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CHEM 2041 FUNDAMENTAL ORGANIC CHEMISTRY II

CPT 1.INFRARED SPECTROSCOPY

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CHEM 2041 FUNDAMENTAL ORGANIC CHEMISTRY II

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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 = Plancks 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 numberSubstance 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 numberSubstance 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 numbergroup 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 numbergroup 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 Hs 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

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