If you have followed this tutorial group-by-group, you will realize that in even rather simple, mono-functional molecules there are so many IR bands that it is not feasible to assign every band in an IR spectrum. Instead, look for tell-tale bands — the region from 4000-1300 cm-1 is particularly useful for determining the presence of specific functional groups. You can rely on the IR correlation charts (linked in the left frame), but we recommend (in fact, in organic chem 1 lab, we require) that you memorize the distinctive bands of the common functional bands:
3500-3300 cm-1 N–H stretch 1&Mac251;, 2&Mac251; amines 3500-3200 cm-1 O–H stretch alcohols, a broad, strong band 3100-3000 cm-1 C–H stretch alkenes 3000-2850 cm-1 C–H stretch alkanes 1760-1665 cm-1 C=O stretch ketones, aldehydes, esters 1680-1640 cm-1 C=C stretch alkenes
Begin by looking in the region from 4000-1300. Look at the C–H stretching bands around 3000:
Indicates: Are any or all to the right of 3000? alkyl groups (present in most organic molecules) Are any or all to the left of 3000? a C=C bond or aromatic group in the molecule
Look for a carbonyl in the region 1760-1690. If there is such a band:
Indicates: Is an O–H band also present? a carboxylic acid group Is a C–O band also present? an ester Is an aldehydic C–H band also present? an aldehyde Is an N–H band also present? an amide Are none of the above present? a ketone (also check the exact position of the carbonyl band for clues as to the type of carbonyl compound it is)
Look for a broad O–H band in the region 3500-3200 cm-1. If there is such a band:
Indicates: Is an O–H band present? an alcohol or phenol
Look for a single or double sharp N–H band in the region 3400-3250 cm-1. If there is such a band:
Indicates: Are there two bands? a primary amine Is there only one band? a secondary amine
Other structural features to check for:
Indicates: Are there C–O stretches? an ether (or an ester if there is a carbonyl band too) Is there a C=C stretching band? an alkene Are there aromatic stretching bands? an aromatic Is there a C≡C band? an alkyne Are there -NO2 bands? a nitro compound
If there is an absence of major functional group bands in the region 4000-1300 cm-1 (other than C–H stretches), the compound is probably a strict hydrocarbon.
Also check the region from 900-650 cm-1. Aromatics, alkyl halides, carboxylic acids, amines, and amides show moderate or strong absorption bands (bending vibrations) in this region.
As a beginning student, you should not try to assign or interpret every peak in the spectrum. Concentrate on learning the major bands and recognizing their presence and absence in any given spectrum.
In the organic chem teaching labs, you usually know what compound you started with and what compound you are trying to make. For instance, if you are oxidizing an alcohol to a ketone, your product should show a carbonyl band but no hydroxyl band. If no carbonyl band is present, the experiment was not successful. If both carbonyl and hydroxyl bands are present, the product is not pure.
example for Study case :
IR Spectroscopy Tutorial: Amines
The N–H stretches of amines are in the region 3300-3000 cm-1. These bands are weaker and sharper than those of the alcohol O–H stretches which appear in the same region. In primary amines (RNH2), there are two bands in this region, the asymmetrical N–H stretch and the symmetrical N–H stretch.
Secondary amines (R2NH) show only a single weak band in the 3300-3000 cm-1 region, since they have only one N–H bond. Tertiary amines (R3N) do not show any band in this region since they do not have an N–H bond.
(A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation. Note the spectrum of aniline, below.)
The N–H bending vibration of primary amines is observed in the region 1650-1580 cm-1. Usually, secondary amines do not show a band in this region and tertiary amines never show a band in this region. (This band can be very sharp and close enough to the carbonyl region to cause students to interpret it as a carbonyl band.)
Another band attributed to amines is observed in the region 910-665 cm-1. This strong, broad band is due to N–H wag and observed only for primary and secondary amines.
The C–N stretching vibration of aliphatic amines is observed as medium or weak bands in the region 1250-1020 cm-1. In aromatic amines, the band is usually strong and in the region 1335-1250 cm-1.
Summary:
- 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
The spectrum of aniline is shown below. This primary amine shows two N–H stretches (3442, 3360); note the shoulder band, which is an overtone of the N–H bending vibration. The C–N stretch appears at 1281 rather than at lower wavenumbers because aniline is an aromatic compound. Also note the N–H bend at 1619.
The spectrum of diethylamine is below. Note that this secondary amine shows only one N–H stretch (3288). The C–N stretch is at 1143, in the range for non-aromatic amines (1250-1020). Diethylamine also shows an N–H wag (733).
Triethylamine is a tertiary amine and does not have an N–H stretch, nor an N–H wag. The C–N stretch is at 1214 cm-1 (non-aromatic).
Thanks for having this up. It was very helpful