Chapter 16: Q19P (page 822)

Explain why each compound is aromatic, antiaromatic, or nonaromatic
(a) Isoxazole
(b) 1,3-thiazole
(c) Pyran
(d) Pyrylium ion
(e) γ - pyrone
(f) 1,2-Dihydropyridine
(g)Cytosine
(h)

Short Answer

Expert verified

(a)

Isoxazole

(Aromatic)

(b)

1,3-thiazole

(Aromatic)

(c)

Pyran

(Nonaromatic)

(d)

Pyrylium ion

(Aromatic)

(e)

γ - pyrone

(Aromatic)

(f)

1,2-Dihydropyridine

(Nonaromatic)

(g)

Cytosine

(Aromatic)

(h)

Antiaromatic

Step by step solution

Aromatic, antiaromatic or nonaromatic:

Aromatic: A molecule must be cyclic, planar, completely conjugated, and must satisfy Huckel’s rule (4n+2 pi electrons) and contains a particular number of pi electrons.

Antiaromatic: A cyclic, planar, completely conjugated compound with 4n pi electrons.

Nonaromatic: A compound that lacks (one or more) of the four requirements to be aromatic.

Classification of given compounds:

(a) Isoxazole is an aromatic compound because it haselectrons and one of the lone pairs of oxygen participates in an aromatic sextet. So, it obeys Huckel’s rule.

Isoxazole

(Aromatic)

(b) 1,3-thiazole is an aromatic compound because it has electrons and one of the lone pairs of sulfur participates in the aromatic sextet. So, it obeys Huckel’s rule.

1,3-thiazole

(Aromatic)

Pyran

(Nonaromatic)

(d) Pyrylium ion is an aromatic compound because it has electrons and one of the lone pairs of oxygen participates in an aromatic sextet. Here, the carbocation also undergoes resonance. So, it obeys Huckel’s rule.

Pyrylium ion

(Aromatic)

(e) γ is an aromatic compound because it has 4π electrons and one of the lone pairs of oxygen participates in the formation of an aromatic sextet. So, it obeys Huckel’s rule.

γ - pyrone

(Aromatic)

(f) 1,2-Dihydropyridine is a nonaromatic compound because it has no conjugated π electron system and it contains one sp³ hybridized carbon atom. So, it does not obey Huckel’s rule.

1,2-Dihydropyridine

(Nonaromatic)

(g) Cytosine is aromatic because it has 4π electrons and one lone pair of nitrogen participates in the formation of an aromatic sextet. So, it obeys Huckel’s rule.