Lecture Notes: Addition Reactions to Alkenes

Introduction to Reaction Mechanisms

Addition Reactions to Alkenes

• Addition reactions involve the reaction of multiply bonded functional groups, such as alkenes.
• General form: electrophile + nucleophile.

Mechanism of Addition Reactions

• Step 1 (Slow): Addition of an electrophile ($H^+$) to the electron-rich double bond.
  • The electrophile approaches perpendicular to the atom plane where electron density is maximum.
  • Forms a carbocation intermediate.
• Step 2 (Fast): Attack by a nucleophile at the carbocation intermediate (similar to SN1 reactions).
• The rate-determining step involves the electrophile (electron-seeking), thus it's an electrophilic addition.
  • Rate equation: $rate = k[H^+][ ]$
• The carbocation intermediate reacts with the most abundant nucleophile.

Addition to Unsymmetrical Alkenes

• Addition to unsymmetrical C=C bonds can lead to multiple products.
• The more stable carbocation intermediate is favored (e.g., a 2° carbocation over a 1° carbocation).
  • Alkyl groups are electron-donating and stabilize the positive charge.

Addition of $Cl<em>2$ and $Br</em>2$

• Nonpolar $Cl<em>2$ and $Br</em>2$ react with alkenes.
• Approach to the electron-rich double bond induces a dipole in the $Br_2$ bond.
• Bromine can share the positive charge, leading to a bromonium ion.
• The bromonium ion is significant for symmetrical alkenes.