Which is not aromatic 1) teopyliumcation 2) chrysene 3) cyclopentadienyl 4) cyclopropenyl

The compound that is not aromatic among the given options is cyclopentadienyl.

Here's why:

Aromatic compounds are cyclic (ring-shaped), planar (flat) structures that follow the Huckel's rule, which states that an aromatic compound should have (4n+2) π electrons, where n is any integer (including zero).

1. Tropylium cation: It is a heptagonal (7-membered) ring with 6 π electrons (3 double bonds), which fits the Huckel's rule (4n+2, where n=1). So, it is aromatic.

2. Chrysene: It is a polycyclic aromatic hydrocarbon with 4 fused benzene rings. Each benzene ring has 6 π electrons, fitting the Huckel's rule. So, it is aromatic.

3. Cyclopentadienyl: It is a 5-membered ring with 4 π electrons (2 double bonds), which does not fit the Huckel's rule (4n+2). So, it is not aromatic.

4. Cyclopropenyl: It is a 3-membered ring with 2 π electrons (1 double bond), which fits the Huckel's rule (4n+2, where n=0). So, it is aromatic.