What is the primary characteristic of an electrophilic addition reaction?

The primary characteristic of an electrophilic addition reaction is that it involves the attack of an electrophile on a nucleophile. In organic chemistry, electrophiles are species that are electron-deficient and seek to gain electrons, while nucleophiles are electron-rich and have a pair of electrons to donate. During an electrophilic addition reaction, the electrophile will react with a nucleophile, often resulting in the formation of a more stable molecule.

For instance, in the addition of hydrogen bromide (HBr) to an alkene, the double bond acts as the nucleophile and attacks the electrophile (the hydrogen from HBr), resulting in the formation of a saturated product. This type of reaction is fundamental in the transformation of unsaturated compounds into more saturated compounds, which is a key process in organic synthesis.

Other options do not accurately represent the primary feature of electrophilic addition reactions. While some electrophilic addition reactions may require a catalyst to speed up the reaction, this is not a defining characteristic across all such reactions. Similarly, not all electrophilic addition reactions result in gaseous products; they can yield solids or liquids as well. Lastly, the loss of a molecule is more characteristic of elimination reactions rather than addition reactions. Therefore, the defining