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CPT 1.INFRARED SPECTROSCOPY
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1. CHEM 2041 FUNDAMENTAL ORGANIC CHEMISTRY II Second Semester Course
By
Dr. Barry Miburo
2. CPT 1. INFRARED SPECTROSCOPY & MASS SPECTROMETRY . Objectives:
1. Describe and explain the basic principles and the operation of mass spectrometry (MS) and infrared spectroscopy (IR).
2. Use MS and IR spectra to identify the structure or structural characteristics of organic compounds.
3. 1.1. Infrared Spectroscopy a. Introduction
Demos:
*Wave: http://www.colorado.edu/physics/2000/waves_particles/waves.html
* Electromagnetic waves & Frequencies: http://www.astronomynotes.com/light/s3.htm
Electromagnetic (EM) radiation: synonym of EM wave
Photons: components of an EM radiation
4. Electromagnetic Radiations (Continued) Features of electromagnetic radiations:
* Wavelength (l): distance between 2 consecutive crests or troughs of one wave.
* Period(p): the distance in time between 2 consecutive crests or troughs of one wave.
* Frequency(n): the number of crests that pass by one point per second.
Note: p = 1/n
* Speed = speed of light (c)
l = c x p = c / n
* Energy: Energy carried by a radiation.
5. Electromagnetic Radiations (Continued 2) Electromagnetic energy: carried by electromagnetic particles (photons)
Relation between the characteristics of a radiation.
* l = cp = c/n
* n = c/ l
* E = hn = hc/ l
Notes: * c = speed of light
* h = Planck’s constant. Refer to CHEM 1211 textbook for additional information
6. Electromagnetic Radiations (Continued 3) Electromagnetic spectrum: http://images.google.com/imgres?imgurl=http://www.nasa.gov/centers/langley/images/content/114284main_EM_Spectrum500.jpg&imgrefurl=http://www.nasa.gov/centers/langley/science/FIRST.html&h=317&w=500&sz=67&tbnid=KDl_2eEm--FMJM:&tbnh=82&tbnw=130&prev=/images%3Fq%3Delectromagnetic%2Bspectrum%26um%3D1&start=2&sa=X&oi=images&ct=image&cd=2
Definition: range of all electromagnetic radiations.
7. b. Infrared absorption spectra Molecules absorb infrared radiation energy.
Result: changes in the vibrations of their bonds.
Illustrations of bond vibrations:
http://www.cmbi.kun.nl/wetche/organic/vibr/
E:\Chapter_12\Present\Animations\IRStretchingandBending.htm
IR Spectroscopic process:
1. Molecules are irradiated by IR photons from l = 2.5E-6 m to l = 2.5E-5 m.
2. Molecules absorb the IR energy and undergo bond vibrations.
3. Absorbed energy is detected by IR spectrometer.
Instrument schematic image: irinstrmt12_04
8. IR absorption spectrum Definition: a graph that shows IR radiations absorbed by a molecule.
Example:
propane: http://webbook.nist.gov/cgi/cbook.cgi?Name=propane&Units=SI&cIR=on
2-propanol: http://webbook.nist.gov/cgi/cbook.cgi?Name=2-propanol&Units=SI&cIR=on
acetone: http://webbook.nist.gov/cgi/cbook.cgi?Name=2-propanone&Units=SI&cIR=on
9. IR absorption spectrum (Continued) IR Spectra features:
X axis:
Top of chart: wavelength (l). Units: mm
Bottom of chart : Wavenumber (n) = inverse of wavelength. Significance: number of waves / length unit. Units: reciprocal cm (rcm): cm- 1
Y axis:
Transmittance: proportion of radiation that passes through. Range: 100% at top of chart, 0% at bottom.
Absorbance: proportion of radiation that does not pass through. Range: 0% at top of chart, 100% at bottom.
Example: http://webbook.nist.gov/cgi/cbook.cgi?Name=2-propanone&Units=SI&cIR=on
10. c. Interpretation of infrared spectra. *1. Characteristic regions:
From 4000 to 2500 rcm: N-H, C-H, O-H single bond stretching
From 2500 to 2000 rcm: CC & CN triple bond stretching
From 2000 to 1500 rcm: C=C, C=N, and C=O vibrations
Below 1500rcm: fingerprint region. different for each molecule.
