Infrared spectrum / infrared frequency and hydrocarbons
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This document provides information about infrared (IR) spectroscopy and analyzing IR spectra of different functional groups. It discusses: 1. The conditions required for IR absorption and the division of the IR spectrum into the functional group and fingerprint regions. 2. The characteristic IR absorptions of common functional groups like alkanes, alkenes, alkynes, alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids, esters, amides, amines, and aromatics. Specific examples and their spectra are provided. 3. Factors that affect IR frequencies, such as bond strength, mass of atoms, resonance, conjugation, and hydrogen bonding.
Infrared spectrum / infrared frequency and hydrocarbons
- 1. Infrared spectrum/Infrared frequency Ravish Yadav Hydrocarbons
- 2. CONDITION FOR ABSORPTION OF IR by BONDS Molecular vibrations must produce a change in dipole moment of bond The frequency of IR radiation = Frequency of bond vibration CONDTION FOR A MOLECULAR VIBRATION (BOND) TO ABSORB IR RADIATION
- 3. REGION OF AN IR SPECTRUM The IR spectrum is divided into 2 regions 1) Functional group region 2) Finger print region
- 4. HYDROGEN REGION 1. O-H 2. N-H i) Primary ii) Secondary 3. C-H i) Sp ii) Sp2 iii) sp3 TRIPLE BOND REGION 1. C≡C 2. C≡N DOUBLE BOND REGION 1. -C=C 2. C=O 3. C=N SINGLE BOND REGION, BENDING 1. C-C 2. C-O 3. C-N 4. C-X FUNCTIONAL GROUP REGION FINGERPRINT REGION 3650c m 2500 cm 2300 cm 2100 cm 1900 cm 1650c m 1500 cm 667 cm
- 5. Factors affecting IR frequency Probable frequency/ wave no. at which particular bond absorbs can be calculated with hook’s law: frequency is directly proportional to bond strength and inversely proportional to masses of atoms
- 6. Other factors 1) Fermi resonance 2) Electronic effects (related to chemical arrangement of atoms in molecule ) i. Positive inductive effect- vibrational frequency decreases ii. Negative inductive effect- vibrational frequency increases iii. Resonance and conjugation- vibrational frequency decreases iv. Field effect- as field increases vibrational frequency decreases v. Bond angle- as bond angle increases vibrational frequency decreases vi. Hydrogen bonding – as strength of OH decreases, vibrational frequency decreases.
- 8. HYDROCARBONS 1.ALKANES: •The spectra of simple alkanes are characterized by absorptions due to C–H stretching and bending •Note the strong bands in the 3000-2850 cm-1 region due to C- H stretch. •Example : octane 8
- 10. 2.ALKENE : •Alkenes are compounds that have a carbon-carbon double bond, –C=C–. The stretching vibration of the C=C bond usually gives rise to a moderate band in the region 1680-1640 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 several bands lower than 3000 cm-1 for –C–H stretch (alkanes). The C=C stretch band is at 1644 cm-1. •Example: 1- octene 10
- 12. 3.ALKYNES: •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. •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). •Example : 1- hexyne 12
- 14. AROMATIC HYDROCARBON • The =C–H stretch in aromatics is observed at 3100-3000 cm-1. • Aromatic hydrocarbons show absorptions in the regions 1600- 1585 cm-1 and 1500-1400 cm-1 due to carbon-carbon stretching vibrations in the aromatic ring. • The pattern of overtone bands in the region 2000-1665 cm- 1 reflect the substitution pattern on the ring.
