Key for Infrared Assignment Summer 2015
Spectrum 1: 1,4,5,8-Tetrahydronaphthalene (R4, C1)
| Frequency | Assignment |
| 3027-3065 |
ν(CH, sp2) |
| 2846-2970 |
ν(CH, sp3) |
| 1630, 1681 |
ν(C=C, alkenes) |
| 1456 |
δ(CH2, bend) |
| 685 |
oop, cis alkene |
Spectrum 2: 4-Fluoronitrobenzene (R1, C3)
| Frequency | Assignment |
| 3066-3121 | ν(CH, sp2) |
| 1525, 1621 |
ν(C=C, aromatic) |
| 1347, 1525 |
ν(NO2) |
| 1246 |
ν(C-F) |
| 854 |
oop, para-subst. arene |
Spectrum 3: 2-Ethylbutylamine (R2, C1)
| Frequency | Assignment |
| 53297, 3376 |
ν(NH2, primary amine) |
| 2875-2962 |
ν(CH, sp3) |
| 1610 |
δ(NH2, bend, scissoring) |
| 1381, 1462 |
δ(CH2,CH3, bend) |
| 829 |
δ(NH2, bend, rocking) |
Spectrum 4: 4-Formylbenzonitrile (R4, C2)
| Frequency | Assignment |
| 3047-3096 |
ν(CH, sp2) |
| 2748, 2850 |
ν(CHO, aldehyde) |
| 2230 |
ν(C≡N, nitrile) |
| 1708 | ν(C=O, conj. aldehyde) |
| 1608 | ν(C=C, aromatic) |
| 832 | oop, para-subst. arene |
Spectrum 5: Cyclopentylacetic acid (R4, C4)
| Frequency | Assignment |
| 2300-3400 | ν(OH, carboxylic acid) |
| 2871-2955 |
ν(CH, sp3) |
| 1708 |
ν(C=O, carboxylic acid) |
| 1452 |
δ(CH2, bend) |
| 1297 |
ν(C-OH) |
Spectrum 6: 4-Toluenesulfonamide (R3, C1)
| Frequency | Assignment |
| 3240, 3327 |
ν(NH2, primary amide) |
| 3034, 3049 |
ν(CH, sp2) |
| 1598 |
δ(NH2, bend, scissoring) |
| 1487, 1574 |
ν(C=C, aromatic) |
| 1389 | δ(CH2, bend) |
| 1153, 1328 | ν(SO2) |
| 809 | oop, para-subst. arene |
Spectrum 7: 1-Chloro-pentyne (R4, C1)
| Frequency | Assignment |
| 2847-2981 |
ν(CH, sp3) |
| 2241 |
ν(C≡C, alkyne) |
| 1378, 1455 |
δ(CH2,CH3, bend) |
| 692 |
ν(C-Cl) |
Spectrum 8: 2-Propylphenol (R5, C2)
| Frequency | Assignment |
| 3200-3600 |
ν(OH, phenol) |
| 3036-3057 |
(ν(CH, sp2) |
| 2872-2960 | ν(CH, sp3) |
| 1503, 1609 |
ν(C=C, aromatic) |
| 1379, 1455 |
δ(CH2, CH3, bend) |
| 1236 | ν(C-OH) |
| 753 | oop, ortho-subst. arene |
Spectrum 9: Phenacyl chloride (R5, C3)
| Frequency | Assignment |
| 3057-3096 |
ν(CH, sp2) |
| 2963-2995 |
ν(CH, sp3) |
| 1696 | ν(C=O, conj. ketone) |
| 1493, 1598 |
ν(C=C, aromatic) |
| 1450 | δ(CH2, bend) |
| 1213 | νas(CCC, ketone) |
|
687, 749 |
oop, mono-subst. arene |
Spectrum 10: 5-Methyl-5-phenyl-2,4-imidazolidinedione (R3, C3)
| Frequency | Assignment |
| 3208, 3282 |
ν(NH, amide) |
| 3059-3093 |
ν(CH, sp2) |
| 1731, 1770 | ν(C=O, amide) |
| 1498 |
ν(C=C, aromatic) |
| 1367, 1444 |
δ(CH3, bend) |
| 696, 734 |
oop, mono-subst. arene |
Note: The designation behind the name is the row and column number the molecule is located in the table.
General comments:
1. The average grade for the assignment is
34.9 points (out of 40). If you scored around or less than 30 points, I would advise you to see your TA or the instructor to seek some help. There is still some time to practice and do better in the final exam.
2. One of the most common mistakes was not to analyze the ν(C-H) region correctly to determine what type of compound (alkane, alkene, aromatic) is present. Just writing "sp3" or "C-H" is not sufficient here.
3. The ν(OH) peaks for carboxylic acids and alcohols look very different and can be easily identified by looking at them (see reader). However, in some cases intramolecular hydrogen bonding broadens and shifts (to lower wavenumbers) the peak of an alcohol.
4. Carbonyl peaks can be shifted to lower wavenumbers when the carbonyl function is conjugated to another π-system i.e., double bond or aromatic ring.
5. The presence of two peaks in the carbonyl range (1630-1850 cm-1) indicates either the presence of an "anhydride type" of function or two different carbonyl functions (i.e., conjugated and saturated ketone ). These peaks can vary greatly in terms of their relative intensities.
6. Peaks due to amine, alkyne or alcohol functions are very different in appearance. Alkyne peaks (CH stretch) are very sharp and fairly intense due to an "isolated" motion. Amine peaks are usually a little broader and less intense. The presence of two peaks suggests a primary amine (NH2) while secondary amines show only one peak in this range. Alcohols (OH-stretch) usually show a rounded peak in this area. There is also a difference between amine and amides in terms of width and intensity of the peaks.
8. If the spectrum shows a lot of peaks (independent from their size), the molecule has to have a lot of atoms and also cannot be very symmetric.
9. Amide and amine are not the same functionality! Many students mislabeled the peaks in these peaks.
10. A significant number of assignments were rather sloppily done: very poor labeling (i.e., "C-H", "aromatic", "sp2/sp3", etc.) structures that are not a choice, benzene rings without the double bonds, five-bonded carbon or nitrogen atoms, etc.
11. In many cases, groups were presented in the labeling that are not part of the final moelcule i.e., CH2 group if the molecule did not have one. Students that did this repeatedly received a deduction due to the inconsistancy of the data and the proposed structure.
12. Note: The abbreviation "n.p." stands for not present in the molecule.