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Calix[4]pyrrole liquid membrane electrodes display a potentiometric response towards the neutral form of phenol derivatives. They do not respond towards the corresponding anionic forms. The mechanism of the generation of the potentiometric signal relies on: (1) The creation of supramolecular complex calix[4]pyrrolenitrophenol at the organic/aqueous interface; (2) The increasing of the acidity of OH group from nitrophenol molecules due to formation of hydrogen bond between calix[4]pyrrolenitrophenol at the interface; (3) The dissociation of the OH group from calix[4]pyrrolenitrophenol complex and the ejection of H+to the aqueous layer adjusted to the membrane organic phase boundary. The sensitivity and selectivity of the calix[4]pyrrole liquid membrane electrodes are governed by the lipophilicity and acidity of the phenolic guests. Also, the affinity to create a hydrogen bond between the host and the target molecule is a parameter which has a strong influence on the phenomena studied.

Calix[4]pyrroles were applied as a new class of ligands of potentiometric sensors for neutral nitrophenol isomers. Calix[4]pyrrole‐containing liquid membranes exhibit a very high affinity to proton uptake. These membranes, in protonated form, showed very high selectivity towards para‐nitrophenol in the presence of other nitrophenols and dihydroxybenzene isomers. The probable mechanism of the potentiometric signal generation of the membrane investigated upon stimulation by nitrophenol isomers existing in neutral form is discussed.

Next to the “normal” calix[4]pyrrole 1, the N-confused calix[4]pyrrole 2 is formed in substantial amounts (up to 22 % yield) as side product in the acid-catalyzed condensation reaction of ketones and pyrrole. In some cases, doubly N-confused calix[4]pyrroles are also formed.

Neben dem “normalen“ Calix[4]pyrrol 1 wird das einfach invertierte Calix[4]pyrrol 2 in bis zu 22 % Ausbeute als Nebenprodukt der säurekatalysierten Kondensation von Cyclohexanon mit Pyrrol isoliert. In einigen Ansätzen der systematisch untersuchten Reaktion wurden außerdem zweifach invertierte Calix[4]pyrrole erhalten.

Regioselective nucleophilic additions to pyridazin‐3(2H)‐ones such as (I) followed by quenching with electrophiles allow double functionalization at C‐4 and C‐5 in a single step, when a leaving group is present at C‐5 [→ (IV), (VII), (X)].

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