Reactions of Alkenes

High Electron Density: The $\pi$ bond in an alkene consists of two lobes of electron density located above and below the plane of the carbon atoms. This concentration of negative charge makes the double bond an attractive site for electrophiles, which are electron-deficient species seeking a pair of electrons.

Bond Strength and Reactivity: While the total bond enthalpy of a $C=C$ double bond is higher than a $C-C$ single bond, the $\pi$ component is significantly weaker and more accessible than the $\sigma$ bond. Consequently, alkenes are far more reactive than alkanes, readily undergoing addition reactions to achieve a more stable, saturated state.

Electrophilic Attack: An electrophile (such as $H^+$ or $Br^{\delta+}$) approaches the double bond, causing the $\pi$ electrons to shift and form a new bond with the electrophile, creating a reactive intermediate.

Step 1: Formation of the Carbocation: The electron pair from the $\pi$ bond moves toward the electrophile, forming a $C-E$ bond. This leaves one of the carbon atoms with only three bonds and a formal positive charge, known as a carbocation.

Step 2: Nucleophilic Attack: A nucleophile (a species with a lone pair of electrons, often generated from the original electrophilic reagent) is attracted to the positive charge of the carbocation. It donates its lone pair to form a new $C-Nu$ bond.

Heterolytic Fission: During the process, bonds in the reagent (like $H-Br$) break heterolytically, meaning one atom retains both electrons from the shared pair, creating the nucleophile (e.g., $Br^-$).

Classification: Carbocations are classified as primary ($1^\circ$), secondary ($2^\circ$), or tertiary ($3^\circ$) based on the number of alkyl groups attached to the positively charged carbon.

The Inductive Effect: Alkyl groups are electron-donating; they "push" electron density toward the positive carbon atom. This spreads the positive charge over a larger volume, reducing the charge density and stabilizing the ion.

Stability Trend: The stability increases as follows: $Primary < Secondary < Tertiary$. Tertiary carbocations are the most stable because they have three electron-donating groups to mitigate the positive charge.

Monomers to Polymers: Alkenes act as monomers that can join together to form long-chain molecules called addition polymers. This occurs when the $\pi$ bonds of thousands of alkene molecules break and link the carbons together into a continuous $\sigma$-bonded chain.

Repeating Units: The structure of a polymer is represented by a repeating unit in square brackets with an '$n$' subscript. The repeating unit has the same atoms as the monomer but replaces the double bond with single bonds extending through the brackets.

Properties and Plasticisers: The physical properties of polymers (like PVC) can be modified using plasticisers. These small molecules sit between polymer chains, weakening the intermolecular forces and allowing the chains to slide, making the plastic more flexible.

Curly Arrow Precision: Always ensure curly arrows start exactly at a bond or a lone pair and point directly to the atom or bond where the electrons are moving. In electrophilic addition, the first arrow MUST start at the $C=C$ double bond.

Intermediate Charges: Never forget to draw the positive charge on the carbocation intermediate. Omitting this charge is a frequent cause of lost marks in mechanism questions.

Major Product Justification: When asked to explain why a specific product is major, always reference the stability of the carbocation intermediate and the inductive effect of the attached alkyl groups.

Symmetry Check: Before applying Markovnikov's rule, check if the alkene is symmetrical. If it is (like but-2-ene), only one product can form, and the rule does not apply.