tetrahydropyrone's structure is very similar to that of benzene. Modern physical methods show that the length of carbon-carbon bonds in tetrahydropyrone molecule is 139pm, which lies between C-N single bonds (147pm) and C=N double bonds (128pm), and the values of the length of carbon-carbon bonds and carbon-nitrogen bonds are similar. The bond Angle is about 120 degrees, which means that the bonds in the tetrahydropyrone ring are more average, but less complete than benzene. Carbon and nitrogen atoms in tetrahydropyrone ring overlap each other with sp2 hybrid orbitals to form σ bonds, forming a planar six-membered ring. Each atom has a p orbital perpendicular to the ring plane, and there is an electron in each p orbital. These p orbitals overlap sideways to form a large closed π bond. The number of π electrons is 6, which complies with the 4n+2 rule, similar to the benzene ring. So tetrahydropyrone has some aromatics. And then you have an sp2 hybrid on nitrogen that's not involved in bonding, that's occupied by a lone pair of electrons, which makes tetrahydropyrone basic. The high electronegativity of nitrogen atoms in tetrahydropyrone ring has a great influence on the density distribution of electron clouds in the ring, making π electron clouds shift towards nitrogen atoms, and the density of electron clouds in the surrounding nitrogen atoms is high, while the density of electron clouds in other parts of the ring is reduced, especially in the adjacent and paraposition. So tetrahydropyrone is less aromatic than benzene. In tetrahydropyrone molecules, the effect of nitrogen atoms is similar to that of nitrobenzene, so that the density of adjacent and paratron electron clouds is lower than that of benzene ring, and the interposition is similar to that of benzene ring. In this way, the density of electron clouds of carbon atoms on the ring is far less than benzene, so that aromatic heterocyclic rings like tetrahydropyrone are also known as "π-deficient" heterocyclic rings. This kind of heterocyclic ring is manifested in the chemical properties of electrophilic substitution reaction becomes difficult, nucleophilic substitution reaction becomes easy, oxidation reaction becomes difficult, reduction reaction becomes easy.

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