updated Wed, May 8, 2002

Answer Key for IR assignment Spring 2002

Spectrum 1: 4-Methylanisole

Frequency Assignment
3001-3012 n(CH, sp2, aromatic)
2834-2952 n(CH, sp3, alkane)
1613, 1512 n(C=C, aromatic)
1248, 1038 n (COC, stretch)
817 oop, para-subst.


Spectrum 2: Catechol
Frequency Assignment
3452, 3326, br n(OH), alcohol
3053 n(CH, sp2, aromatic)
1620, 1514 n(C=C, aromatic)
1281, 1256 n (COC, stretch)


Spectrum 3: trans-Cinnamic Acid
Frequency Assignment
2500-3400, br n(OH), acid
1682 n(C=O)
1629 n(C=C, alkene)
1577, 1495 n(C=C, aromatic)
1285 n(COC, stretch)
979 oop, trans alkene
710, 683 oop, mono subst.



Spectrum 4: Menthol
Frequency Assignment
3000-3400, br n(OH), alcohol
2872-2960 n(CH, sp3, alkane)
1442 d(CH2, bend)
1026 1046 n (CO, stretch)

Remark: The unknown cannot be propanol, because the compound is a solid!!!



Spectrum 5: m-Nitrobenzonitril
Frequency Assignment
3080 n(=CH, sp2, aromatic)
2237 n(CN, nitrile)
1629 n(C=C, stretch, aromatic)
1535 1356 n (NO2, asym + sym.)


Spectrum 6: 1,8-Naphthalic anhydride
Frequency Assignment
3067 n(=CH, sp2, aromatic)
1771, 1741 n(C=O, anhydride!!!)
1580, 1512 n(C=C, stretch, aromatic)
1122 n (COC, stretch)


Spectrum 7: Cycloheptene
Frequency Assignment
3018 n(=CH, sp2, alkene)
2851-2921 n(CH, sp3, alkane)
1654 n(C=C, stretch, alkene)
1445 d(CH2, bend)
688 oop, cis-alkene


Spectrum 8: Diphenylamine
Frequency Assignment
3310 n(NH, secondary amine)
3026-3084 n(CH, sp2, aromatic)
1752-1949 aromatic overtones
1603, 1494 n(C=C, stretch, aromatic)
677, 735 oop, mono subst.


Spectrum 9: Benzyl acetate
Frequency Assignment
3455 overtone of C=O
3034-3080 n(CH, sp2, aromatic)
2894-2955 n(CH, sp3, alkane)
1742 n(C=O, ester)
1455, 1363 d(CH2,CH3, bend)
1279, 1027 n (COC, stretch)
698, 730 oop, mono-subst.


Spectrum 10: iso-Butyraldehyde
Frequency Assignment
2879, 2973 n(CH, sp3, alkane)
2711, 2828 n(CH, sp2, aldehyde)
1693 n(C=O,aldehyde)


General comments:

1. The average grade for the assignemnt is 34 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 and 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 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