5.3: Spectroscopic Properties of Alkyl Halides

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Organohalogen compounds give rise to strong absorptions in the infrared arising from stretching vibrations of the carbon-halogen bond. The frequency of absorption decreases as the mass of the halogen increases. For monohaloalkanes the absorptions useful for identification are those of \(\ce{C-F}\) at \(1100\)-\(1000 \: \text{cm}^{-1}\) and \(\ce{C-Cl}\) at \(850\)-\(550 \: \text{cm}^{-1}\). The \(\ce{C-Br}\) and \(\ce{C-I}\) absorptions are below \(690 \: \text{cm}^{-1}\) and therefore are out of range of most commercial spectrophotometers. Because these bands are in the fingerprint region or far infrared, it is difficult to infer the presence of halogen in a molecule solely from its infrared spectrum.

Apart from fluorine, the magnetic properties of halogen nuclei do not complicate proton or \(\ce{^{13}C}\) nuclear magnetic resonance spectra of organohalogen compounds. But fluorine \(\left( \ce{^{19}F} \right)\) has a spin of 1/2 and causes spin-spin splitting of the resonances of neighboring magnetic nuclei (\(\ce{^{13}C}\), \(\ce{^1H}\), and other \(\ce{^{19}F}\) nuclei). Proton chemical shifts are influenced strongly by the presence of halogen, which serves to deshield neighboring protons by electronegativity effects (see Section 9-10E).

The mass spectra of chlorine- and bromine-containing compounds clearly show the abundance ratios of the stable isotopes \(\ce{^{35}Cl}\):\(\ce{^{37}Cl} =\) 3:1 and \(\ce{^{79}Br}\):\(\ce{^{81}Br} =\) 1:1 in the molecular ions and those ionic fragments which contain halogens (Section 9-11).

Contributors and Attributions

John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."