Question

Provide a general overview of the use of infrared spectroscopy for functional group identification in structure determination.

1. Explain the degree of unsaturation calculation for compounds with C,H only; C,H,X; C,H,O; and C,H,N.

Answer 1

Infrared spectroscopy is the absorption measurement of different IR frequencies obtained by positioning the compound (sample) in the path of IR beam. It is the most common technique used to determine the functional groups of chemicals and compounds, which in turn help in the identification in structure determination. Peaks in the region of 4000-1500 cm^-1 are characteristic of specific bonds, and can be used to identify the presence of a that particular functional group. And peaks in 1500-400 cm^-1 region is characteristic of combination arising from simultaneous deformation of multiple bonds. IR absorption of common functional groups is given below:

Functional Group	Characteristic Absorption(s)(cm-1)
Alkyl C-H Stretch	2950 - 2850 (m or s)
Alkenyl C-H Stretch Alkenyl C=C Stretch	3100 - 3010 (m) 1680 - 1620 (v)
Alkynyl C-H Stretch Alkynyl C=C Stretch	~3300 (s) 2260 - 2100 (v)
Aromatic C-H Stretch Aromatic C-H Bending Aromatic C=C Bending	~3030 (v) 860 - 680 (s) 1700 - 1500 (m,m)
Alcohol/Phenol O-H Stretch	3550 - 3200 (broad, s)
Carboxylic Acid O-H Stretch	3000 - 2500 (broad, v)
Amine N-H Stretch	3500 - 3300 (m)
Nitrile C=N Stretch	2260 - 2220 (m)
Aldehyde C=O Stretch Ketone C=O Stretch Ester C=O Stretch Carboxylic Acid C=O Stretch Amide C=O Stretch	1740 - 1690 (s) 1750 - 1680 (s) 1750 - 1735 (s) 1780 - 1710 (s) 1690 - 1630 (s)
Amide N-H Stretch	3700 - 3500 (m)

The presence of peaks in these regions suggests which groups might be present. Further, examination of IR can also suggest the identity of an unknown compound by the process of elimination i.e., if the peak is absent in that region, the likelyhood of that functional group is nil.

Calculation of degree of unsaturation. A simple alkane containing only the single bonds with no rings is considered saturated. Since, an alkane has the formula C_nH_2n+2, the number of H atoms can be represented by 2C+2. Now, a molecule with a ring or a double bond is formed by losing two hydrogens. The double bond or a ring represent one degree of unsaturation and the triple bond represents two degrees of unsaturation. A general formula for calculating the degrees of unsaturation (DBE) from given formula is:

DBE = [(Number of Carbons x 2) + 2 - Number of Hydrogens] / 2 = C + 1- H/2

For C,H,O i.e., the presence of oxygen—it makes no effect in the degree of unsaturation and hence is not considered.

For C,H,X i.e., the presence of halides (F, Cl, Br, I)—these can be counted as a hydrogen.

Therefore, DBE = C + 1- H/2 – X/2

and for C,H,N i.e, the presence of nitrogen. It is counted as one half of a carbon.

DBE = C + 1- H/2 + N/2