IR.pptx
- 2. Introduction Infrared spectroscopy (IR) is an absorption method widely used in both qualitative and quantitative analyses. The infrared region of the spectrum includes electromagnetic radiation that can alter the vibrational and rotational states of covalent bonds in organic molecules.
- 3. IR Spectroscopy Tutorial: Alcohols Alcohols have characteristic IR absorptions associated with both the O-H and the C-O stretching vibrations. When run as a thin liquid film, or "neat", the O– H stretch of alcohols appears in the region 3500-3200 cm-1 and is a very intense, broad band. The C–O stretch shows up in the region 1260-1050 cm-1. O–H stretch, hydrogen bonded 3500-3200 cm-1 C–O stretch 1260-1050 cm-1 (s)
- 4. The spectrum of ethanol is shown below. Note the very broad, strong band of the O–H stretch (3391) and the C–O stretches (1102, 1055).
- 5. Oxygen forms two bonds. An oxygen atom could be found in between two carbons, as in dibutyl ether. Ether If you look at an IR spectrum of dibutyl ether, you will see: there are the usual sp3 C-H stretching and CH2 bending modes at 2900 and 1500 cm-1. there is a strong peak near 1000 cm-1. This peak is due to the C-O stretching vibration.
- 7. The N–H stretches of amines are in the region 3300-3000 cm-1. These bands are weaker and sharper than those of the alcohol O–H stretches which appear in the same region. In primary amines (RNH2), there are two bands in this region, the asymmetrical N–H stretch and the symmetrical N–H stretch. Amines
- 8. Secondary amines (R2NH) show only a single weak band in the 3300-3000 cm-1 region, since they have only one N–H bond. Tertiary amines (R3N) do not show any band in this region since they do not have an N–H bond. (A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation. Note the spectrum of aniline, below.)
- 9. Carbonyl Compounds All carbonyl compounds absorb in the region 1760-1665 cm-1 due to the stretching vibration of the C=O bond. This distinctive carbonyl band is particularly useful for diagnostic purposes because it has a characteristic high intensity and few other functional groups absorb in this region. Different carbonyl compounds absorb in narrow ranges within the general carbonyl region. The exact wavenumber of the C=O stretch can give you clues as to whether the compound is a ketone, aldehyde, ester, or carboxylic acid; furthermore it can tell you whether it is an alpha, beta- carbonyl.
- 10. Range Type of compound Such as 1750-1735 cm-1 Saturated aliphatic esters 1740-1720 cm-1 Saturated aliphatic aldehydes 1730-1715 cm-1 𝜶, 𝜷-unsaturated esters 1715 cm-1 Saturated aliphatic ketones 1710-1665 cm-1 𝜶, 𝜷-unsaturated aldehydes and ketones
- 11. 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. Alkyl Halides
- 12. In terminal alkyl halides, the C–H wag of the –CH2X group is seen from 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 these 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 o C–Cl stretch 850-550 cm-1 o C–Br stretch 690-515 cm-1
- 13. 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.
- 14. Even though both 1-bromopropane and 1-chloro-1-methylpropane have similar spectra and the bands that distinguish one from the other are in the fingerprint region, if the two spectra are overlayed, examination of the fingerprint region readily shows that they are different compounds.