Lesson 15: Alkynes and Benzene Reactions - Comprehensive Review (2 hours)

Alkynes

1. Hydrogenation

   • Full Hydrogenation to Alkanes:

     • Reagents: H₂, Pd/C or Pt.

     • Mechanism:

       1. Hydrogen molecules adsorb onto the surface of the catalyst.

       2. The alkyne approaches the catalyst surface and binds.

       3. Hydrogen atoms are added syn (on the same side) to the alkyne, converting it to an alkane.

     • Example: Ethyne (acetylene) to ethane.

   • Partial Hydrogenation to cis-Alkenes:

     • Reagents: H₂, Lindlar’s catalyst (Pd/CaCO₃ poisoned with Pb(OAc)₂ and quinoline).

     • Mechanism:

       1. Hydrogen adsorbs onto Lindlar’s catalyst.

       2. The alkyne approaches and binds to the catalyst.

       3. Syn addition of hydrogen atoms to the alkyne, forming a cis-alkene.

     • Example: Ethyne to cis-ethylene.

   • Partial Hydrogenation to trans-Alkenes:

     • Reagents: Na or Li in liquid NH₃.

     • Mechanism:

       1. The metal donates an electron to the alkyne, forming a radical anion.

       2. The radical anion is protonated by NH₃, forming a trans-alkene.

       3. This sequence repeats to convert the alkyne to a trans-alkene.

     • Example: Ethyne to trans-ethylene.

2. Hydrohalogenation:

   • Reagents: HX (X = Cl, Br, I).

   • Mechanism:

     1. Protonation of the alkyne by HX forms a vinyl cation intermediate.

     2. The halide ion attacks the more substituted carbon of the vinyl cation.

   • Example: Addition of HBr to ethyne forms bromoethene and then dibromoethane.

3. Hydration:

   • Reagents: H₂O, H₂SO₄, HgSO₄.

   • Mechanism:

     1. Protonation of the alkyne by H₂SO₄ forms a vinyl cation.

     2. Nucleophilic attack by water forms an enol intermediate.

     3. The enol undergoes tautomerization to form a ketone.

   • Example: Ethyne to acetaldehyde.

4. Halogenation:

   • Reagents: X₂ (Cl₂, Br₂).

   • Mechanism:

     1. Formation of a halonium ion intermediate.

     2. Anti addition of a second halide ion to form a dihaloalkene.

     3. Further addition can occur to form a tetrahaloalkane.

   • Example: Addition of Br₂ to ethyne forms 1,2-dibromoethene and then 1,1,2,2-tetrabromoethane.

5. Oxidative Cleavage:

   • Reagents: O₃ followed by H₂O.

   • Mechanism:

     1. Ozone adds to the alkyne, forming an ozonide.

     2. Hydrolysis of the ozonide produces carboxylic acids.

   • Example: Ethyne to formic acid.

Benzene Reactions: Electrophilic Aromatic Substitution (EAS)

1. Nitration:

   • Reagents: HNO₃ and H₂SO₄.

   • Mechanism:

     1. Generation of the nitronium ion (NO₂⁺) from HNO₃ and H₂SO₄.

     2. Electrophilic attack by NO₂⁺ on the benzene ring, forming a sigma complex (arenium ion).

     3. Deprotonation restores aromaticity, yielding nitrobenzene.

   • Example: Benzene to nitrobenzene.

2. Sulfonation:

   • Reagents: H₂SO₄ or SO₃.

   • Mechanism:

     1. Generation of the electrophile (HSO₃⁺ or SO₃H⁺).

     2. Electrophilic attack by HSO₃⁺ on the benzene ring, forming a sigma complex.

     3. Deprotonation restores aromaticity, yielding benzene sulfonic acid.

   • Example: Benzene to benzene sulfonic acid.

3. Halogenation:

   • Reagents: X₂ (Cl₂, Br₂) with a Lewis acid catalyst (FeX₃, AlX₃).

   • Mechanism:

     1. Generation of the halonium ion (Cl⁺, Br⁺) with the Lewis acid.

     2. Electrophilic attack by Cl⁺ on the benzene ring, forming a sigma complex.

     3. Deprotonation restores aromaticity, yielding halobenzene.

   • Example: Benzene to chlorobenzene.

4. Friedel-Crafts Alkylation:

   • Reagents: R-Cl (alkyl halide) and AlCl₃.

   • Mechanism:

     1. Generation of the alkyl cation (R⁺) with AlCl₃.

     2. Electrophilic attack by R⁺ on the benzene ring, forming a sigma complex.

     3. Deprotonation restores aromaticity, yielding alkylbenzene.

   • Example: Benzene to toluene (using CH₃Cl).

5. Friedel-Crafts Acylation:

   • Reagents: RCOCl (acyl chloride) and AlCl₃.

   • Mechanism:

     1. Generation of the acylium ion (RCO⁺) with AlCl₃.

     2. Electrophilic attack by RCO⁺ on the benzene ring, forming a sigma complex.

     3. Deprotonation restores aromaticity, yielding acylbenzene.

   • Example: Benzene to acetophenone (using CH₃COCl).