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IR Spectroscopy Tutorial: Alkanes
The spectra of simple alkanes are characterized by absorptions due to C–H stretching and bending (the C–C stretching and bending bands are either too weak or of too low a frequency to be detected in IR spectroscopy). In simple alkanes, which have very few bands, each band in the spectrum can be assigned.
•C–H stretch from 3000–2850 cm-1
•C–H bend or scissoring from 1470-1450 cm-1
•C–H rock, methyl from 1370-1350 cm-1
•C–H rock, methyl, en only in long chain alkanes, from 725-720 cm-1
The IR spectrum of octane is shown below. Note the strong bands in the 3000-2850 cm-1region due to C-H stretch. The C-H scissoring (1470), methyl rock (1383), and long-chain methyl rock (728) are noted on this spectrum. Since most organic compounds have the features, the C-H vibrations are usually not noted when interpreting a routine IR spectrum.
The region from about 1300-900 cm-1is called the fingerprint region. The bands in this region originate in interacting vibrational modes resulting in a complex absorption pattern. Usually, this region is quite complex and often difficult to interpret; however, each organic compound has its own unique absorption pattern (or fingerprint) in this region and thus an IR spectrum be ud to identify a compound by matching it with a sample of a known compound.
IR Spectroscopy Tutorial: Alkenes
Alkenes are compounds that have a carbon-carbon double bond, –C=C–. The stretching vibration of the C=C bond usually gives ri to a moderate band in the region 1680-1640 cm-1.
Stretching vibrations of the –C=C–H bond are of higher frequency (higher wavenumber) than tho of the –C–C–H bond in alkanes.
This is a very uful tool for interpreting IR spectra: Only alkenes and aromatics show a C-H stretch slightly higher than 3000 cm-1. Compounds that do not have a C=C bond show C-H stretches only below 3000 cm-1.
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The strongest bands in the spectra of alkenes are tho attributed to the carbon-hydrogen bending vibrations of the =C–H group. The bands are in the region 1000-650 cm-1(Note: this overlaps the fingerprint region).
Summary:
•C=C stretch from 1680-1640 cm-1
•=C–H stretch from 3100-3000 cm-1
辅助的英文•=C–H bend from 1000-650 cm-1
The IR spectrum of 1-octene is shown below. Note the band greater than 3000 cm-1 for the =C–H stretch and the veral bands lower than 3000 cm-1 for –C–H stretch (alkanes). The C=C stretch band is at 1644 cm-1. Bands for C–H scissoring (1465) and methyl rock (1378) are marked on this spectrum; in routine IR analysis, the bands are not specific to an alkene and are generally not noted becau they are prent in almost all organic molecules (and they are in the fingerprint region). The bands at 917 cm-1 and 1044cm-1 are attributed to =C&Mac173;H bends.
It is interesting to overlay a spectrum of 1-octene with octane (shown below). Note how similar they are, except for the bands due to C=C or C=C–H stretches/bends.
IR Spectroscopy Tutorial: Alkynes
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Alkynes are compounds that have a carbon-carbon triple bond (–C≡C–). The –C≡C– stretch appears as a weak band from 2260-2100 cm-1. This can be an important diagnostic tool becau very few organic compounds show an absorption in this region.
A terminal alkyne (but not an internal alkyne) will show a C–H stretch as a strong, narrow band in the range 3330-3270 cm-1. (Often this band is indistinguishable from bands resulting from other functional groups on the same molecule which absorb in this region, such as the O-H stretch.)
A terminal alkyne will show a C–H bending vibration in the region 700-610 cm-1.
Summary:
•–C≡C– stretch from 2260-2100 cm-1
上诉书•–C≡C–H: C–H stretch from 3330-3270 cm-1
•–C≡C–H: C–H bend from 700-610 cm-1
The spectrum of 1-hexyne, a terminal alkyne, is shown below. Note the C–H stretch of the C–H bond adjacent to the carbon-carbon triple bond (3324), the carbon-carbon triple bond stretch (2126), and t
he C–H bend of the C-H bond adjacent to the carbon-carbon triple bond (636). The other bands noted are C–H stretch, scissoring, and methyl rock bands from the alkane portions of the molecule.
IR Spectroscopy Tutorial: Alkyl Halides
英语教案Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine (usually Br or Cl in the organic chemistry teaching labs). In general, C–X vibration frequencies appear
in the region 850-515 cm-1, sometimes out of the range of typical IR instrumentation. C–Cl stretches appear from 850–550 cm-1, while C–Br stretches appear at slightly lower wavenumbers from 690-515 cm-1. In
X group is en from terminal alkyl halides, the C–H wag of the –CH
2
1300-1150 cm-1. Complicating the spectra is a profusion of absorptions throughout the region 1250-770 cm-1, especially in the smaller alkyl halides. Note that all of the bands are in the fingerprint region. In summary, the following bands are specific to alkyl halides: •C–H wag (-CH2X) from 1300-1150 cm-1
•C–X stretches (general) from 850-515 cm-1
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o C–Cl stretch 850-550 cm-1
o C–Br stretch 690-515 cm-1
The spectra of 1-bromopropane and 1-chloro-2-methylpropane are shown below. Note the C–Br or C–Cl stretches in the region 850-515 cm-1. They also show C–Br or C–Cl wag in the region 1300-1150 cm-1.
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