12-1 Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson William H. Brown Christopher S. Foote Brent L. Iverson

12-2 Infrared Spectroscopy Chapter 12

12-3 Electromagnetic Radiation  Electromagnetic radiation:  Electromagnetic radiation: light and other forms of radiant energy  Wavelength ( ):  Wavelength ( ): the distance between consecutive peaks on a wave  Frequency ( ):  Frequency ( ): the number of full cycles of a wave that pass a given point in a second  Hertz (Hz):  Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read “per second”)

12-4 Electromagnetic Radiation  Common units used to express wavelength

12-5 Molecular Spectroscopy  Molecular spectroscopy:  Molecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure we study three types of molecular spectroscopy

12-6 Infrared Spectroscopy  The vibrational IR extends from 2.5 x m (2.5  m) to 2.5 x m (25  m) the frequency of IR radiation is commonly expressed in wavenumbers wavenumber :wavenumber : the number of waves per centimeter, with units cm -1 (read reciprocal centimeters) expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 400 cm -1 = = 400 cm = 4000 cm  mcm 2.5 x m10 -2 mcm 2.5 x m

12-7 Infrared Spectroscopy  IR spectrum of 3-methyl-2-butanone

12-8 Molecular Vibrations atoms joined by covalent bonds undergo continual vibrations relative to each other the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm -1

12-9 Molecular Vibrations  For a molecule to absorb IR radiation the bond undergoing vibration must be polar and its vibration must cause a periodic change in the bond dipole moment  Covalent bonds which do not meet these criteria are said to be IR inactive the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds H 3 C CC CH 3 H 3 CCH 3 H 3 C-CC-CH 3 2,3-Dimethyl-2-butene2-Butyne

12-10 Molecular Vibrations n 3n - 6  For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibrations  For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex  The simplest vibrational motions are bending and stretching

12-11 Molecular vibrations  Fundamental stretching and bending vibrations for a methylene group

12-12 Molecular Vibrations  Consider two covalently bonded atoms as two vibrating masses connected by a spring the total energy is proportional to the frequency of vibration the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring K = a force constant, which is a measure of the bonds’ strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 10 5 dynes/cm  = reduced mass of the two atoms, (m 1 m 2 )/(m 1 + m 2 ), where m is the mass of the atoms in grams

12-13 Molecular Vibrations position  From this equation, we see that the position of a stretching vibration is proportional to the strength of the vibrating bond is inversely proportional the masses of the atoms connected by the bond intensity  The intensity of absorption depends primarily on the polarity of the vibrating bond

12-14 Correlation Tables  Table 12.4  Table 12.4 Characteristic IR absorptions for the types of bonds and functional groups we deal with most often

12-15 Hydrocarbons-Table 12.5

12-16 Alkanes  IR spectrum of decane (Fig 12.4)

12-17 Alkenes  IR spectrum of cyclohexene (Fig 12.5)

12-18 Alkynes  IR spectrum of 1-octyne (Fig 12.6)

12-19 Aromatics  IR spectrum of toluene (Fig 12.7)

12-20 Alcohols IR spectrum of 1-hexanol (Fig 12.8)

12-21 Ethers  IR spectrum of dibutyl ether (Fig 12.9)

12-22 Ethers  IR spectrum of anisole (Fig 12.10)

12-23 Amines  IR spectrum of 1-butanamine (Fig 12.11)

12-24 IR of Molecules with C=O Groups

12-25

12-26 Aldehydes and Ketones  IR spectrum of menthone (Fig 12.12)

12-27 Carbonyl groups  The position of C=O stretching vibration is sensitive to its molecular environment as ring size decreases and angle strain increases, absorption shifts to a higher frequency conjugation shifts the C=O absorption to lower frequency 1850 cm 1780 cm 1745 cm 1715 cm O O O O 1690 cm 1700 cm 1717 cm OO H O

12-28 Carboxylic acids  IR spectrum of pentanoic acid (Fig 12.13)

12-29 Esters  IR of ethyl butanoate (Fig 12.14)

12-30 InfraredSpectroscopy End Chapter 12