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We have these nomenclatures of the same family :

4H-chromene 2H-chromene Why do we use 2H and 4H to name them? And how ?

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There are two isomers of chromene (1-benzopyran): 2​H-chromene and 4​H-chromene.

2H-chromene        4H-chromene

According to Subsection P-14.7 of the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the name can be made specific by indicating the position of the extra hydrogen atom in the structure, i.e. the position where no multiple bond is attached. This is accomplished by adding to the front of the name an italic capital ‘H’ preceded by the appropriate numerical locant for this atom.

In 2​H-chromene, the ‘indicated hydrogen’ locates one extra hydrogen atom in position ‘2’ of the chromene structure; and in 4​H-chromene, the ‘indicated hydrogen’ indicates an extra hydrogen atom at position ‘4’, i.e. one hydrogen atom more than the number present if there were a double bond in the ring at that position.

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