Molecular structure is a major theme in chemistry, and is especially important in organic chemistry. Have you ever wondered how chemists determine the molecular structures of new reaction products or new materials isolated from natural sources? The instruments most directly concerned with molecular structure are the spectrometers. We are familiar with various kinds of electromagnetic radiation: light (visible, ultraviolet, infrared), microwaves, x-rays, radio and radar waves. When a beam of electromagnetic radiation is passed through a substance, the radiation can be either absorbed or transmitted, depending on its frequency and the structure of the molecules it encounters. The energy gained by a molecule in this way may bring about increased vibration or rotation of the atoms, or may raise electrons to higher energy levels. The particular frequency of radiation that a given molecule can absorb depends upon the changes in vibrations or rotations or electronic states that are permitted to a molecule of that structure. This curriculum unit will focus on infrared spectroscopy as one way of knowing or determining the structure of organic compounds. The unit is designed to be used in a second year high school chemistry class during the study of organic chemistry. The students would have previously studied atomic structure and bonding.
Molecules are made of atoms joined by chemical bonds. The movement of the atoms and bonds can be modeled using balls connected by springs. The movements can be divided into bending and stretching of bonds. The frequency of vibration involved depends on the masses of atoms involved, the nature of the bonds and the geometry of the molecule. Each organic molecule, with the exception of enantiomers, has a unique infrared spectrum. Like fingerprints, IR spectra can be compared with the spectra of known molecules and be used for identification purposes.
Of more importance to the organic chemist is the fact that certain atomic vibrations give rise to absorptions or bands which appear at approximately the same frequency in a large variety of molecules. These bands are characteristic of the presence of certain functional groups. By examining these key regions the chemist can determine which functional groups are present or absent.
The infrared region of the electromagnetic spectrum falls between the visible and microwave portions and corresponds to wavelengths between 0.7 (m and 100 (m (1014 –1012 Hz or14,000-100 cm-1 ). Infrared radiation in this range is absorbed and converted by an organic molecule into energy of molecular vibration. The frequency of absorption depends on the relative masses of the atoms, the force constants of the bonds, and the geometry of the atoms.
Upon completing this unit the student will be able to:
Describe the factors affecting the frequency of vibration.
Understand terms used in infrared (IR) spectroscopy.
Understand the reasons for compounds exhibiting infrared activity and how light can interact with matter.
Use a correlation table to determine the presence or absence of functional groups.
Determine if a given IR spectrum is consistent with a structural formula.
Determine the identity of certain unknown organic compounds given their IR spectra, molecular mass, and other physical and/or chemical properties.