Antiaromatic compounds, often being very unstable, can be highly reactive in order to relieve the antiaromatic destabilization. Cyclobutadiene, for example, rapidly dimerizes with no potential energy barrier via a 2 + 2 cycloaddition reaction to form tricyclooctadiene. While the antiaromatic character of cyclobutadiene is the subject of debate, the relief of antiaromaticity is usually invoked as the driving force of this reaction.
Antiaromaticity can also have a significant effect on pK. The linear compound propene has a pK of 44, which is relatively acidic for an sp carbon center because the resultant allyl anion can be resonance stabilized. The analogous cyclic system appears to have even more resonance stabilized, as the negative charge can be delocalized across three carbons instead of two. However, the cyclopropenyl anion has 4 π electrons in a cyclic system and in fact has a substantially higher pK than 1-propene because it is antiaromatic and thus destabilized. Because antiaromatic compounds are often short-lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation.
Some antiaromatic compounds are stable, especially larger cyclic systems (in which the antiaromatic destabilization is not as substantial). For example, the aromatic species 1 can be reduced to 2 with a relatively small penalty for forming an antiaromatic system. The antiaromatic  2 does revert to the aromatic species 1 over time by reacting with oxygen in the air because the aromaticity is preferred.
The loss of antiaromaticity can sometimes be the driving force of a reaction. In the following keto-enol tautomerization, the product enol is more stable than the original ketone even though the ketone contains an aromatic benzene moiety (blue). However, there is also an antiaromatic lactone moiety (green). The relief of antiaromatic destabilization provides a driving force that outweighs even the loss of an aromatic benzene.