11. Noticeable peaks (from table 12-2, pg 531) Wave- Absorbing Features
number Substance
3300 Alcohol O-H Strong, broad
Amine, amide
N-H broad, with 1 or 2 spikes
Alkynes ?C-H sharp, may
be strong
12. Noticeable peaks (Continued) Wave- Absorbing Features
number Substance
3000 rcm Alkanes C-H Just below 3000
Alkenes =C-H Just above 3000
Carboxylic
acid O-H very broad
2300 alkyne -C ?C- just below 2300
nitriles -C ?N- just above 2300
13. Noticeable peaks (Continued 2) Wave- Absorbing Features
number group
1710 rcm carbonyl C=O very strong
Aldehydes
ketones
Esters around 1735
conjugated C=O around 1650
Examples: butanone
http://webbook.nist.gov/cgi/cbook.cgi?ID=C78933&Units=SI&Type=IR-SPEC&Index=1#IR-SPEC
Butanal: http://webbook.nist.gov/cgi/cbook.cgi?ID=C123728&Units=SI&Mask=80#IR-Spec
14. Noticeable peaks (Continued 2) Wave- Absorbing Features
number group
1660 Alkenes C=C
conjugated C=C below 1660
Amides C=O Stronger than C=C
Examples: 2-methylbutene: http://webbook.nist.gov/cgi/cbook.cgi?Name=2-methylbutene&Units=SI&cIR=on
propanamide
http://webbook.nist.gov/cgi/cbook.cgi?Name=propanamide&Units=SI&cIR=on
15. d. Typical IR Spectra 1. Hydrocarbons
Example 1: butane, represents alkanes http://webbook.nist.gov/cgi/cbook.cgi?ID=C106978&Units=SI&Type=IR-SPEC&Index=1#IR-SPEC
* Strong peak around 2900 rcm: alkane
C-H stretch
* Peaks below 1450 rcm: fingerprint region
16. Hydrocarbons (Continued: Alkenes) Example 2: 2-methylbutene http://webbook.nist.gov/cgi/cbook.cgi?Name=2-methylbutene&Units=SI&cIR=on#IR-Spec
* Sharp peak at 3100 rcm: =C-H stretch
* Strong peak at 2980 rcm: -C-H stretch
* Sharp peak at 1620 rcm: C=C stretch
* Peaks below 1420 rcm: fingerprint region
17. Hydrocarbons (Continued: aromatic compounds) Example:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C108883&Units=SI&Type=IR-SPEC&Index=2#IR-SPEC
* Above 3000 rcm: =C-H stretches
* Around 1600 rcm: C=C stretches
18. Hydrocarbons (Continued: Alkynes) Example: 1-octyne
http://webbook.nist.gov/cgi/cbook.cgi?ID=C629050&Units=SI&Type=IR-SPEC&Index=1#IR-SPEC
* Around 3350 rcm: ?C-H stretch
* Around 2100 rcm: C?C stretch
* 1560 rcm & below: fingerprint region
19. *2. Alcohols Example:
* 1-butanol: http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_12/Text_Images/FG12_09.JPG
Around 3310 rcm: OH strecth
Around 2900 rcm: C-H stretch
20. Carboxylic Acids Example: Propanoic acid
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
* Around 3000 rcm: OH stretch mixed with C-H stretch
* Around 1710 rcm: C=O stretch
21. Amines Example: Butanamine: primary amine, RNH2 http://webbook.nist.gov/cgi/cbook.cgi?ID=C109739&Units=SI&Type=IR-SPEC&Index=2#IR-SPEC
Around 3300 rcm: N-H stretch. Two spikes for the 2 H’s on N
Around 2900 rcm: C-H stretch
22. Amines (2) Example: secondary amine, R2NH 2: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
Around 3200 rcm: N-H stretch. One spike -> one H on N.
Around 2900 rcm: C-H stretch
23. *3. Carbonyl Compounds General characteristic: C=O group
Example: hexanal (aldehyde) http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
* Around 2700 rcm: C-H stretch characteristic of aldehydes
* Around 1730 rcm: C=O stretch
Note: conjugated C=O groups absorb at lower frequencies
24. Carbonyl Compounds (Esters & Conjugated Ketones ) Example: ethyl butanoate (ester) http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
* At 1739 rcm: C=O group stretching
Example 2: 1-penten-3-one (conjugated ketone)
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
* At 1685 rcm: C=O stretch
25. 1.2. Mass Spectrometry (MS) Purpose: Determination of the structure of a compound by recombination of its fragments.
