Alkyne, Alkene, and Addition Reactions: A Detailed Analysis

Alkynes

Alkynes are hydrocarbons containing at least one triple bond, forming a homologous series with the empirical formula CnH2n-2. The C-H bond in ethyne is more polar than in other C-H bonds, allowing ethyne to act as a weak acid. The simplest and most important alkyne is ethyne, which has a triple bond, a high electron density, and sp-hybridized orbitals. These sp-hybrid orbitals have a higher s-character than sp² and sp³ orbitals. The bond length of the C-C and C-H bonds is shorter than in ethene and ethane molecules. The pi electrons are strongly attracted by the nuclei, resulting in slower addition reactions.

Alkenes

Alkenes are hydrocarbons with one or more double bonds, forming a homologous series with the molecular formula CnH2n. Multiple double bonds can be found in dienes (two double bonds) and polyenes (more than two double bonds). These double bonds can be directly adjacent (cumulative), alternating (conjugated), or isolated. The carbon atoms in alkenes are sp²-hybridized, and the pi electrons are perpendicular to the plane of the sigma bonds, forming an electron cloud. This makes alkenes susceptible to electrophilic attack. A characteristic reaction is the addition reaction, where a double bond is cleaved to form two single bonds, resulting in stable products. The bond angle in the double bond is approximately 120 degrees, making it highly reactive.

Key Reactions of Alkenes

• Halogenation: The addition of two halogen atoms to the C=C bond, forming dihalogenated alkanes. Example: CH₂=CH₂ + Br₂ -> CH₂Br-CH₂Br
• Hydrohalogenation: The addition of one hydrogen halide molecule to the C=C bond, forming haloalkanes. Example: CH₂=CH₂ + HCl => CH₃-CH₂Cl
• Hydrogenation: The addition of two hydrogen atoms to the C=C bond, forming saturated alkanes. This reaction requires a suitable catalyst, such as nickel. Example: CH₃-CH=CH-CH₃ + H₂ – Ni, 500°C -> CH₃-CH₂-CH₂-CH₃
• Polymerization: The formation of large molecules from smaller repeating units.

Properties of Alkenes

1. Alkenes are non-polar molecules, with physical properties and combustibility similar to alkanes. They are insoluble in water, burn with a smoky flame, and are generally unreactive.
2. The double bond provides a target for electrophilic reagents.
3. Alkenes react with halogens to form halogenated alkanes.

Addition Reactions

Addition reactions involve the addition of reagents to a substrate, typically at a multiple bond. The multiple bond is broken, and two new sigma bonds are formed. These reactions are associated with energy release, making them favored for alkenes and alkynes.

Electrophilic Addition

Electrophilic addition is a reaction mechanism in organic chemistry where unsaturated hydrocarbons (alkenes and alkynes) react with various compounds. The reaction is initiated by the attack of an electron-seeking particle, the electrophile, on the double or triple bond. Electrophiles are attracted to negative charges, are positively charged or polarized, and have a high affinity for electrons. Typical electrophiles include carbocations, H⁺, and halogens.

Addition of Halogens

Halogens can be added to alkenes, forming haloalkanes. The reaction proceeds in multiple steps:

1. The proton acts as an electrophile, attacking the double bond and forming a carbocation. The proton adds to the carbon with the most hydrogen atoms (Markovnikov’s rule).
2. The carbocation reacts with a halide ion, such as bromide, to form the haloalkane.

Hydrobromination of Propene

Example: CH₃-CH=CH₂ + HBr -> CH₃-CHBr-CH₃ (2-bromopropane)

Addition of Bromine

The bromine molecule approaches the double bond and forms a pi complex. The bond in the bromine molecule is polarized. The bromine molecule is then heterolytically cleaved, forming a bromonium ion and a bromide anion. The bromide ion attacks the bromonium ion from the rear, forming a 1,2-dibromoalkane. This reaction is competitive with other chemical reactions. No substitution occurs because the addition is preferred. Other halogens can be used in this reaction. Bromine is very reactive and destroys all double bonds.

Electrophiles are electron-seeking particles (+ charged), while nucleophiles are nucleus-seeking particles (- charged). Heterolytic bond cleavage results in a cation and an anion.

Radical Substitution Reactions

Radical substitution reactions proceed through a mechanism involving free radicals (uncharged species with unpaired electrons). These reactions involve homolytic bond cleavage, where the bond is split equally. In substitution reactions, one radical replaces another.