Benzene and Aromaticity

Spectroscopy of Aromatic Compounds

Aromatic compounds have two characteristic stretches in infrared spectroscopy, 3,000 to 3,100 cm–1 for carbon-hydrogen bonds and 1,450 to 1,650 cm–1 for aromatic carbon-carbon bonds. In 1H NMR, aromatic protons usually appear between 6.5 and 8 ppm. In 13C NMR, aromatic carbons appear between 100 and 150 ppm.
Infrared (IR) light is a part of the electromagnetic spectrum, with wavelengths longer than red light at the end of the visible spectrum. IR light comprises electromagnetic radiation of wavelengths from 750 nm to 1 mm. Infrared (IR) spectroscopy is the method that observes the vibrations of bonds and provides evidence of the functional groups present. The absorption, emission, or reflection is measured. The vibration of atoms and functional groups is measured by IR spectroscopy. Atoms and functional groups have unique vibrations. The carbon-hydrogen bonds of a benzene ring have an IR absorbance between 3,000 and 3,100 cm–1. The aromatic carbon-carbon bonds of the benzene ring have stretching and vibrational stretches in the 1,450–1.650 cm–1 range.

Infrared Spectrum of Benzene

The IR spectrum of benzene (C6H6) shows C(sp2)H{\rm{C}}(sp^2){-}{\rm{H}} bond stretches. The hydrogens attached to sp2 carbons show an absorbance between 3,000 and 3,100 cm-1. The double bonds of benzene ring have stretching and vibrational stretches in the 1,450-1,650 cm-1 range.
Nuclear magnetic resonance, NMR, measures the magnetic resonance of a nucleus. In the context of NMR, resonance refers to a hydrogen atom responding to a matching frequency of radio waves. All nuclei carry a charge, and some nuclei also have a nuclear spin. This spin creates a magnetic dipole than can interact with an applied magnetic field. 1H has a spin of 1/2, and in an applied field, each 1H is either aligned with or against the applied field. In 1H NMR spectroscopy, the energy in the radio frequency (Rf) range of the electromagnetic spectrum required to "flip" these 1H spins is measured. The chemical environment of the hydrogen atoms is indicated by the chemical shifts, corresponding to slight differences in the Rf energy of each distinct group of hydrogen atoms in the molecule. Chemical shift is the position on the delta (x-axis) scale and is measured in parts per million (ppm). Chemical shifts are affected by the decreasing of the electron cloud around an atom because of the proximity of electronegative atoms (deshielding) or π\pi bonds (anisotropy). The amount and types of hydrogens are indicated by the intensity (integral) and number of groups of signals in the 1H NMR spectra.

Hydrogen-1 NMR Spectrum of Benzene

Benzene (C6H6) has six equivalent hydrogen atoms that show up as one signal in the 1H NMR spectrum of benzene. Aromatics hydrogen atoms have a chemical shift between 6.5 and 8 ppm. The aromatic benzene hydrogen atoms have a chemical shift of 7.34 ppm.
Disubstituted benzenes will have unique splitting of these signals that is different for ortho, para, and meta. Hydrogen atoms on an sp3-carbon that is attached to a benzene usually appear between 2 and 3 ppm. 13C NMR spectroscopy is the application of NMR to the isotope of carbon-13, 13C. Carbon-12, 12C, is not detectable by NMR because it has a net zero spin. 13C has a spin of 1/2. It is less sensitive than 1H because of the significantly less natural amount of 13C (only about 1% of the carbons in a molecule).

Carbon-13 NMR Spectrum of Benzene

Benzene (C6H6) has six equivalent carbon atoms that show up as one signal in the 13C NMR spectrum of benzene. Aromatics carbon atoms have a chemical shift between 100 and 150 ppm. The aromatic benzene hydrogen atoms have a chemical shift of 128.5 ppm.