Chapter_9

Organic Chemistry

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P1: PCX/PBR P2: PBU Printer: Hamilton JWDD052-09 JWDD052-Solomons-v2 May 29, 2007 22:23 9 NUCLEAR MAGNETIC RESONANCE AND MASS SPECTROMETRY: TOOLS FOR STRUCTURE DETERMINATION SOLUTIONS TO PROBLEMS 9.1 The presence of two signals in the 1 H NMR spectrum signiFes two unique proton environ- ments in the molecule. The chemical shift of the downFeld signal, (a), is consistent with protons bearing a chlorine on the same carbon. Its triplet nature indicates two hydrogens on the adjacent carbon. The upFeld quintet (b) indicates four adjacent and equivalent hydrogens, which requires two hydrogens on each of two carbons. Integration data indicate a ratio of (approximately) 2:1, which is actually 4 : 2 in this case. Of the isomeric structures with the formula C 3 H 6 Cl 2 , only ClCH 2 CH 2 CH 2 Cl ( a )( a ) ( b ) Fts the evidence. 9.2 The sharp singlet of the 1 H NMR spectrum indicates that all six hydrogens are equivalent. The chemical shift is appropriate for chlorine substitution on a carbon adjacent to the hydrogens producing the signal. Only CH 3 CCl 2 CH 3 would give the spectrum shown. 9.3 We see below that if we replace a methyl hydrogen by Cl, then rotation of the methyl group or turning the whole molecule end-for-end gives structures that represent the same compound. This means that all of the methyl hydrogens are equivalent. rotate CH 2 Cl group turn C H H H H H H C C H H HH C Cl H C Cl H H replace H with Cl H C 150
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P1: PCX/PBR P2: PBU Printer: Hamilton JWDD052-09 JWDD052-Solomons-v2 May 29, 2007 22:23 NUCLEAR MAGNETIC RESONANCE AND MASS SPECTROMETRY 151 rotate CH 2 Cl group C C1 H HH H turn All of these are the same compound. C H Cl H turn C H H HC l H C H C H H H Cl C C H C H Replacing a ring hydrogen by Cl gives another compound, but replacing each ring hydrogen in turn gives the same compound. This shows that all four ring hydrogens are equivalent and that 1,4-dimethylbenzene has only two sets of chemical shift equivalent protons. All of these are the same compound. replace H by Cl CH 3 CH 3 Cl CH 3 CH 3 Cl CH 3 CH 3 Cl CH 3 CH 3 Cl CH 3 CH 3 9.4 (a) One (d) One (b) Two (e) Two (c) Two (f) Two 9.5 (a) Diastereomers C C replacement by Q CH 3 CH 3 C HO H HO H HO H C H Q H H Q H CH 3 CH 3 C C CH 3 CH 3
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P1: PCX/PBR P2: PBU Printer: Hamilton JWDD052-09 JWDD052-Solomons-v2 May 29, 2007 22:23 152 NUCLEAR MAGNETIC RESONANCE AND MASS SPECTROMETRY (b) Six CO H H CH 3 CH 3 (b) (c) CH H (d) (e) (a) (f) (c) Six signals 9.6 (a) CH 3 (a) (a) (c) CH 2 (b) CH 2 CH 3 (a) Two, (e) CH 3 (a) CHBr Br C H H (c) Four, (c) Four, (b) CH 3 (a) CH 2 (b) O H (d) H CH 3 H (a) (a) C H CH 3 CH 3 H C H (c) Three, (d) Two, (f ) Two, (d) H H H H (a) (a) (b) CH 3 CH 3 CC CH 3 (a) (d) (e) (c) (a) CH 2 (g) Three, (i) Six, (c) (e) C HH CH 2 H C (h) Four, (j) Five, H CH 3 Cl H OH H H H H H (a) (a) (c) (c) CH 3 CH 3 H H (a) (a) (d) (c) CH 3 CH 3 9.7 The determining factors here are the number of chlorine atoms attached to the carbon atoms bearing protons and the deshielding that results from chlorine’s electronegativity. In 1,1, 2- trichloroethane the proton that gives rise to the triplet is on a carbon atom that bears two chlorines, and the signal from this proton is down±eld. In 1,1,2,3,3-pentachloropropane,
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Chapter_9 - P1: PCX/PBR JWDD052-09 P2: PBU...

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