3AL+Lecture+10_NMR_.revised - Complex Coupling singlet...

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Unformatted text preview: Complex Coupling singlet doublet triplet quartet ? Successive Applica9on of the N + 1 Rule Two different hydrogen atoms coupling to a third hydrogen atom What if: JAB = 3 Hz JAC = 8 Hz JBC = 6 Hz doublet of doublets (dd) doublet of doublets (dd) doublet of doublets (dd) Successive Applica9on of the N + 1 Rule Two different hydrogen atoms coupling to a third hydrogen atom What if: JAB = 6 Hz JAC = 6 Hz JBC = 8 Hz doublet of doublets (dd) doublet of doublets (dd) doublet of doublets (dd) Successive Applica9on of the N + 1 Rule Doublet of Quartets (dq) Successive Applica9on of the N + 1 Rule Three different hydrogen atoms coupling to a fourth hydrogen atom doublet of doublet of doublets (ddd) doublet of doublet of doublets (ddd) SpinSpin Coupling with Hydrogen Atoms AQached to Heteroatoms Ethanol (dq) Predicted (t) (t) (q) Observed (broad singlet) (t) Proton exchange is fast [email protected] to the NMR @me scale Determining a Structure From NMR 4.5 ppm (d, J = 2.5) A B C D Count carbons: 4 Unique resonances: 9.8 (1H), 200(13C) (t, J = 2.5) HD HC J = 7.3 Hz J = 7.3 Hz HB HA J = 2.5 Hz CHCl3 7.27 ppm J = 2.5 Hz integral 1 2 2 3 A B C D CDCl3 77.0 ppm * * O R" C H C O R' R"' 3.6-4.8 ppm C7H12O4 Degrees of unsatura9on: ((7 x 2) + 2) 12 = 2 2 How many different carbon atoms? 7 Unique resonances? Can't be an amide based on the molecular formula No hydroxy proton observed: probably not a carboxylic acid Most likely an ester CDCl3( NMR solvent) A doublet or two singlets? Integral of 6 suggests two singlets: i.e. two CH3 groups Conclusion: two different methyl ester func9ons H3C O O HO C R' RO O C R' R2N O C R' O C R' 150-185 ppm carboxylic acids, esters and amides * * What do we know as of now? Two "different" methyl ester groups Both degrees of unsatura9on (DOU) are accounted for All oxygen atoms accounted for: C7H12O4 3 carbons and 6 hydrogens unaccounted for O O H H H C C C H H H C O CH3 DOU H3C O O C O C DOU O CH3 Can't be: H3C O C Symmetry in this compound does not match NMR data (e.g. one would only see 4 peaks in the carbon spectrum) O O H CH3 C C H H C O CH3 How about? H3C O C ddq? dd dd d HC = ddq ? But it's a sextet! Recall that if coupling constants are iden9cal or similar, then it makes the mul9plet appear to be coupled to N equivalent hydrogen atoms! JAC, JBC and JCH3C must all be the same or very similar, making it look like 5 equivalent hydrogens (impossible!) are coupled to HC. O H3C O C HA CH3 C C HB HC O C O CH3 Predic9ons about mul9plicity for HA, HB, HC and CH3 HA = dd HB = dd CH3 = d HC = ddq E,F or F,E Reasonable Assignments A B O or C B A D E C H3C O D O HA CH3 F C O CH3 C C HB HC D O H CH3 A C C H3C O B C C C E F O CH3 G H H O B,F or F,B A,G or G,A C,E or E,C D ...
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This note was uploaded on 02/07/2012 for the course CHEM 3AL taught by Professor Li during the Fall '08 term at University of California, Berkeley.

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