Key Concepts—Conjugation, Resonance, and Dienes

Rules on Evaluating the Relative Stability of Resonance Structures (16.4) 

Markovnikov s rule is followed. Addition of forms the more stable allylic carbocation. 

2-product is the kinetic product. When adds to the double bond, X adds to the end of the allylic carbocation to which it is closer (C2 not C4). The kinetic product is formed faster at low temperature. 

The thermodynamic product has the more substituted, more stable double bond. The thermodynamic product predominates at equilibrium. With 1,3-butadiene, the thermodynamic product is the 1,4-product. 

The mechanism is concerted All bonds are broken and formed in a single step. The diene must react in the s-cis conformation (16.13A). 

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