Infrared Spectroscopy

Transcription

1 Infrared Spectroscopy (Chapter 12) 1 This reaction from Ochem 1 How do we know if it worked? The reactant is cyclohexene; the product is cyclohexanol. How can we tell the difference? Infrared Spectroscopy (IR) is an easy way to do it. 2

2 3 Electromagnetic Radiation Electromagnetic radiation: light and other forms of radiant energy Wavelength (λ):( the distance between consecutive peaks on a wave Frequency (ν):( the number of full cycles of a wave that pass a given point in a second Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read per second ) 4

3 Electromagnetic Radiation Common units used to express wavelength λ Relation Unit to Meter Meter (m) ---- Millimeter (mm) Micrometer (μm) 1 mm = 10-3 m 1 μm = 10-6 m Nanometer (nm) 1 nm = 10-9 m Angstrom (Å) 1 Å = m c = λν E = hν E is kj/mol h= 3.99 X kj s mol -1 ν = frequency in Hz 5 E=hν 6

4 Infrared Spectroscopy The vibrational IR extends from 2.5 x 10-6 m (2.5 μm) to 2.5 x 10-5 m (25 μm) the frequency of IR radiation is commonly expressed in wavenumbers Wavenumber ν: 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 ν = 10-2 m cm x 10-6 m = 4000 cm m cm -1 ν = = 400 cm x 10-5 m 25 μm to 2.5 μm 7 Sections of an IR Spectrum This is the most common scale. 8

5 IR spectrum of 3-methyl-2-butanone Strong absorption 9 IR spectrum of 3-methyl-2-butanone C-H Stretch C=O Stretch 10

6 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 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 polarized bonds H 3 C CH 3 C C H 3 C-C C-CH 3 H 3 C CH 3 2,3-Dimethyl-2-butene 2-Butyne 12

7 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 ν = 4.12 K μ 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 μ = reduced mass of the two atoms, (m 1 m 2 )/(m 1 + m 2 ), where m is the mass of the atoms in grams 13 The simplest vibrational motions are bending and stretching. Here are the fundamental stretching and bending vibrations for a methylene group: 14

8 Symmetric Asymmetric Scissoring Rotation Wagging Twisting 15 Molecular Vibrations ν = 4.12 K μ From this equation, we see that the position (i.e. wavenumber) 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 The intensity (i.e. weak, s, m) of absorption depends primarily on the polarity of the vibrating bond 16

9 Correlation Tables Table 12.4 Characteristic IR absorptions for the types of bonds and functional groups encountered most often: Bond Stretching Frequency (cm -1 ) Intens ity O-H weak to strong N-H medium C- H weak to medium C= C weak to medium C= O strong C- O strong 17 Hydrocarbons-Table 12.5 Hydrocarbon Alkane C- H CH 3 C-C Alkene C- H C= C Alkyne C- H C C Arene C- H C= C C- H Vibration Intensity Stretching Medium Bend ing Mediu m Bend ing 1375 and 1450 Weak to medium (N ot useful for interpretation - too man y b ands Stretching Stretching Frequency (cm -1 ) Weak to medium Weak to medium Stretching 3300 Medium to strong Stretchin g Weak Stretching 3030 Weak to medium Stretching Medium Bend ing Strong 18

10 Alkanes IR spectrum of decane (Fig 12.4) Alkenes IR spectrum of cyclohexene (Fig 12.5)

11 Alkynes IR spectrum of 1-octyne (Fig 12.6) Aromatics IR spectrum of toluene (Fig 12.7)

12 Alcohols Bond O- H (free) O- H (H bonded) C-O Frequency, cm -1 Inten sity Weak Medium, broad Medium 23 Effect of Concentration Upon Hydrogen Bonding 24

13 Effect of Concentration Upon Hydrogen Bonding 25 Ethers IR spectrum of dibutyl ether (Fig 12.9)

14 Ethers IR spectrum of anisole (Fig 12.10) Amines IR spectrum of 1-butanamine (Fig 12.11) 28

15 IR of Molecules with C=O Groups Carbonyl Group O RCR' O RCH Ketones C=O Aldehydes C=O C-H Vibration Frequency (cm -1 ) Intensity Stretching Strong Stretching Strong Stretching 2720 Weak O RCOH Carboxylic acids C=O Stretching Strong OH Stretching Strong (broad) 29 O RCNH 2 IR of Molecules with C=O Groups Amides C= O N H Stretching Stretching , 3400 Strong Medium (1 amides have two N-H stretches) (2 amides have one N-H stretch) O RCOR' O O RCOCR RC N Carboxylic esters C= O sp 2 C O Stretching Stretching Strong Strong sp 3 C O Stretching Strong Acid anhydrides C= O Stretching and Strong C O Stretching Strong Nitriles C N Stretching Medium 30

16 Aldehydes and Ketones IR spectrum of menthone (Fig 12.12) 31 Carboxylic acids IR spectrum of pentanoic acid (Fig 12.13) 32

17 Amide IR of N-methylpropanamide (Fig 12.14) 33 Esters IR of Ethyl butanoate (Fig 12.15) 34

18 Strategies for IR Interpretation Inspect the spectrum from left to right. If there is a strong, but broad band 3500 cm -1 then, OH is present. One or two weak peaks in this area are indicative of amines (N H stretch). Examine the 3000 cm -1 C H aliphatic stretches are to the right and C H from alkenes & aromatics are to the left. Aldehyde C H stretch will be ~ 2720 cm -1 Check the area from 1820 to 1630 cm -1. Strong peaks in this area indicate C=O and this is often the strongest peak in the spectrum. The area from 1250 to 1000 cm -1 are the C O stretches of ethers, esters, acids. 35