Massachusetts Institute of Technology 4 Organic Chemistry 5.13 September 13, 2006 Prof. Timothy F. Jamison Notes for Lecture #4 Organic Structure Determination: Infrared Spectroscopy (IR) and MS Fragmentation Patterns O O Molecules of the Day Isoamyl acetate – banana Benzaldehyde – almonds (R)–Limonene – lemon, lime 1,4-Diaminobutane – a.k.a. putrescine Imagine that four unlabelled vials, each containing one of our four "Molecules of the Day", had somehow become mixed up. How could IR spectroscopy (instead of your sense of smell) be employed to identify the contents of each vial? O H NH2 H2N Figure by MIT OCW. Three-Stage Strategy for Organic Structure Determination • Determine the molecular formula using elemental analysis and mass spectrometry. • Identify the functional groups using infrared spectroscopy (IR) and nuclear magnetic resonance spectroscopy (NMR). • Elucidate the connectivity using 1H NMR (“proton NMR”) and 13C NMR (“carbon NMR”) spectroscopy. 4-2 Absorption Spectroscopy Type of Radiation Energy (kcal/mol) Frequency Molecular Interaction Wavelength Translation Microwave 0.001 - 1 1 - 400 cm-1 Molecular Rotations Rotation of the molecular as a whole about its center of mass 400 - 4,000 cm-1 Molecular Vibrations Stretching and binding of bonds Infrared 1.1 - 11 Visible 50 - 75 400 - 600 nm Ultraviolet 75 - 150 200 - 400 nm Electronic Excitation Promotion of electrons to higher energy levels Infrared Absorption Spectroscopy 4000 cm-1 2000 cm-1 X-H Region 2500 - 4000 cm-1 C-H N-H O-H Stretching SP X=Y Region Region 2000 2500 cm-1 1500 2000 cm-1 C=C C=N Stretching 400 cm-1 1500 cm-1 C=C C=O C=N Stretching Fingerprint Region 400 - 1500 cm-1 Single Rond Stretching Bond Bending Polyatomic Vibrations Figure by MIT OCW. 4-3 Infrared Spectra: Tables of Reference X-H Region Phenols and Alcohols ROH 3700-3500 sharp or 3200-3600 broad(H-bonded) Acids RCO2H 2800-3600 very broad Amides and Amines RCONHR R2NH 3300-3500 C-H bonds C C H C C H C C H RCHO 3100-3300 3000-3200 2850-3000 2700-2800 sp Region Acetylenes Nitriles Ketenes Allenes C C C C C N C O C C 2100 2200 2150 1950 Double Bond Region Alkenes Imines Nitro C C C N NO2 1600-1670 weak unless conjugated 1600-1700 1350-1550(two bands) Carbonyl Groups Note: subtract ca. 30 cm-1 for conjugation (e.g. with a double bond or aromatic ring) Anhydrides RC(O)OCOR Acid Chlorides RCOCI Esters Acids 1710 RCO2H 1700-1725 1740 O 1715 1780 1740 O NR O O O O 1770 O 1700 1720-1740 1680 O O 1630-1700 O O 1790-1815 1725-1755 Urethanes R2NCO2R Aldehydes RCHO O 6-membered and larger cyclic ketones 1740-1780, 1800-1840 (two bands) RCO2R Amides RCONR2 Ketones (subtract ca. 30 cm-1 for R2C O 1710 conjugation) 1690-1740 O 1730 NR 1650 Figure by MIT OCW. 4-4 Infrared Spectra of 2-hexanone and 3-hexanone Images of Infrared Spectra of 2-hexanone and 3-hexanone removed due to copyright restrictions. Please see: http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng Mass Spectrometry data for 2-hexanone and 3-hexanone Mass Spectrometry data for 2-hexanone and 3-hexanone 100 2-hexanone Relative Intensity 80 60 58 40 20 0 10 20 30 40 50 60 70 80 90 100 m/z 100 57 3-hexanone Relative Intensity 80 60 71 40 20 0 10 20 30 40 50 m/z 60 70 80 90 100 Figures by MIT OCW. 4-5