-Dinitrobenzene Analysis

For 1,3-dinitrobenzene, the meta position of the \(NO_2\) groups produces distinct multiplet patterns, as the protons are influenced differently by two nitro groups, providing three different aromatic environments.

Step 9: 1,4-Dinitrobenzene Analysis

In 1,4-dinitrobenzene, the para position of the nitro groups leads to simplified spectra compared to ortho and meta configurations. It typically shows two doublets corresponding to the symmetric environments of the protons.

Key Concepts

Chemical Equivalence

In NMR Spectroscopy, the concept of **chemical equivalence** refers to protons experiencing the same electronic environment. Protons that are chemically equivalent will resonate at the same frequency in the NMR spectrum and thus appear as a single peak. For example, in the molecule propanone \((CH_3COCH_3)\), the two methyl \(CH_3\) groups are equivalent as they are in identical environments. This results in a single peak on the NMR spectrum. The understanding of chemically equivalent protons simplifies the analysis of NMR data.

Identifying chemically equivalent protons is crucial, as it determines the number of signals or peaks you will observe. The symmetry in the molecular structure often suggests equivalence. Look for repeating groups or protons that are indistinguishable from one another. This enables you to map out the initial framework of the spectrum you'll analyze.

Proton Splitting

The splitting of NMR signals, known as **proton splitting**, gives insight into how protons interact with their neighbors. Proton splitting occurs because protons in different environments "see" different magnetic fields from their neighbor protons. This interaction is known as spin-spin coupling.

The N+1 Rule is a simple way to predict splitting patterns. If a proton has \(N\) neighboring protons, its signal will be split into \(N+1\) parts, called multiplets. For example, in 1-bromopropane \(CH_3CH_2CH_2Br\), the terminal \(CH_3\) group has two adjoining protons from the neighboring \(CH_2\) group, resulting in a triplet (splitting into three peaks).

Understanding this rule helps interpret the connectivity in a molecule just from the NMR signals and how many protons are adjacent to each other, providing invaluable insight into complex structures.

Molecular Structure Analysis

NMR spectroscopy is a powerful tool for **molecular structure analysis**. It allows chemists to deduce the structure of a molecule by examining the NMR spectrum. Each peak in the spectrum corresponds to protons in different chemical environments, and their chemical shift reveals information about these environments.

For instance, in analyzing nitrobenzene \(C_6H_5NO_2\), the influence of the nitro group causes shifts in the resonance of hydrogen atoms on the benzene ring. The presence of electron-withdrawing groups such as \(NO_2\) affects the chemical environment of the nearby protons, shifting their signals in the spectrum, usually downfield (higher ppm).

Using this data, chemists can draw conclusions about the electronic structure, substituent effects, and symmetry of the molecule, providing a detailed understanding of how atoms are pieced together in three-dimensional space.

Multiplet Patterns

**Multiplet patterns** in NMR spectra arise from complex spin-spin coupling among protons. The interaction between non-equivalent neighboring protons can result in various multiplet patterns such as doublets, triplets, and more complex shapes depending on the neighboring group's size.

Analyzing these patterns is crucial because they inform you of the number and proximity of protons around a given set of protons. For 1,3-dinitrobenzene, the patterns are influenced by two nitro groups' presence, leading to intricate multiplets due to the various electronic environments around the aromatic protons.

Understanding how to predict and interpret multiplet patterns involves recognizing how complex they can get as the molecule's structure becomes more complicated. This skill is essential for distinguishing between isomers or identifying substituents in more complex organic structures effectively.