Instrument used: mass spectrometer.
Procedure:
*Decomposition of a molecule into ionized fragments
* Separation and identification of the resulting fragments.
26. a. MS: Fundamental Principles Most common type of mass spectrometers: electron ionization spectrometers.
Image of MS instrument: http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_12/Text_Images/FG12_15.JPG
Operation process:
* A molecule is bombarded by an electron beam. The molecule loses a bond electron to form an cation radical.
* The cation-radical breaks down further into charged and neutral fragments.
* The charged fragments are attracted into and deflected by the magnetic field in the MS. Angle of deflection: according to their masses and charges.
* The position and abundance of the fragments in the detector part of the MS provides information about the mass & structure of the fragments.
27. Mass Spectrum Definition: a bar graph indicating the fragments generated and their abundance as peaks of different heights.
* X Axis: m/z (m/e) = fragment mass related info
* Y axis: Abundance = info about stability of the fragment.
Parent peak or Molecular ion: due to cation from molecule - 1 electron.
Base Peak: the tallest peak (given 100% intensity), due to the most stable fragment.
Isotopic peaks:
* due to presence of isotopes of C, H, O, N, ... in the sample molecule.
* appear around the main peaks.
28. Mass Spectrum (Illustration) Example: 2-methylpentane
http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_12/Text_Images/FG12_16.JPG
Molecular ion: m/z = 100
Base peak: m/z = 41
Other remarkable peaks:
* m/z = 85 : M(+) – 15
* m/z = 57: m/z 85 - 28
29. b. MS fragmentation patterns of some functional groups General rule: most favored fragmentation routes are the ones that:
* produce most stable cations
* lose the most stable radicals.
b1. Alkanes
Most visible losses:
* ethyl radical, more stable than methyl radical
* Ethene molecule
Example: Hexane: http://webbook.nist.gov/cgi/cbook.cgi?Name=hexane&Units=SI&cMS=on
30. Hexane MS
31. Hexane MS (Continued)
32. Alkenes Most stable fragments: allylic cations
Reason for stability: delocalization of the charge by resonance
Example: 2-hexene http://webbook.nist.gov/cgi/cbook.cgi?Name=2-hexene&Units=SI&cMS=on
33. 2-Hexene MS
34. 2-Hexene MS (Continued)
35. b3. Alcohols Two major fragmentation patterns
a-cleavage: loss of a C-C bond nexte to the OH group. Result: a neutral radical and a O containing cation
Dehydration: elimination of h2o. Result: a alkene radical cation + H2O.
Notes:
* Presence of a even numbered peak = Hint of loss of
neutral molecule.
* Loss of H2O is so frequent that M(+) peak of alcohols is low or absent.
Example: 2-methylbutanol
http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_12/Text_Images/FG12_21.JPG
36. Alcohols Fragmentation (Illustration)
37. Alcohol MS (2-methylbutanol)
38. b4. Amines Fragmentation General feature: odd MW
Most common fragmentation pattern: alpha cleavage. Result: N-containing fragment with an even m/z
Example: N-methyl-2-pronanamine
http://webbook.nist.gov/cgi/cbook.cgi?ID=C4747211&Units=SI&Mask=200#Mass-Spec
39. MS of N-methyl-2-pronanamine
40. b5. Carbonyl compounds Major fragmentation patterns
*1. McLafferty rearrangement
Structural condition: minimum 3-C chain
Next to the C=O group.
MS Event: Transfer of the H 3 C's away from the O.
Result: an alkene radical and O-containing fragment with an even m/z.
41. McLafferty Rearrangement
42. *2. Alkyl-carbonyl cleavage General structure: R-CO-R’
Bond breaks between the C=O group and the R group. Result: Acylium ion, R'-(C=O)(+)
Example: 2-hexanone:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C591786&Units=SI&Mask=200#Mass-Spec
43. Alkyl-carbonyl cleavage (in general)
44. MS of 2-Hexanone (Ketone)
45. b6. Carboxylic acids *1. Acyl-alkyl cleavage
Bond breaks between C=O group and alkyl group. Result Acylium-type of ion
*2. Alkyl loss to produce an allylic system in resonance with the two O atoms.
*2. McLafferty rearrangement when possible
Example: Hexanoic acid
http://webbook.nist.gov/cgi/cbook.cgi?ID=C142621&Units=SI&Mask=200#Mass-Spec
46. Carboxylic Acids Example: hexanoic acid