Select the reaction "Nucleophilic aromatic substitution." Predict the \({ }^{1} \mathrm{H}\) NMR spectra of the starting material (1-chloro-2,4-dinitrobenzene) and the product ( \(N-\) methyl-2,4-dinitroaniline). How would the IR data differ for the starting material and the product?

Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to determine the structure of organic molecules by observing the interactions of nuclear spins when placed in an external magnetic field. In \({ }^{1} \rm H\) NMR spectroscopy, we specifically look at the environment of hydrogen (proton) atoms in a molecule.
For the starting material, 1-chloro-2,4-dinitrobenzene, the \({ }^{1} \rm H\) NMR spectrum shows a multiplet around 7.5-8.5 ppm. This is due to the aromatic protons impacted by the electron-withdrawing effects of the nitro and chloro groups.
The product, N-methyl-2,4-dinitroaniline, has distinct NMR signals:

• A singlet around 4.0-4.5 ppm for the N-methyl protons (3 protons)
• A multiplet around 7.5-8.5 ppm for the aromatic protons (4 protons)
• An additional singlet around 6.0-6.5 ppm for the N-H proton (1 proton)

Differences in chemical shift give insights into the molecular environment around the protons, helping us understand the structure and substituent effects in a molecule.

Infrared (IR) spectroscopy identifies functional groups in a molecule by measuring the absorption of infrared light, which causes vibrations in the molecules. In IR spectra, different functional groups absorb characteristic frequencies of IR radiation.
For 1-chloro-2,4-dinitrobenzene, the IR spectrum shows prominent peaks for nitro groups (NO2) around 1500-1600 cm⁻¹ and 1300-1400 cm⁻¹. These peaks correspond to the symmetric and asymmetric stretching vibrations of the NO2 group.
In the product, N-methyl-2,4-dinitroaniline, in addition to the nitro group peaks, we see an N-H stretch around 3200-3500 cm⁻¹ and C-N stretching vibrations. The N-H stretch is a broad peak due to hydrogen bonding, and it usually appears at a higher frequency compared to other stretches.

Functional group analysis helps in identifying different chemical groups within a molecule, which dictates its physical and chemical properties. In this exercise, we are dealing with nitro, chloro, and amine functional groups.
In the starting material, 1-chloro-2,4-dinitrobenzene, we have:

• A nitro group (NO2) at positions 2 and 4
• A chloro group (Cl) at position 1

In the product, N-methyl-2,4-dinitroaniline, we have:

• NO2 groups at positions 2 and 4 remain unchanged
• A methylated amine group (N-methyl) at position 1

This functional group transformation from a chloro to an amine group affects the molecule's reactivity and spectroscopic properties, as observed in both NMR and IR spectra.

Organic reaction mechanisms explain how and why reactions occur at a molecular level. In nucleophilic aromatic substitution (NAS), an electron-rich nucleophile replaces an electron-withdrawing group on an aromatic ring.
In this exercise, the mechanism involves:

• A nucleophile (such as a methylamine) attacking the electron-deficient carbon attached to the chloro group in 1-chloro-2,4-dinitrobenzene.
• This results in the displacement of the chloro group, forming N-methyl-2,4-dinitroaniline.

Understanding these mechanistic steps helps in predicting the products and their spectroscopic characteristics, as seen with the differing NMR and IR spectra of the starting material and the product.