Answer Key for IR assignment Winter 2002
Spectrum 1: 1,2-Diaminododecane (solid!)
| Frequency | Assignment |
| 3167-3332 | v(NH2), primary amine |
| 2851, 2921 | v(CH, alkane, sp3) |
| 1583 | d(NH2, bending) |
| 1463 | d(CH2, bending) |
| 721 | r((CH2)4, rocking) |
Spectrum 2: 4,4'-Dimethylbenzophenone (solid!)
| Frequency | Assignment |
| 3020 | v(CH, aromatic, sp2) |
| 2921 | v(CH, alkane, sp2) |
| 1646 (s) | v(C=O) |
| 1605 | v(C=C, aromatic) |
Note: The carbonyl frequency is shifted to lower frequency due to conjugation.
Spectrum 3: Benzhydrol (solid!)
| Frequency | Assignment |
| 3386 (br, s) | v(OH, alcohol) |
| 3027, 3080 | v(CH, aromatic, sp2) |
| 2890 | v(CH, alkane, sp3) |
| 1760-1948 | aromatic overtone, mono |
| 1597 | v(C=C, aromatic) |
| 1018 | v(C-O) |
| 697-754 | oop, mono |
Spectrum 4: p-Bromobenzoic acid (solid!)
| Frequency | Assignment |
| 3500-2500 (br, m) | v(OH, acid) |
| 1679 (s) | v(C=O) |
| 1599 | v(C=C, aromatic) |
Spectrum 5: p-Methyl hydroxybenzoate (solid!)
| Frequency | Assignment |
| 3349 (br, s) | v(OH, alcohol) |
| 2963 | v(CH, alkane, sp3) |
| 1682 (s) | v(C=O) |
| 1608 | v(C=C, aromatic) |
| 1279, 1232, 1105 | v(C-O-C, alcohol, ester) |
Spectrum 6: 3,4-Dihydro-2H-pyran (neat= pure liquid)
| Frequency | Assignment |
| 3064 | v(CH, alkene, sp2) |
| 1851-2931 | v(CH, alkane, sp3) |
| 1649 (s) | v(C=C, alkene) |
| 1242, 1070 | v(C-O-C, alcohol, ester) |
Note: The C=C stretching is strong because the attached oxygen in the ring induces a bigger dipole moment.
Spectrum 7: Bromobenzene (liquid)
| Frequency | Assignment |
| 3065 | v(CH, aromatic, sp2) |
| 1732-1942 | aromatic overtone, mono |
| 1578 | v(C=C, aromatic) |
| 671-750 | oop, mono |
Spectrum 8: Cyclohexane (liquid)
| Frequency | Assignment |
| 2852, 2929 | v(CH, alkane, sp3) |
| 1450 | d(CH2, bending) |
Note: The spectrum is missing the for longer chanins (n>4) characteristic peak ~720 cm-1.
Spectrum 9: Nitrobenzene (liquid)
| Frequency | Assignment |
| 3107, 3076 | v(CH, aromatic, sp2) |
| 1523 (s) | v(NO2, asym) |
| 1347 (s) | v(NO2, sym) |
| 681-723 | oop, mono |
Spectrum 10: Triethylamine (liquid)
| Frequency | Assignment |
| 2799-2972 | v(CH, alkane, sp3) |
| 1467, 1382 | d(CH2, bending) |
Spectrum 11: Xylene (liquid)
| Frequency | Assignment |
| 3036 | v(CH, aromatic, sp2) |
| 2872, 2965 | v(CH, alkane, sp3) |
| 1745-1933 | aromatic overtone,disubst. |
| 1613 | v(C=C, aromatic) |
Spectrum 12: Benzaldehyde
| Frequency | Assignment |
| 3031-3086 | v(CH, aromatic, sp2) |
| 2738, 2850 | v(CH, aldehyde) |
| 1703 (s) | v(C=O) |
| 1597 | v(C=C, aromatic) |
| 688-745 | oop, mono |
General comments:
1. The average grade for the assignemnt is 30 points (out of 36). If you scored significantly less than 25 points, I would advise you to see your TA or the instructor to get a better insight on IR spectroscopy.
2. One of the most common mistakes was not to analyze the v(C-H) region correctly to determine what type of compound (alkane, alkene, aromatic, or mixed) is present.
3. The v(OH) peaks for acids and alcohols look very different and can be easily identified by looking at them.
4.Carbonyl peaks can be shifted to lower wavenumbers when the carbonyl function is conjugated to a pi-system e.g. double bond or aromatic ring.
5. Even though C-C modes are usually fairly weak in intensity, they can appear big if an additional dipole moment is induced by elements with high electronegativity.