- 16. CARBONYL CARBON COMPOUNDS 1)Carboxylic acid: Carboxylic acid can be easily detected by IR spectroscopy .It mainly considered two units like C=O and O-H. • C=O shows very strong bands that appears in the range of 1730-1700cm-1 for simple aliphatic carboxylic acids. O-H stretching occurs at broad band in the range of 3000- 2500cm-1 Example : hexanoic acid , benzoic acid 16
- 17. Hexanoic acid O-H 3300-2500cm-1 C-H 2950-2850cm-1 C=O 1721cm-1
- 18. Benzoic acid O-H occurs at 3400-2400cm-1( broad peak) C=O Occurs at 1730-1700 cm-1
- 19. 2) ESTER Esters show a very strong band for the C=O group that appears in the range of 1750-1735cm-1 for simple aliphatic esters. The C=O band is shifted to lower frequency when it is conjugated to C=C or phenyl group. Ex:, ethyl benzoate, ethyl acetate. 19
- 21. C-H 2980-3000 cm-1 C=O 1752 cm-1 C-O 1250 cm-1
- 23. 4) ALDEHYDES The carbonyl stretch C=O of saturated aliphatic aldehydes appears from 1740-1720 cm-1. If the carbons adjacent to the aldehyde group are unsaturated, this vibration is shifted to lower wavenumbers, 1710-1685 cm-1 H–C=O stretch 2830-2695 cm-1 The spectra of benzaldehyde and butyraldehyde are shown. Note that the O=C stretch of the alpha, beta-unsaturated compound -- benzaldehyde -- is at a lower wavenumber than that of the saturated butyraldehyde. Note the O=C–H stretches in both aldehydes in the region 2830-2695 cm-1, especially the shoulder peak at 2725 cm-1 in butyraldehyde and 2745 cm-1 in benzaldehyde.
- 26. 5) KETONES The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears at 1715 cm-1. Conjugation of the carbonyl group with carbon-carbon double bonds or phenyl groups, shifts this band to lower wavenumbers, 1685-1666 cm-1. The spectrum of 2-butanone is shown below. This is a saturated ketone, and the C=O band appears at 1715. Note the C–H stretches (around 2991) of alkyl groups.
- 29. 7) AMIDES The carbonyl stretching vibration band C=O of saturated aliphatic amides appear in between 1650 to 1690 cm-1. EXAMPLE- PROPANAMIDE
- 31. 5.ALCOHOL AND PHENOL: Alcohol and phenol show strong and broad hydrogen bonded O- H stretching bands centering between 3400cm-1 and 3300cm-1. In solution it will also be possible to observe a free O-H stretching band at 3600cm-1. Example: ethanol , p-cresol 3
- 33. O-H =3500-3300 cm-1 C-O = 1200 cm-1
- 36. 6.ETHERS: Ether is derivative of alcohol and show characteristics c-o-c bonds . Ether show prominent C-O stretching band at 1300 to 1000cm-1. Absence of C=O and O-H is required to ensure that C-O stretch is not due to an ester and phenol .phenyl alkyl ether gives two strong bands at 1250cm-1 and 1040cm-1. Example: di butylether, anisole 3
- 38. AMINES Amines show characteristic N-H stretching in the hydrogen region. Occurring between 3300-3500 cm-1. C-N stretching in the single bond region. Occurring between 1350-1000cm-1
- 39. AMINES: N-H stretch: 3300- 3500cm-1 occurs to the left of C-H absorptions. Primary amines:3500- 3400cm-1 Less intensity of absorption Primary amines display two small peaks. Bands represent the asymmetric and symmetric vibrations. Secondary amines:3350-3310cm-1 Very less intensity Consist of only a single peak small peak Tertiary amine: Peaks are non existent.
- 40. Aromatic amines: High intensity of absorption. Single & long peak. C-N stretch: Occurs at 1350-1000cm-1, varying intensity. To the right of C-H bonds. 1250-1000cm-1 for aliphatic, Primary amine: 2peaks with very less intensity. Secondary amine: single, peak with high intensity.
- 41. Tertiary amine: single peak with comparatively less intensity. 1350-1250cm-1 for aromatics. Single , sharp peak with higher intensity The higher frequency in aromatics is from resonance increasing double-bond character between the ring and attached nitrogen.
- 42. AMINES: primary amine e.g. Butyl amine
- 43. Secondary amine : egg: dibutyl amine
- 44. Tertiary amine : egg: Tributylamine
- 45. Aromatic amine: egg: N-methyl aniline