Q.2. (b)How would you identify various functional groups by IR spectroscopy provide supporting examples.
Ans.2. (b)Identify various functional groups by IR spectroscopy:
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When identify various functional groups by IR spectroscopy analysing the spectrum of an unknown, concentrate your first efforts on determining the presence of (or absence) of a few major functional groups. The C = O, O – H, N – H, C – O, C = C, C= C, C=_N, and NO2 peaks are the most conspicuous and give immediate structural information if they are present. Do not try to make a detailed analysis of the C – H, absorption near 3000cm-1, almost all compounds have these absorptions. Do not worry about subtleties of the exact environment in which the functional group is found. Following is a major checklist of the important gross features.
1. Is a carbonyl group present? The C = O group gives rise to a strong absorption in the region 1820 – 1660 cm-1. The peak is often the strongest in the spectrum and of medium width. You can’t miss it.
2. If C = O is present, check the following types
(If it is absent, go to step 3):
|
ACIDS |
Is O – H also present? Broad absorption near 3400-2400cm-1 (Usually overlaps C - H). |
|
AMIDES |
Is N – H also present? Medium absorption near 3400cm-1, sometimes a double peak with equivalent halves. |
|
ESTERS |
Is C – O also present? Strong-intensity absorption near 1300-1000cm-1. |
|
ANHYDRIDES |
Two C = O absorptions near 1810 and 1760cm-1. |
|
ALDEHYDES |
Is aldehyde C – H present? Two weak absorption near 2850 and 2750cm-1on right side of the aliphatic C – H absorptions. |
|
KETONES |
The preceding five choices have been eliminated. |
3. If C = O is absent:
|
ALCOHOLS,PHENOLS |
Check for O – H. Broad absorption near 3400-3300cm-1. Confirm this by finding C – O near 1300-1000cm-1. |
|
AMINES |
Check for N – H. Medium absorptions near 3400cm-1. |
|
ETHERS |
Check for C- O near 1300-1000cm-1(and absence of O – H near 3400cm-1). |
4. Double bonds and/or aromatic rings.
C = C is a weak absorption near 1650 cm.
Medium to strong absorption in the region 1600-1450cm-1, these often imply an aromatic ring.
Confirm the double bond or aromatic ring by consulting the C – H region; aromatic and vinyl C – H occur to the left of 3000cm-1 (Aliphatic C – H occurs to the right ofthis value).
5. Triple bonds.
C ≡ N is a medium, sharp absorption near 2250cm-1.
C ≡ C is a weak, sharp absorption near 2150cm-1.
Check also for acetylenic C – H near 3300-1.
6. Nitro groups.
Two strong absorptions at 1600-1530cm-1,and 1390-1300cm-1
7. Hydrocarbons.
None of the preceding is found.
Major absorptions are in C – H region near 3000cm-1.
Very simple spectrum, the only other absorption appears near 1460 and 1375 cm-1.
