Organic Chemistry William H Brown Christopher S Foote
Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson 12 -1
Infrared Spectroscopy Chapter 12 12 -2
Electromagnetic Radiation u Electromagnetic radiation: light and other forms of radiant energy u Wavelength ( ): the distance between consecutive peaks on a wave u Frequency ( ): the number of full cycles of a wave that pass a given point in a second u Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”) 12 -3
Electromagnetic Radiation u Common units used to express wavelength 12 -4
Molecular Spectroscopy u 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 -5
Infrared Spectroscopy u The vibrational IR extends from 2. 5 x 10 -6 m (2. 5 m) to 2. 5 x 10 -5 m (25 m) • the frequency of IR radiation is commonly expressed in wavenumbers • 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 -2 -1 10 m • cm = = 4000 cm-1 2. 5 x 10 -6 m = 10 -2 m • cm -1 2. 5 x 10 -5 m = 400 cm-1 12 -6
Infrared Spectroscopy u IR spectrum of 3 -methyl-2 -butanone 12 -7
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 -8
Molecular Vibrations u 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 u 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 CH 3 C- CH 3 2, 3 -Dimethyl-2 -butene 2 -Butyne 12 -9
Molecular Vibrations u For a nonlinear molecule containing n atoms, there are 3 n - 6 allowed fundamental vibrations u For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex u The simplest vibrational motions are bending and stretching 12 -10
Molecular vibrations u Fundamental stretching and bending vibrations for a methylene group 12 -11
Molecular Vibrations u 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 105 dynes/cm = reduced mass of the two atoms, (m 1 m 2)/(m 1 + m 2), 12 -12 where m is the mass of the atoms in grams
Molecular Vibrations u From this equation, we see that the position of a stretching vibration u • is proportional to the strength of the vibrating bond • is inversely proportional the masses of the atoms connected by the bond The intensity of absorption depends primarily on the polarity of the vibrating bond 12 -13
Correlation Tables u Table 12. 4 Characteristic IR absorptions for the types of bonds and functional groups we deal with most often 12 -14
Hydrocarbons-Table 12. 5 12 -15
Alkanes u IR spectrum of decane (Fig 12. 4) 12 -16
Alkenes u IR spectrum of cyclohexene (Fig 12. 5) 12 -17
Alkynes u IR spectrum of 1 -octyne (Fig 12. 6) 12 -18
Aromatics u IR spectrum of toluene (Fig 12. 7) 12 -19
Alcohols • IR spectrum of 1 -hexanol (Fig 12. 8) 12 -20
Ethers u IR spectrum of dibutyl ether (Fig 12. 9) 12 -21
Ethers u IR spectrum of anisole (Fig 12. 10) 12 -22
Amines u IR spectrum of 1 -butanamine (Fig 12. 11) 12 -23
IR of Molecules with C=O Groups 12 -24
IR of Molecules with C=O Groups 12 -25
Aldehydes and Ketones u IR spectrum of menthone (Fig 12. 12) 12 -26
Carbonyl groups u 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 O O 1715 cm-1 1745 cm-1 1780 cm-1 1850 cm-1 • conjugation shifts the C=O absorption to lower frequency O O -1 1717 cm O -1 1690 cm H -1 1700 cm 12 -27
Carboxylic acids u IR spectrum of pentanoic acid (Fig 12. 13) 12 -28
Esters u IR of ethyl butanoate (Fig 12. 14) 12 -29
Infrared Spectroscopy End Chapter 12 12 -30
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