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Mild and efficient esterification reactions of carboxylic acids with neo-pentanol were carried out using task-specific ionic liquids with hydrogen sulphate counteranion under microwave irradiation. The latent acidity of the ionic liquid was introduced by anion metathesis from hydrogen sulphate with the corresponding imidazolium or pyridinium halides. The catalyst [C4mim][HSO4] modified with 5% of concentrated sulfuric acid was reused three times without considerable loss of activity in esterification using classical heating in oil bath.

Crotonaldehyde semicarbazone and crotonaldehyde thio­semicarbazone show the same E conformation around the imine C=N bond. Each mol­ecule has an intra­molecular N—H⋯N hydrogen bond, which generates an S(5) ring. Inter­molecular N—H⋯O hydrogen bonds in the semicarbazone link the mol­ecules into layers parallel to the bc plane, while weak inter­molecular N—H⋯S hydrogen bonds in the thio­semicarbazone link the mol­ecules into chains propagating in [110]., Crotonaldehyde semicarbazone {systematic name: (E)-2-[(E)-but-2-en-1-yl­idene]hydrazinecarboxamide}, C5H9N3O, (I), and crotonaldehyde thio­semi­carba­zone {systematic name: (E)-2-[(E)-but-2-en-1-yldene]hydra­zinecarbo­­thio­amide}, C5H9N3S, (II), show the same E conformation around the imine C=N bond. Compounds (I) and (II) were obtained by the condensation of crotonaldehyde with semicarbazide hydro­chloride and thio­semicarbazide, respectively. Each mol­ecule has an intra­molecular N—H⋯N hydrogen bond, which generates an S(5) ring. In (I), the crotonaldehyde fragment is twisted by 2.59 (5)° from the semicarbazide mean plane, while in (II) the corresponding angle (with the thio­semicarbazide mean plane) is 9.12 (5)°. The crystal packing is different in the two compounds: in (I) inter­molecular N—H⋯O hydrogen bonds link the mol­ecules into layers parallel to the bc plane, while weak inter­molecular N—H⋯S hydrogen bonds in (II) link the mol­ecules into chains propagating in [110].