Flashpoint measurements of various types are used as one measure of the flammability of liquid materials … Many manufacturing processes involve flammable chemicals; therefore, flashpoints and flammability limits are essential to maximise safety in process design and operational procedures - VCE - SSCE Chemistry - Question 9 - 2022 - Paper 1

The variation in flashpoints among organic molecules is significantly influenced by their molecular structure and bonding characteristics.

1. Molecular Weight and Size: Generally, larger molecules with greater molecular weights have higher flashpoints. This is largely due to the increased difficulties in vaporization as molecular size increases, which raises the temperature needed to reach the flashpoint.

2. Intermolecular Forces: The strength of intermolecular forces, such as hydrogen bonding, van der Waals forces, and dipole-dipole interactions, play a crucial role. Molecules exhibiting strong intermolecular attractions often have higher flashpoints because more energy (i.e., heat) is required to overcome these forces during vaporization.

3. Structure (Chain Length and Branching): Aliphatic compounds with long straight chains typically have higher flashpoints compared to their branched counterparts due to the enhanced surface area and better packing, leading to stronger van der Waals interactions. In contrast, branched molecules tend to have lower flashpoints due to lower boiling points and vaporization ease.

4. Functional Groups: The presence of specific functional groups can either increase or decrease flashpoint. For example, alcohols, due to their ability to form hydrogen bonds, tend to have higher flashpoints compared to non-alcoholic hydrocarbons of similar molecular weight. Conversely, the addition of electronegative atoms (such as chlorine) might lead to a reduction in flashpoint due to altered molecular interactions.

Overall, understanding these factors allows for better predictions of the flammability characteristics of various organic substances.