1 / 26

CHEM 344

CHEM 344. Spectroscopy of Organic Compounds Lecture 3 11 th and 12 th September 2007. Review of Lecture 2. Spin-spin splitting leads to multiplicity in NMR spectra The size of the splitting between two hydrogen atoms is the coupling constant, J ( measured in Hz ).

valentine-reilly
Download Presentation

CHEM 344

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CHEM 344 Spectroscopy of Organic Compounds Lecture 3 11th and 12th September 2007

  2. Review of Lecture 2 • Spin-spin splitting leads to multiplicity in NMR spectra • The size of the splitting between two hydrogen atoms is the coupling constant, J (measured inHz). • n+1 rule - doublet, triplet, quartet…..Pascal’s triangle • Infrared radiation excites molecular vibrations • IR bands depend on bond type, strength etc. • IR spectroscopy good for functional group assignment

  3. NMR: Exceptions to the n+1 Rule • The n+1 rule does not apply when a set of equivalent protons is split by two or more other non-equivalent sets with different coupling constants. • The n+1 rule does not apply to spectra in which the chemical shift difference between two sets of protons is not much larger than the coupling constant (e.g. butane, hexane). • These are termed “second-order” spectra and usually have to be simulated. The signals typically appear to be very poorly resolved. • You are very unlikely to encounter 2nd order spectra in CHEM 344

  4. NMR: Some Specific Functional Group Characteristics • O-H and N-H protons often show broad peaks with no resolved splitting (acidic exchange broadening), and their chemical shifts vary greatly. • An aldehyde C-H is strongly deshielded (d = 9-10 ppm) and coupling to alkyl protons on adjacent carbon atom is small (i.e. J is very low). • Carboxylic acid O-H is very strongly deshielded. (d = 10-12 ppm)

  5. In this example, the O-H proton (labeled c) gives a sharp signal but does not influence the splitting pattern of the Hb protons.

  6. NMR: Some Specific Functional Group Characteristics • Cis and trans protons on alkenes usually show strong coupling, but geminal protons on alkenes show little or no resolved coupling. • Jcis≈ 8 Hz Jtrans ≈ 15 Hz Jgem ≈ 0 Hz • Ortho splitting on aromatic rings is often resolved, but meta and para splitting is not. • Jo≈ 8 Hz Jm ≈ 4 Hz Jp ≈ 0 - 2 Hz

  7. NMR: Mixtures of Compounds • Unlike most textbook examples, “real world” NMR samples usually contain mixtures of compounds. • It is common to see extra signals due to impurities. • Impurities can be side products, un-reacted starting materials, solvent contaminants etc. • In some cases it is useful (or necessary) to analyze mixtures that contain comparable amounts of two or more compounds.

  8. NMR: Mixtures of compounds Synthesis of ethyl acetate by Fischer Esterification (you will do this in CHEM 344). At equilibrium, all 4 components are present. Remove H2O by drying agent, remove RCO2H by neutralization. Can analyze NMR spectrum of mixture to determine extent of reaction.

  9. Unknown A(Figure 14.27 Solomons 8th ed.) • Formula = C9H12 • IHD = 4 • IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1 • 1H NMR d: 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H)

  10. Unknown B(Figure 14.27 Solomons 8th ed.) • Formula = C8H11N • IHD = 4 • IR shows two medium peaks between 3300 and 3500 cm-1 . No other medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1 • 1H NMR d: 1.4 (d, 3H), 1.7 (s, br, 2H), 4.1(quart., 1H), 7.2-7.4 (m, 5H)

  11. Unknown C(Figure 14.27 Solomons 8th ed.) • Formula = C9 H10 • IHD = 5 • IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1 • 1H NMR d: 2.05 (pent., 2H), 2.90 (trip., 4H), 7.1-7.3 (m, 4H)

More Related