Introduction The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants
Spectroscopy and the Electromagnetic Spectrum Unlike mass spectrometry, infrared (IR), ultraviolet (UV) and nuclear magnetic resonance (NMR) spectroscopies: are nondestructive involve interaction of molecules with electromagnetic energy rather than with high-energy electron beam
The Electromagnetic Spectrum The electromagnetic spectrum is the range of electromagnetic energy, including IR, UV and visible radiation
The electromagnetic spectrum covers a continuous range of wavelengths and frequencies, radio waves to g rays High n Low n Low l High l
Wavelength, Frequency and Amplitude
Wavelength (l) is the distance from one wave maximum to the next Frequency (n) - is the # of waves that pass by a fixed point per unit time (s -1 or Hz) Amplitude - is the height of a wave, measured from midpoint to peak Wavelength x Frequency = Speed l (m) x n (s -1 ) = c l = c n n = c l Speed of light: C vacuum = 3.00 x 10 8 m/s
The Planck equation gives: e = hn = hc l where e = Energy of 1 photon (1 quantum) h = Planck s constant (6.62 x10-34 J.s) n = Frequency (s -1 ) l = Wavelength (m) c = Speed of light (3.00 x 10 8 m/s) Radiant energy is proportional to its frequency and inversely proportional to its wavelength
The Planck equation can be rewritten: N A hc E = N A e = = l 1.20 x 10-4 kj/mol l where E = Energy of Avogadro s number of photons N A = Avogadro s number e = Energy of 1 photon (1 quantum) h = Planck s constant (6.62 x10-34 J.s) c = Speed of light (3.00 x 10 8 m/s) l = Wavelength (m)
An absorption spectrum shows the wavelength on the x-axis and the intensity of the various energy absorptions expressed in % transmittance on the y-axis. Ethyl alcohol CH 3 CH 2 OH
Infrared Spectroscopy of Organic Molecules The infrared (IR) region is lower in photon energy than visible light Only 2.5 10-6 m to 2.5 10-5 m region is used by organic chemists for structural analysis
Absorption Spectrum IR energy in a spectrum is usually measured as wavenumber ~ Wavenumber (n) is the inverse of wavelength is proportional to frequency is expressed in cm -1 Wavenumber (cm -1 ) = 1 l (cm) Specific IR absorbed by organic molecule is related to its structure
Infrared Energy Modes Molecules are in constant motion (i.e bond stretching, contracting, bending ) Their energy is quantized
Infrared Energy Modes Combinations of atomic movements, such as bending and stretching of bonds between groups of atoms, are called normal modes IR energy absorption corresponds to specific modes
Infrared Energy Modes When a molecule is irradiated with electromagnetic radiation, energy is absorbed if the frequency of the radiation matches the frequency of the vibration. Energy absorption increases amplitude for the vibration
Infrared Energy Modes IR energy - is characteristic of the atoms in the group and their bonding - corresponds to the amount of energy needed to increase the amplitude of specific molecular vibrations
Interpreting Infrared Spectra Most functional groups absorb at about the same energy and intensity independent of the molecule in which they are. Characteristic IR absorptions can be used to confirm the presence of a functional group in a molecule are listed in Table 12.1
Fingerprint Region of Infrared Absorption Spectrum IR spectrum has a lower energy region characteristic of molecule as a whole known as fingerprint region. Its range goes from 1500 cm -1 to 400 cm -1
Hexane 1-hexene 1-hexyne
Regions of Infrared Absorption Spectrum 4000-2500 cm -1 N-H, C-H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm -1 C C and C N (stretching)
Regions of Infrared Absorption Spectrum 2000-1500 cm -1 double bonds C=O, C=C C=N (stretching) C=O 1680-1750 C=C 1640-1680 cm -1 Below 1500 cm -1 fingerprint region
Differences in Infrared Absorptions Molecules vibrate and rotate in normal modes, which are combinations of motions These are related to force constants Bond stretching dominates higher energy (frequency) modes
Differences in Infrared Absorptions Light objects connected to heavy objects vibrate fastest (at higher frequencies): C-H, N-H, O-H > C-O, C-N For two heavy atoms, stronger bond requires more energy (higher frequency): C C, C N > C=C, C=O, C=N > C-C, C-O, C-N, C-X
8. Infrared Spectra of Hydrocarbons C-H, C-C, C=C, C C have characteristic peaks
Example: Hexane
Alkenes
Example: 1-hexene 3100 1660
Alkynes (Terminal alkyne)
Example: 1-hexyne 3300 2100
Practice Problem: The IR spectrum of phenylacetylene is shown below. What absorption bands can you identify?
Infrared Spectra of Some Common Functional Groups Spectroscopic behavior of functional groups is discussed in later chapters Brief summaries are presented here
Alcohols Example: Cyclohexanol
Amines Example: Cyclohexylamine
Aromatic Compounds
Example: phenylacetylene Ring bonds 1450-1600 cm -1
Carbonyl Compounds Strong, sharp C=O peak at 1670 to 1780 cm -1 The exact position of absorption within the range is characteristic of each type of carbonyl compound. It can often be used to identify aldehydes, ketones, and esters.
Aldehydes 1730 cm -1 in saturated aldehydes 1705 cm -1 in aldehydes next to double bond or aromatic ring
Example: Phenylacetaldehyde C=O 1725 cm -1
Ketones 1715 cm -1 in six-membered ring or acyclic ketones 1750 cm -1 in five-membered ring ketones 1690 cm -1 in ketones next to a double bond or an aromatic ring
Example: cyclohexanone 1715 cm -1 Ring bonds 1450-1600 cm -1
Esters 1735 cm -1 in saturated esters 1715 cm -1 in esters next to aromatic ring or a double bond
Problem 1: Cyclohexane or Cyclohexene?
Problem 2: Propose structure(s) for unknown hydrocarbon
Problem 3: Propose structure(s) for unknown hydrocarbon