Reactions of Alkenes

• Electronic Structure: The double bond consists of one strong $\sigma$ bond and one weaker $\pi$ bond. The $\pi$ electrons are located above and below the plane of the nuclei, making them highly accessible to electron-seeking species.

• Nucleophilic Character: Because the $\pi$ cloud is electron-dense, alkenes behave as nucleophiles (Lewis bases). They react with electrophiles (Lewis acids) to undergo addition reactions where the $\pi$ bond is broken to form two new $\sigma$ bonds.

• Bond Strength and Reactivity: The $\pi$ bond is significantly weaker than the $\sigma$ bond (approx. 264 kJ/mol vs 347 kJ/mol). This energy difference is the thermodynamic driving force for addition reactions, as the conversion of a $\pi$ bond into two $\sigma$ bonds is generally exothermic.

• The Rule: In the addition of $HX$ to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms. This ensures that the more stable carbocation is formed as an intermediate.

• Carbocation Stability: Stability increases with substitution: $Tertiary (3^\circ) > Secondary (2^\circ) > Primary (1^\circ)$. Alkyl groups stabilize the positive charge through inductive effects and hyperconjugation.

• Anti-Markovnikov Addition: Certain reaction conditions, such as hydroboration-oxidation or the presence of peroxides with $HBr$, can force the substituent to add to the less substituted carbon. This is achieved by changing the mechanism to avoid a standard carbocation intermediate.

Common Transformations

• Halogenation: This reaction proceeds via a cyclic halonium ion intermediate. The nucleophilic attack of the second halide occurs from the opposite face, resulting in anti-addition.

• Hydrogenation: The reaction occurs on the surface of a metal catalyst. Both hydrogen atoms are added to the same face of the double bond, resulting in syn-addition.

• Ozonolysis: Reaction with $O_3$ followed by a reducing agent (like $Zn$ or $DMS$) cleaves the $C=C$ bond entirely. This produces carbonyl compounds (aldehydes or ketones) depending on the substitution of the original alkene.

• Dihydroxylation: The addition of two hydroxyl ($-OH$) groups across the double bond. Reagents like $OsO_4$ or cold, alkaline $KMnO_4$ yield 1,2-cis-diols via a syn-addition mechanism.

• Epoxidation: Reaction with peroxyacids (like $mCPBA$) creates a three-membered cyclic ether called an epoxide. This is a stereospecific syn-addition that preserves the relative geometry of the alkene substituents.

• Check for Rearrangements: Whenever a carbocation is formed (e.g., in hydration or $HX$ addition), always check if a 1,2-hydride or 1,2-methyl shift can produce a more stable carbocation.

• Stereochemical Awareness: In cyclic systems, distinguish between syn and anti products. For example, bromination of a cyclohexene derivative will always place the two bromine atoms in a trans relationship.

• Reagent Specificity: Do not confuse hydration ($H_3O^+$) with hydroboration-oxidation. Hydration gives the Markovnikov alcohol and allows rearrangements, while hydroboration gives the anti-Markovnikov alcohol without rearrangements.

• Count Your Carbons: In ozonolysis problems, students often lose or gain carbons. Always map the carbons of the starting alkene to the resulting carbonyl fragments to ensure the structure is consistent.