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Logic Puzzle #40: What Fragment is This? … Solution

April 2, 2013

Logical reasoning and pattern recognition form the basis for making a pattern-to-fragment connection.

For an unknown, the 1H NMR spectrum below shows 5 multiplets in the aromatic region. The multiplet at 7.57 ppm shows two overlapping multiplets: a broad signal overlapping a ‘doublet’ (noted as d*).

Based on the multiplets’ tilt/lean and integrals, a 1,2-disubstituted benzene ring can be inferred from the multiplets at 7.11, 7.17, 7.34 and 7.57(d*) ppm. The multiplet at 6.96 ppm is a doublet with a small coupling. The small coupling can come from an adjacent NH group. If this is the case, then the broad signal at 7.57 ppm may be attributed to the NH group.

A good candidate for the unknown is an indole group (IUPAC: 1H-indol-3-yl).

Thank you Serge and Walter for your comments.

One Reply to “Logic Puzzle #40: What Fragment is This? … Solution”

  1. Dear sir,
    I have carefully checked the proton NMR spectrum of 3-methylindole and found 2-H signal is quartet rather than doublet as Logic Puzzle #40:decribed.The quartet is resulted of the coupling between methyl proton and 2-H through 3-bonds.
    best wishes
    Guangren Li


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