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Logic Puzzle #5: Coupling + Tilting + Integration = Benzene … Solution

July 22, 2010

The goal of this puzzle is to determine the respective benzene ring systems that would exhibit the following 1H NMR spectrum.

The number of substituted benzene ring systems that would exhibit the following 1H NMR spectrum is 3 (assuming no repeating units). The keys to solving this puzzle lie with the determination of which multiplets are coupled in combination with the integral information.

 

Logic#5H1Coupling_1hArrow_Jul212010

 

 

Starting with the easy set of multiplets, M08 and M07 are coupled to each other. The second order effects (evident by tilting towards each other) and the similar coupling constants (roughly +/- 0.2 Hz) support this claim. Along side the integral ratio of 2:2, this pattern is intrinsic for a para-substituted benzene ring. Next, the multiplets M01, M03 and M05 are coupled. The 1:2:2 integral ratio and identical coupling constants support a mono-substituted benzene ring. Finally, M02, M04 and M06 are coupled. The 1:1:1 integral ratio and meta-coupling pattern (post#2 link) indicate the presence of a 1,2,4-trisubstituted benzene ring system.

 

 

Logic#5H1Coupling_Str_Jul212010

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