2. If a C::O is present you want to determine if it is part of an acid, an ester, or an aldehyde or ketone. At this time you may not be able to distinguish aldehyde from ketone and you will not be asked to do so. How to Analyze an IR Spectrum • Look first for the carbonyl C=O band. Look for a strong band at 1820-1660 cm-1. • This band is usually the most intense absorption band in a spectrum. It will have a medium width. If you see the carbonyl band, look for other bands associated with functional groups that contain the carbonyl by going to step 2. If no C=O band is present, go to step 3 and check for alcohols. 2. If a C=O is present determine if it is part of an acid, an ester, aldehyde, ketone or anhydride. anhydrides show two C=O absorptions near 1810 and 1760 cm-1
3. If the C=O band is absent, look for the following: ALCOHOLS, PHENOLS Broad absorption near 3400-3300 cm-1. Confirm by finding C-O stretch near 1300-1000 cm-1 AMINES Check for N-H stretch. Medium absorption(s) Near 3400 cm-1. ETHERS Check for a C-O stretch near 1300-1000 cm-1 (and an absence of O-H near 3400 cm-1). • If no functionalities in step 3 are present, check for double bonds and/or aromatic rings. • C=C is a weak absorption near 1650 cm-1. • Aromatic rings show medium to strong absorptions from 1600-1450 cm-1. • Consult the C-H region to confirm. Aromatic and vinyl • C-H absorptions occur to the left of 3000 cm-1. Aliphatic C-H occurs to the right of 300 cm-1.
5. Triple bonds CN shows a medium, sharp absorption near 2250 cm-1. CC shows a weak, sharp absorption near 2150 cm-1. 6. Nitro groups Shows two strong absorptions at 1600-1530 cm-1 and 1390-1300 cm-1. 7. Hydrocarbons This may be indicated if none of the other functional groups are present Major absorptions are found to the right of 3000 cm-1 to C-H. Hydrocarbons show very simple spectra. The only other absorptions appear near 1460 and 1375 cm-1.
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, seen only in long chain alkanes, from 725-720 cm-1
C=C stretch from 1680-1640 cm-1 =C–H stretch from 3100-3000 cm-1 =C–H bend from 1000-650 cm-1
–C≡C– stretch from 2260-2100 cm-1 –C≡C–H: C–H stretch from 3330-3270 cm-1 –C≡C–H: C–H bend from 700-610 cm-1
C–H wag (-CH2X) from 1300-1150 cm-1 • C–X stretches (general) from 850-515 cm-1 • C–Cl stretch 850-550 cm-1 • C–Br stretch 690-515 cm-1
C–H stretch from 3100-3000 cm-1 overtones, weak, from 2000-1665 cm-1 C–C stretch (in-ring) from 1600-1585 cm-1 C–C stretch (in-ring) from 1500-1400 cm-1 C–H "oop" from 900-675 cm-1
O–H stretch, hydrogen bonded 3500-3200 cm-1 C–O stretch 1260-1050 cm-1 (s)
C=O stretch: aliphatic ketones 1715 cm-1 α, β-unsaturated ketones 1685-1666 cm-1
H–C=O stretch 2830-2695 cm-1 • C=O stretch: • aliphatic aldehydes 1740-1720 cm-1 • alpha, beta-unsaturated aldehydes 1710-1685 cm-1
Asymmetric C=O stretch Symmetric C=O stretch
O–H stretch from 3300-2500 cm-1 C=O stretch from 1760-1690 cm-1 C–O stretch from 1320-1210 cm-1 O–H bend from 1440-1395 and 950-910 cm-1
C=O stretch • aliphatic from 1750-1735 cm-1 • α, β-unsaturated from 1730-1715 cm-1 • C–O stretch from 1300-1000 cm-1
N–H stretch 3400-3250 cm-1 • 1° amine: two bands from 3400-3300 and 3330-3250 cm-1 • 2° amine: one band from 3350-3310 cm-1 • 3° amine: no bands in this region • N–H bend (primary amines only) from 1650-1580 cm-1 • C–N stretch (aromatic amines) from 1335-1250 cm-1 • C–N stretch (aliphatic amines) from 1250–1020 cm-1 • N–H wag (primary and secondary amines only) from 910-665 cm-1
N–O asymmetric stretch from 1550-1475 cm-1 N–O symmetric stretch from 1360-1290 cm-1
Effect of Conjugation on C=O absorptions Conjugation moves the C=O absorption to a smaller wave number.
Match the following spectra with the structures shown below. Support your answers with specific approximate frequencies.
aliphatic C-H stretch arom C-H stretch C-O stretch arom C=C O-H stretch
Propose a structure for a compound with molecular formula C7H12O, whose IR spectrum appears below.
Propose a structure for a compound with molecular formula C6H12O, with the following IR spectrum:
Propose a structure for a compound with molecular formula C6H13N, whose IR spectrum appears below.
993 cm-1 912 cm-1
965 cm-1 (trans)
698 cm-1 (cis)
Limitations of IR in Identifying Alkenes What would be a better way to identify these?
Combination and overtone bands Benzenoid Aromatics C-H out-of-plane bending C=C out-of-plane ring bending monosubstituted: strong absorption near 690 cm-1 ortho-disubstituted: strong absorption near 750 cm-1 meta-disubstituted: the 690 cm-1 band, one near 780 cm-1, and often one near 880 cm-1 para-disubstituted: strong absorption in the region 800-850 cm-1
monosub- stitution oop aromatic C=C stretch
ortho-di- substitution aromatic C=C stretch oop
meta-di- substitution aromatic C=C stretch oop
para-di- substitution aromatic C=C stretch oop
Propose a structure for an aromatic compound with the molecular formula C9H12 which gives the following IR spectrum. Justify your answer.
Propose a structure for an aromatic compound with the molecular formula C10H14 which gives the following IR spectrum. Justify your answer.
overlap N-H bend C=O stretch NH2 stretch (primary amide) C-N stretch
Overtone of 1540 band (3080 cm-1) NH stretch (secondary amide) N-H Bend (1540 cm-1) C=O stretch