**7. Organic compounds**

Elucidation of the molecular structure is especially important in organic chemistry. An analytical method for the identification of functional groups from organic compounds uses one of the most physical properties of a chemical compound: the infrared absorption spectrum. Compared with other physical properties: melting point, refractive index, or specific gravity which offer only a single point of comparison with other substances, the IR spectrum of a specific compound, gives a multitude of important information (position of bands, band intensity). The intensity is indicative of the number of a particular group contributing to absorption.

Organic Compounds FT-IR Spectroscopy 161

absorption pattern in this region is complex, with bands originating in interacting vibrational modes. Absorption in this intermediate region is probably unique for every molecular species. For example, in the cases of hydrocarbons, organic compounds classified as saturated or unsaturated based on the absence or presence of multiple bonds, the energy of the infrared light absorbed by a C-H bond depends on the hybridization of the hybrid orbital, in the order of sp3>sp2>sp. The sp3-hybridized C-H bonds in saturated hydrocarbons absorb in the 2850-3000 cm-1 region. The sp2-hybridized C-H bonds from alkenes absorbs at **3080** cm-1. A sp-hybridized C-H bond in a molecule, alkyne absorbs infrared at 3320 cm-1. Another classification of hydrocarbons can be made based on absorptions due to the carboncarbon bond. Carbon-carbon bond strength increases in the order of single>double>triple. Saturated hydrocarbons all contain carbon-carbon single bonds that absorb in the 800-1000 cm-1 region. But, unsaturated hydrocarbons also contain carbon-carbon single bonds that absorb in this same region. So, this interval can not be considered as fingerprint region

The alkanes give an IR spectrum with relatively few bands because there are only CH bonds

C-C 800-1000

C=C 1450-1600

C=C 1630-1670

C-C 2100-2140

C-O 1050-1200

C-H 2700 -2900

C=O 1700-1725 O-H 2500-3300 C-O 1100-1300

O-C 1040-1100

C-O 1000-1300 (2 bands)

The next table present the characteristic group frequencies of organic molecules.

**Class Group Wavenumber (cm-1)**

because most organic compounds have carbon-carbon single bonds.

Alkane C-H 2850-3000

Aromatic C-H 3000-3100

Alkene C-H 3080-3140

Alkyne C-H 3300-3320

Alcohol O-H 3300-3600

Ether C-O 1070-1150 Aldehyde C=O 1720-1740

Ester C=O 1735-1750

Ketone C=O 1700-1725 Acyl halides C=O 1785-1815

Amides C=O 1630-1695

Anhydrides C=O 1750;1820 (2 bands)

that can stretch or bend.

**Hydrocarbons** 

**Oxygen Compounds**

Carboxylic Acids

It is well known that molecules absorb a unique set of IR light frequencies, because the frequency of vibration involved depends on the masses of atoms involved, the nature of the bonds and the geometry of the molecule. A molecule absorbs only those frequencies of IR light that match vibrations that cause a change in the dipole moment of the molecule. Each organic molecule, with the exception of enantiomers, has a unique infrared spectrum. This is because symmetric structures and identical groups at each end of one bond will not absorb in the IR range. The spectrum has two regions. The *fingerprint* region is unique for a molecule and the *functional group* region is similar for molecules with the same functional groups.

The entire spectral pattern is unique for a given compound. The bands that appear depend on the types of bonds and the structure of the molecule.

In a complicated molecule many fundamental vibrations are possible, but not all are observed movements which do not change the dipole moment for the molecule or the those which are so much alike that they coalesce into one band.

IR is usually preferred when a combination of qualitative and quantitative analysis is required. It is often used to follow the course of organic reactions allowing the researcher to characterize the products as the reaction proceeds.

For the analysis, the samples can be liquids, solids, or gases. The only molecules transparent to IR radiation under ordinary conditions are monatomic and homonuclear molecules such as Ne, He, O2, N2, and H2. One limitation of IR spectroscopy is that the solvent water is a very strong absorber and attacks NaCl sample cells.

Computerized spectra data bases and digitized spectra are widely used today especially in research, chemistry, medicine, criminology, etc
