Question

Three isomeric chloro-derivatives of pyridine \((\mathbf{A}, \mathbf{B}\) and \(\mathbf{C}\) ) analyse as containing \(40.58 \%\) C \(, 2.04 \%\) H and \(9.46 \%\) N. The \(^{1}\) H NMR spectroscopic data for the compounds are as follows where \(\mathrm{d}=\) doublet, \(\mathrm{d} \mathrm{d}=\) doublet of doublets and \(\mathrm{t}=\) triplet: $$\begin{array}{ll} \hline \text { Compound } & ^{1} \mathbf{H} \text { NMR } \delta / \text { ppm } \\ \text { A } & 7.66(\mathrm{t}, J=7.6 \mathrm{Hz}) \\ & 7.31(\mathrm{d}, J=7.6 \mathrm{Hz}) \\ \text { B } & 8.64(\mathrm{d}, J=2.1 \mathrm{Hz}) \\ & 8.25(\mathrm{t}, J=2.1 \mathrm{Hz}) \\ \text { C } & 8.70(\mathrm{dd}, J=3.0 \text { and } 0.3 \mathrm{Hz}) \\ & 8.13(\mathrm{dd}, J=9.0 \text { and } 3.0 \mathrm{Hz}) \\ & 7.68(\mathrm{dd}, J=9.0 \text { and } 0.3 \mathrm{Hz}) \\ & \\ \hline \end{array}$$ In each isomer, \(\mathrm{Cl}\) atoms are in either the 2 - or \(3-\) position with respect to the \(\mathrm{N}\) atom. Suggest structures for \(\mathbf{A}, \mathbf{B}\) and \(\mathbf{C}\).

Short answer

A: 4-chloropyridine, B: 2-chloropyridine, C: 3-chloropyridine.

Step-by-step solution

Determine Possible Structures for Chloro-Pyridines

Given that the chlorine atoms can be in the 2-, 3-, or 4-positions relative to the nitrogen atom in pyridine, we have three potential isomers: 2-chloropyridine, 3-chloropyridine, and 4-chloropyridine.

Analyze the 1H NMR Spectra for Compound A

For compound A, the presence of a triplet at 7.66 ppm with a coupling constant of 7.6 Hz suggests protons that are meta to each other, typical for 3- and 5-positions in 4-chloropyridine. The doublet at 7.31 ppm supports this by indicating proton-proton coupling consistent with 4-chloropyridine structure.

Analyze the 1H NMR Spectra for Compound B

Compound B has a doublet at 8.64 ppm and a triplet at 8.25 ppm both with a coupling constant of 2.1 Hz. This pattern is typical for protons in vicinal ortho positions, suggesting the structure of 2-chloropyridine where protons at positions 3, 4, and 5 couple with the nitrogen atom.

Analyze the 1H NMR Spectra for Compound C

The NMR data for compound C displays two doublets of doublets which suggests a more complex coupling usually associated with 3-chloropyridine. Here, the three protons interact via both ortho and meta couplings, consistent with the position of chlorine in 3-chloropyridine.

Confirm Structures based on NMR Data

Based on the NMR data interpretations: Compound A is 4-chloropyridine, Compound B is 2-chloropyridine, and Compound C is 3-chloropyridine.

Key concepts

Understanding NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy is an essential analytical tool in organic chemistry. It helps us understand the molecular structure by observing the interactions of atomic nuclei with an external magnetic field. The nuclei, typically hydrogen or carbon, absorb and re-emit electromagnetic radiation, providing information about chemical environments and neighboring atoms. For isomeric chloro-derivatives of pyridine, NMR spectroscopy distinguishes between different isomers by analyzing the magnetic environment of hydrogen atoms. Since protons in a molecule will have different chemical shifts based on the local magnetic environment, the NMR spectrum is like a molecular fingerprint.

• The chemical shift, denoted by \(\delta\), is measured in parts per million (ppm).
• The splitting pattern in spectra, such as singlet, doublet, or triplet, reveals proton interactions.
• Coupling constants, represented as \(J\), indicate the strengths of these interactions and are measured in hertz (Hz).

By deciphering these features, chemists can deduce the structure of a compound like pyridine isomers.

Isomeric Compounds Overview

Isomers are compounds that share the same molecular formula but differ in the arrangement of their atoms. They can be classified broadly into structural isomers, which differ in connectivity, and stereoisomers, which differ in spatial arrangement. For chloro-derivatives of pyridine, we are dealing with structural isomers. The position of the chlorine atom relative to the nitrogen in the pyridine ring changes the compound's chemical properties. All isomers have the same molecular formula, which can result in different NMR spectra.

• 2-Chloropyridine has chlorine in the ortho position relative to the nitrogen atom.
• 3-Chloropyridine places chlorine in the meta position.
• 4-Chloropyridine situates the chlorine para to the nitrogen atom.

These position shifts lead to unique NMR spectra, as observed in proton coupling and chemical shift changes.

The Structure of 2-Chloropyridine

In 2-chloropyridine, the chlorine atom is located at the second position on the pyridine ring, directly adjacent to the nitrogen atom. This positioning significantly affects the electronic environment and, consequently, the NMR data. Protons closer to the chlorine atom, particularly those in the 3 and 5 positions, experience different magnetic environments.

• The presence of a doublet at 8.64 ppm in this case suggests protons affected by the electron-withdrawing nature of the chlorine.
• A triplet at 8.25 ppm indicates further coupling, illustrating the proximity and influence of adjacent hydrogen atoms.

The coupling constant between these protons and their neighbors is relatively small type, consistent with the ortho arrangement. This data confirms the identification of compound B as 2-chloropyridine.

Exploring 3-Chloropyridine

3-Chloropyridine features a chlorine atom in the meta position relative to the nitrogen atom, which leads to a more complex NMR pattern. This complexity arises from multiple non-equivalent hydrogen atoms in proximity to chlorine.

• The NMR spectrum for compound C displays doublets of doublets. Such patterns indicate extensive splitting, showing the meta coupling between protons.
• Specific chemical shifts provided (at 8.70 ppm, 8.13 ppm, and 7.68 ppm) reveal intricate interactions between hydrogens spread across the pyridine ring.

The multiple coupling constants hint at both ortho and meta coupling mechanisms. These features accurately point to compound C being 3-chloropyridine.

Identifying 4-Chloropyridine

In 4-chloropyridine, the chlorine atom is positioned para to the nitrogen atom, presenting a clearer, recognizable NMR pattern. Compound A aligns with this structure.

• The presence of a triplet at 7.66 ppm signifies symmetrical coupling between hydrogen atoms at positions 3 and 5.
• The ensuing doublet at 7.31 ppm supports the para-position arrangement, indicating minimal skew in the electronic environment due to the para positioning.

In 4-chloropyridine, the distance between chlorine and the influencing hydrogen atoms in the spectrum is maximized, resulting in distinct and less overlapped signals compared to 2- or 3-chloropyridines. This pattern encloses the identity of compound A as 4-chloropyridine.