Chemistry - How explosive is benzene compared to TNT?

All flammable organic liquids could, in principle, create large fuel-air explosions but the conditions are very hard to achieve accidentally

The first important point to note is that benzene should not be a particular worry. There are plenty of other, widely used, chemicals and mixtures that could, in principle, cause large explosions. Standard automotive petrol (gasoline) for example is present in most vehicles (non-diesel ones anyway) and is transported in 40 tonne tankers on most roads. Fires and explosions happen, but rarely, and they are never known to be of the thermobaric type you are worried about. Most motor vehicles have 40kg or so of fuel that is just as flammable as benzene but after accidents they rarely catch fire or explode and never do so with the power of a fuel air thermobaric explosion (the hollywood trope that all crashed cars explode is pure nonsense).

But big fuel air explosions are known to be possible and have been developed into some very effective military weapons. These are known as thermobaric bombs and the largest are just about the biggest non-nuclear explosions possible in existing military arsenals. But a quick read of how the weapons work (see this wikipedia entry) suggests that they require some fairly notable technology to get right. Largely this is because the fuel has to be dispersed at just the right concentration in the air to get a detonation or deflagration rather than a fiery but unimpressive "phut". It isn't easy to achieve this by accident with a large volume of fuel.

Bad maintenance and safety procedures in chemical and oil plants can lead to accidental fuel-air explosions. The Buncefield explosion in a UK oil storage depot was caused by one. It was a huge explosion probably involving about 300 tons of fuel (but that is only about 0.1% of the fuel stored in the depot a lot of which caught fire but didn't explode). Another famous UK example is the Flixborough disaster caused by a fuel-air explosion caused by a leaky pipe in a cyclohexane plant. In both incidents poor management and engineering contrived to make a dangerous event possible and standards were rewritten to prevent future mistakes. That such event are rare should be of significant comfort if you are worried about the dangers of transporting tankers full of benzene. It is worth noting that none of these (very bad0 accidents came close to involving the majority of the flammable compounds stored nearby. It is hard to get the right fuel-air mixture with very large volumes of fuel, thank goodness.

A more common form of fuel air explosion might also put the rare chemical plant incidents in perspective. This is explosions caused in flour mills and grain elevators. a mixture of finely divided flour or grain dust is explosive and plants that handle flour and grain have to be specially designed to avoid dangerous dust explosions. These are far more common that explosions in chemical plants. While this may not be much comfort, it does suggest that worrying about benzene is not that big a real world risk.