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Topic 8 Summary: "Spectroscopic Structural Elucidation"

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Western University
Chemistry 1027A/B
Mel Usselman

Key Concepts In Organic Chemistry: Spectroscopic Structural Elucidation (Usselman Version) -Energy is transmitted in the form of electromagnetic radiation understood as oscillating waves of electric and magnetic fields. Humans can detect radiation of wavelengths 400-800nm known as visible light. An average C-C bond absorbs UV light, π bonds absorb visible light and the ΔE for molecular conformations absorbs IR light. In IR spectroscopy, a chart of “% transmittance” of light through the sample beam against “wavenumber” (defined as the number of wavelengths in 12 cm) is produced. -1 -There are normally two regions in IR spectra investigated: the region from 4000-1400cm (where absorptions are characteristic of several major functional groups) and the “fingerprint” region (1400- 400cm ), which contains a complex absorption pattern dependent upon each specific molecular structure. Each of the functional group have characteristic absorption “peaks”. These peaks are identified by frequency (in wavenumbers) and intensity of absorption, whether weak (w), medium (m), or strong (s). -Saturated alkanes have a strong peak with multiple splittings between 2850-3000cm (C-H stretch). -1 These peaks are common to all organic compounds that contain saturated alkyl groups. Alkenes have two possible peaks due to C = C bonds: a weak/medium peak around 1600-1680cm (C=C stretch) -1 and/or a weak/medium peak around 3000-3100cm (=C-H stretch). Alkynes also have two characteristic -1 absorptions: a weak peak between 2100-2260cm (C = C stretch) and/or a medium/weak peak at 3300cm (= C-H). -Alcohols have a strong, broad O-H (H-bonded) stretch at 3200-3500cm . Ethers have a strong C-O -1 stretch between 1070 and 1150cm (they are just like alcohols, but lack the O-H stretch). Arenes have 4 2 -1 -1 characteristic strong absorptions: sp C-H stretch at 3030cm , sharp peaks between 1450 and 1600cm (C=C stretching), aromatic ring peaks between 690-900cm (C-H bending), and benzene peaks between -1 1700-2000cm (indicator of a benzene ring). Amines have strong N-H stretching peaks in the 3100-3500 region: two peaks in 1° amines, one peak in 2° amines and no peaks in 3° and 4° amines. -All nuclei with an odd mass number have two spin energy states of + ½ and – ½. Examples of this phenomenon are readily observed in H and C atoms. Although C may have two spin energy states, due to the even number of neutrons, the opposite spins cancel out. Opposite spins are not observed in oxygen because O atom has no stable isotope with ± ½ spin. Nuclei with spin will absorb radiation and the energy field of the absorbed radiation will equal the energy difference between the two states. -The absorption of radiation by nuclei in a magnetic field is termed Nuclear Magnetic Resonance (NMR) 13
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