Left fire radical, the indexing element — pictograph of a flame with rising tongues. Marks 烷 as a combustible: alkanes are the saturated hydrocarbons that fuel modern flames. Same family as 烧, 烤, 灯, plus the organic-chemistry coinages 烯, 炔, 烃.
Right 完 supplies the sound directly: wán → wán. Strong semantic flavour too — 完 means 'complete, saturated,' which is exactly what alkanes are at the molecular level (every carbon bond fully filled with hydrogen). The phonetic encodes the chemistry. Same phonetic in 玩, 院, 莞.
