Alpha-pinene is an organic compound of the terpene class, found in turpentine oil of pine trees. Structure of this strained alkene with reactive four-membered ring is shown bellow:

When turpentine or alpha-pinene is poured over powdered iodine a very intensive emission of violet iodine vapors occurs. Step back immediately, because iodine and its vapors are harmful, so the experiment must be performed within a fume hood or in a place with adequate and strong ventilation or an open space.

Iodine does not react easily with most of common alkenes, which can be demonstrated by pouring cyclohexene on iodine - the reaction is reversible. However, iodine reacts considerably exothermic with alpha-pinene, with heat liberated enough to cause iodine sublimation.

Iodine is added to the double bond of the pinene system, thus forming an intermediate iodonium ion that is in resonance with the secondary carbocation, as shown bellow:

As it can be seen, the system contains a four membered ring, and is therefore under tension. It is relieved of tension by a rearrangement, during which a norbornane derivate is formed (bicyclo[2.2.1.]heptane):

A great deal of energy from relief of ring strain in the alpha-pinene molecule is liberated during this rearrangement, and this leads to immediate iodine sublimation.