Addition reactions Knowledge-Based Study

Study notes

1. What is an Addition Reaction?

   In organic chemistry, an addition reaction occurs when two or more reactant molecules combine to form a single product without the loss of any atoms. This process is fundamentally characteristic of unsaturated organic compounds—molecules that contain multiple carbon-carbon bonds, such as alkenes (double bonds) and alkynes (triple bonds), as well as polar functional groups like carbonyls (carbon-oxygen double bonds).

   During an addition reaction, the relatively weak $\pi$ (pi) bond of the multiple bond is broken, and two new, stronger $\sigma$ (sigma) bonds are formed to accommodate the incoming atoms or groups. Because $\sigma$ bonds possess lower potential energy and greater bond dissociation energy than $\pi$ bonds, addition reactions are generally thermodynamically favorable (exothermic).

   Mechanisms and Common Types of Addition

   The electron-rich $\pi$ electron cloud above and below the plane of an alkene makes it an attractive target for electron-deficient species known as electrophiles. The general reaction proceeds via an electrophilic addition mechanism.

   Common electrophilic additions include:

   • Halogenation: Reaction with halogens such as $\text{Br}_2$ or $\text{Cl}_2$ yields vicinal dihalides. The rapid decolorization of a reddish-brown bromine solution serves as a classic diagnostic laboratory test for unsaturation.
   • Hydrogenation: Addition of molecular hydrogen ($\text{H}_2$) across the double bond in the presence of a finely divided transition metal catalyst (such as $\text{Pt}$, $\text{Pd}$, or $\text{Ni}$) converts alkenes to saturated alkanes.
   • Hydrohalogenation: Addition of hydrogen halides ($\text{HCl}$, $\text{HBr}$, or $\text{HI}$) converts an alkene into a haloalkane.
   • Hydration: In the presence of an acid catalyst such as dilute sulfuric acid, water ($\text{H}_2\text{O}$) adds across the double bond to yield an alcohol.

   Markovnikov's Rule and Carbocation Stability

   When an unsymmetrical reagent (such as $\text{HBr}$ or $\text{H}_2\text{O}$) reacts with an unsymmetrical alkene (such as propene), two constitutional isomers could theoretically form. However, one product is obtained in significantly higher yield.

   This outcome is governed by Markovnikov's Rule: When an unsymmetrical reagent adds to an unsymmetrical alkene, the hydrogen atom attaches to the double-bonded carbon that already bears the greater number of hydrogen atoms.

   The electronic basis for this empirical rule lies in the stability of the intermediate carbocation:

   • The initial attack of the electrophile ($\text{H}^+$) creates a carbocation intermediate.
   • Alkylation stabilizes carbocations via inductive electron donation and hyperconjugation: tertiary carbocations are more stable than secondary, which are far more stable than primary carbocations.
   • The reaction proceeds predominantly through the more stable carbocation intermediate, yielding the Markovnikov product as the major constitutional isomer.

   Stepwise and Nucleophilic Additions

   Addition reactions are not restricted to carbon-carbon double bonds:

   • Alkynes: Containing two $\pi$ bonds, alkynes can undergo addition in two sequential steps. Addition of one equivalent of reagent converts the alkyne into an alkene derivative, while addition of a second equivalent leads to a fully saturated product. Markovnikov's rule continues to direct the orientation at each step for unsymmetrical alkynes.
   • Carbonyl Compounds: The carbonyl group ($\text{C}=\text{O}$) is strongly polarized due to the high electronegativity of oxygen relative to carbon. Consequently, the carbonyl carbon acts as an electrophilic center. These compounds undergo nucleophilic addition, where an electron-rich nucleophile attacks the carbonyl carbon, followed by protonation of the oxygen atom.