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The proton signal of the O=C–H group in ortho-substituted benzaldehydes shows a coupling with a ring proton. The analysis of the spectra of 2,4-dichlorobenzaldehyde, 2,4-dimethoxybenzaldehyde, and 2,4-dihydroxybenzaldehyde shows that the observed splitting is due to a coupling with a ring proton situated five bonds away.
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Kowalewski et al. (1962) studied this question.
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