Chapter 6 and 7: The Reactions of Alkenes and Alkynes

Introduction to π Bonds in Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons characterized by the presence of π bonds. These π bonds are regions of high electron density, making them reactive toward electrophiles in addition reactions.

• Alkene Structure: Contains a carbon-carbon double bond (C=C), with one σ bond and one π bond.

• Alkyne Structure: Contains a carbon-carbon triple bond (C≡C), with one σ bond and two π bonds.

• Reactivity: The π bond is more reactive than the σ bond due to its higher electron density and accessibility.

• Example: Ethylene (CH2=CH2) and acetylene (HC≡CH).

Compounds That Can Be Formed From an Alkene

Alkenes can undergo a variety of addition reactions, leading to diverse products depending on the reagents used.

• Halogenation: Addition of Br2 or Cl2 yields vicinal dihalides.

• Hydrohalogenation: Addition of HX (HBr, HI, HCl) forms alkyl halides.

• Hydration: Addition of H2O (with acid catalyst) forms alcohols.

• Hydroboration-Oxidation: Sequential addition of BH3/THF and H2O2 yields alcohols with anti-Markovnikov selectivity.

• Ozonolysis: Cleavage of the double bond with O3 produces aldehydes or ketones.

• Hydrogenation: Addition of H2 (with Pd/C catalyst) reduces alkenes to alkanes.

Electrophilic Addition of Hydrogen Halides to Alkenes

Hydrogen halides (HX) add to alkenes via an electrophilic addition mechanism, converting the alkene into an alkyl halide.

• General Reaction:

• Example:

• Application: Synthesis of alkyl halides from alkenes.

Mechanism of Electrophilic Addition

The addition of HX to an alkene proceeds via a two-step mechanism involving carbocation formation.

1. Step 1 (Slow): The electrophile (H+) adds to the sp2 carbon of the alkene, generating a carbocation intermediate.

2. Step 2 (Fast): The nucleophile (X-) attacks the carbocation, forming the final alkyl halide product.

Equation:

Regioselectivity: Markovnikov's Rule

Electrophilic addition reactions are regioselective. Markovnikov's rule states that the hydrogen atom adds to the sp2 carbon bonded to the most hydrogens.

• Major Product: The halide attaches to the more substituted carbon.

• Minor Product: The halide attaches to the less substituted carbon.

• Markovnikov's Rule: "H+ is added to the sp2 C that has most H."

• Example: (major) + (minor)

Relative Stabilities of Carbocations

The stability of carbocation intermediates determines the rate and outcome of electrophilic addition reactions.

• Order of Stability: Tertiary > Secondary > Primary > Methyl

• Alkyl Groups: Electron-donating alkyl groups stabilize carbocations by dispersing positive charge.

• Visualization: Tert-butyl cation is most stabilized; methyl cation is least stabilized.

Rate of Electrophilic Addition

The rate of electrophilic addition is influenced by the stability of the carbocation intermediate. More stable carbocations form more rapidly, leading to faster reactions.

• Key Point: Formation of a secondary carbocation is faster than a primary carbocation.

• Example: Addition of HCl to propene forms a secondary carbocation, which reacts faster than if a primary carbocation were formed.

Equation:

Carbocation Rearrangements: 1,2-Hydride Shift

Carbocations can rearrange to form more stable intermediates via hydride or alkyl shifts.

• 1,2-Hydride Shift: A hydrogen atom with its bonding electrons moves from an adjacent carbon to the carbocation center, increasing stability.

• Result: Formation of a more substituted (and thus more stable) carbocation.

• Example: (after hydride shift)

Surprising Major Products Due to Rearrangement

Sometimes, the major product of an electrophilic addition is not the expected Markovnikov product, but rather the result of a carbocation rearrangement.

• Example: Addition of HBr to 3-methyl-1-butene yields 2-bromo-2-methylbutane (major) due to a hydride shift, and 2-bromo-3-methylbutane (minor).

• Mechanistic Insight: Rearrangement occurs if it leads to a more stable carbocation intermediate.

Summary Table: Electrophilic Addition to Alkenes

Key Takeaways

• Electrophilic addition to alkenes and alkynes is a fundamental reaction in organic chemistry.

• Regioselectivity and carbocation stability dictate the major product.

• Carbocation rearrangements (hydride or alkyl shifts) can alter product distribution.

• Understanding these mechanisms is essential for predicting reaction outcomes in synthesis.