Which hydrocarbon is expected to have the lowest autoignition temperature (AIT)?

The lowest autoignition temperature (AIT) is associated with hydrogen. This is primarily due to its physical and chemical properties, particularly its low molecular weight and high diffusibility. The AIT is the minimum temperature at which a substance will spontaneously ignite in air without an external ignition source. Hydrogen's small molecular structure allows for a rapid response to heat within an existing atmosphere of oxygen, facilitating quicker ignition.

In the case of hydrocarbons like ethane, propane, and butane, their molecular structures are larger and more complex compared to hydrogen. As a result, they generally require a higher temperature to reach the energy threshold necessary for autoignition. Hydrogen's high flammability, coupled with its low ignition energy, plays a significant role in its lower AIT. Understanding these distinctions is crucial for assessing explosion hazards and implementing appropriate safety measures in environments where these substances might be present.