Answer Key for IR assignment Spring 2003

Spectrum 1: Dimethylphthalate (liquid)

Frequency Assignment
3017 n(CH, sp2)
2844, 2961 n(CH, sp3)
1602 d(NH2)
1470 d (CH3, bend)
1387, 1366 d (-C(CH3)2, stretch)

Spectrum 2:Azelaic Acid (solid)

Frequency Assignment
2500-3500 n(OH, acid)
2934 n(CH, sp3)
1695 n(C=O)
1469 d (CH3, bend)
1252 n(C-OH)

Spectrum 3: 3-Methylcyclohexanone (liquid)

Frequency Assignment
3406 n(C=O, overtone)
2955, 2928, 2871 n(CH, sp3)
1713 n(C=O, stretch)
1456 d (CH3, bend)

Spectrum 4: Acetanilide (solid)

Frequency Assignment
3295 n(NH)
3021-3195 n(CH, sp2)
1665 n(C=O)
1598, 1500 n(C=C, aromatic)
694, 757 oop bending, monosubst.

Spectrum 5: Squalene (liquid)

Frequency Assignment
2854-2966 n(CH, sp3)
1668 n(C=C, alkene)
1447, 1381 d (CH2, CH3, bend)
835 oop bending, trisubst. alkene

Spectrum 6: 1-chloro-2,4-dinitrobenzene (solid)

Frequency Assignment
3107 n(CH, sp2)
2880 n(CH, sp3)
1545, 1350 n(NO2)
1605 n(C=C, aromatic)

Spectrum 7: Tetradecane (liquid)

Frequency Assignment
2871-2957 n(CH, sp3)
1466, 1378 d (CH2, CH3, bend)
721 rocking mode (n>4)

Spectrum 8: 2-Octanol (liquid)

Frequency Assignment
3351 n(OH, alcohol)
2858-2962 n(CH, sp3)
1465, 1375 d (CH2, CH3, bend)
1114 n (C-OH)
724 rocking mode (n>4)

Spectrum 9: 4-Nitrobenzaldehyde (solid)

Frequency Assignment
3066-3107 n(CH, sp2)
2849, 2732 n(CH, aldehyde)
1708 n (C=O)
1606 n(C=C, aromatic)
1541, 1346 n(NO2)
816 oop, para substituted

Spectrum 10: 4-Methylbenzylamine (liquid)

Frequency Assignment
3292, 3370 n(NH2)
3018-3046 n(CH, sp2)
2860-2921 n(CH, sp3)
1799, 1902 aromatic overtones (para)
1613 d (NH2,bend)
1514 n(C=C, aromatic)
1453, 1380 d (CH2, CH3, bend)
802 oop, para substituted

General comments:

1. The average grade for the assignment is 36 points (out of 40). If you scored significantly less than 30 points, I would advise you to see your TA or the instructor to seek some help.

2. One of the most common mistakes was not to analyze the
n(C-H) region correctly to determine what type of compound (alkane, alkene, aromatic, or mixed) is present.

3. The
n(OH) peaks for acids and alcohols look very different and can be easily identified by looking at them (see reader).

4.Carbonyl peaks can be shifted to lower wavenumbers when the carbonyl function is conjugated to a
p-system e.g. double bond or aromatic ring.

5. Nitro groups show two intense peaks in the IR spectrum: one between 1300-1400 cm-1for the symmetric stretching mode, the other one between 1500-1600 cm-1 for the asymmetric stretching mode.

6. 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.

7. 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.

8. A lot of noise of the left side of the spectrum usually indicates that the spectrum was acquired as a KBr pellet, often times indicating that the compound is a solid.