What is an Electrophilic Addition Reaction?
An electrophilic addition reaction is a type of chemical reaction where an electrophile (an electron-deficient species) is added to a carbon-carbon double bond (alkene) or triple bond (alkyne), breaking the pi bond and forming two new sigma bonds. This process results in a saturated or more saturated product. The reaction is initiated by the electrophile attacking the electron-rich pi bond.
Key Principles and Steps
The reaction proceeds in two main steps. First, the electrophile attacks the pi bond, forming a carbocation intermediate (or a cyclic bromonium ion in halogenation). The carbon atom that doesn't bond to the electrophile becomes positively charged. In the second step, a nucleophile (an electron-rich species) attacks the positively charged carbocation, completing the addition across the original double or triple bond. This mechanism is common for alkenes and alkynes.
A Practical Example: Addition of HBr to Propene
A classic example is the reaction of propene (CH₃-CH=CH₂) with hydrogen bromide (HBr). The H⁺ ion (electrophile) from HBr adds to one of the carbons of the double bond, typically forming a more stable secondary carbocation according to Markovnikov's Rule. Subsequently, the bromide ion (Br⁻, nucleophile) attacks this carbocation, resulting in 2-bromopropane (CH₃-CHBr-CH₃) as the major product. The original pi bond is broken, and new sigma bonds to H and Br are formed.
Importance and Applications
Electrophilic addition reactions are crucial in organic synthesis for converting unsaturated hydrocarbons into a variety of functionalized compounds. They are widely used in the industrial production of alcohols, alkyl halides, and other intermediates from alkenes. Understanding this reaction type is fundamental for predicting reaction products and mechanisms in organic chemistry, forming the basis for many synthesis strategies involving carbon-carbon multiple bonds.