Key Concepts of Alkenes and Alkynes to Know for Organic Chemistry

Alkenes and alkynes are key players in organic chemistry, featuring carbon-carbon double and triple bonds, respectively. Understanding their structure, reactivity, and unique properties helps in grasping essential concepts like isomerism, addition reactions, and polymerization.

1. Structure and nomenclature of alkenes and alkynes

   • Alkenes contain at least one carbon-carbon double bond (C=C), while alkynes have at least one carbon-carbon triple bond (C≡C).
   • The longest carbon chain containing the double or triple bond determines the base name (e.g., "butene" for alkenes, "butyne" for alkynes).
   • Number the carbon chain to give the double or triple bond the lowest possible number.
   • Use prefixes (e.g., "cis-", "trans-") to indicate the geometry of alkenes and specify the position of the double or triple bond.

2. Geometric isomerism (cis-trans) in alkenes

   • Geometric isomers occur due to restricted rotation around the double bond.
   • "Cis" isomer has substituents on the same side of the double bond, while "trans" has them on opposite sides.
   • The presence of different substituents on the double-bonded carbons is necessary for geometric isomerism to occur.

3. Electrophilic addition reactions

   • Alkenes and alkynes react with electrophiles to form more saturated products.
   • The double or triple bond acts as a nucleophile, attacking the electrophile.
   • Common electrophiles include halogens, hydrogen halides, and water.

4. Hydrogenation of alkenes and alkynes

   • Hydrogenation involves the addition of hydrogen (H₂) across the double or triple bond.
   • This reaction typically requires a catalyst, such as palladium, platinum, or nickel.
   • The result is the conversion of alkenes to alkanes and alkynes to alkenes.

5. Hydration of alkenes and alkynes

   • Hydration is the addition of water (H₂O) across the double or triple bond.
   • This reaction often requires an acid catalyst and follows Markovnikov's rule.
   • The product is an alcohol for alkenes and a ketone or aldehyde for alkynes.

6. Halogenation of alkenes and alkynes

   • Halogenation involves the addition of halogens (e.g., Cl₂, Br₂) to the double or triple bond.
   • The reaction typically occurs in an inert solvent and results in vicinal dihalides.
   • The addition is anti, meaning the halogens add to opposite sides of the double bond.

7. Hydrohalogenation of alkenes and alkynes

   • Hydrohalogenation is the addition of hydrogen halides (HX) to alkenes and alkynes.
   • The reaction follows Markovnikov's rule, where the hydrogen atom adds to the carbon with more hydrogen substituents.
   • The product is an alkyl halide.

8. Markovnikov's rule

   • Markovnikov's rule states that in the addition of HX to alkenes, the hydrogen atom will attach to the carbon with the most hydrogen atoms already attached.
   • This rule helps predict the major product in electrophilic addition reactions.
   • It is based on the stability of the carbocation intermediate formed during the reaction.

9. Anti-Markovnikov addition (hydroboration-oxidation)

   • Hydroboration-oxidation is a two-step reaction that adds water across the double bond in an anti-Markovnikov fashion.
   • In the first step, borane (BH₃) adds to the alkene, forming a trialkylborane.
   • In the second step, oxidation with hydrogen peroxide (H₂O₂) converts the boron to an alcohol, resulting in the alcohol being added to the less substituted carbon.

10. Oxidation reactions (ozonolysis, dihydroxylation)

    • Ozonolysis involves the cleavage of double bonds using ozone (O₃), producing carbonyl compounds (aldehydes or ketones).
    • Dihydroxylation is the addition of two hydroxyl groups (OH) across the double bond, typically using osmium tetroxide (OsO₄) or potassium permanganate (KMnO₄).
    • Both reactions are useful for functionalizing alkenes.

11. Reduction of alkynes to alkenes

    • Alkynes can be reduced to alkenes using hydrogenation or metal catalysts.
    • Partial hydrogenation can be achieved using Lindlar's catalyst, which selectively converts alkynes to cis-alkenes.
    • This reaction is important for synthesizing specific alkenes from alkynes.

12. Acidity of terminal alkynes

    • Terminal alkynes (alkynes with a triple bond at the end of the carbon chain) are more acidic than alkenes and alkanes.
    • The acidity is due to the stability of the resulting anion after deprotonation.
    • Terminal alkynes can react with strong bases to form acetylide ions.

13. Addition polymerization of alkenes

    • Alkenes can undergo addition polymerization to form long-chain polymers.
    • This process involves the repeated addition of monomer units (alkenes) to form a polymer.
    • Common examples include polyethylene and polypropylene, which are widely used in plastics.

14. Diels-Alder reaction

    • The Diels-Alder reaction is a [4+2] cycloaddition between a diene and a dienophile.
    • This reaction forms a six-membered ring and is a key method for constructing cyclic compounds.
    • It is stereospecific and can produce both syn and anti products depending on the substituents.

15. Stability and reactivity trends

    • Alkenes and alkynes exhibit different stability and reactivity based on their structure (e.g., degree of substitution).
    • More substituted alkenes are generally more stable due to hyperconjugation and inductive effects.
    • Alkynes are typically more reactive than alkenes due to the presence of the triple bond, which is more electron-rich and can participate in various reactions.