Slide 1: Chemistry of p-Block Elements - Group 13 - Trihalides - Lewis acids

  • Group 13 elements include boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).
  • These elements have three valence electrons in their outermost shell.
  • The trihalides of these elements, namely boron trihalides (BX₃), aluminum trihalides (AlX₃), gallium trihalides (GaX₃), indium trihalides (InX₃), and thallium trihalides (TlX₃), where X represents a halogen (F, Cl, Br, or I), are important compounds.
  • In these trihalides, the central atom is surrounded by three halogen atoms, forming a trigonal planar structure.
  • These compounds act as Lewis acids due to the presence of an incomplete octet on the central atom.

Slide 2: Lewis Acids and Bases

  • Lewis acids are electron pair acceptors, while Lewis bases are electron pair donors.
  • Lewis acid-base reactions involve the formation of coordinate bonds, where the Lewis acid accepts a lone pair of electrons from the Lewis base.
  • In the context of group 13 trihalides, the central atom acts as the Lewis acid by accepting a lone pair of electrons from a Lewis base.
  • The Lewis acid-base interaction leads to the formation of a coordinate covalent bond.

Slide 3: Boron Trihalides (BX₃)

  • Boron trihalides include boron trifluoride (BF₃), boron trichloride (BCl₃), boron tribromide (BBr₃), and boron triiodide (BI₃).
  • These compounds are Lewis acids due to the electron deficiency on the boron atom.
  • Examples:
    • BF₃ + :NH₃ → BF₃:NH₃ (Ammonia donates a lone pair of electrons to boron trifluoride)
    • BCl₃ + :NH₃ → BCl₃:NH₃ (Ammonia donates a lone pair of electrons to boron trichloride)
    • BBr₃ + :NH₃ → BBr₃:NH₃ (Ammonia donates a lone pair of electrons to boron tribromide)
    • BI₃ + :NH₃ → BI₃:NH₃ (Ammonia donates a lone pair of electrons to boron triiodide)

Slide 4: Aluminum Trihalides (AlX₃)

  • Aluminum trihalides include aluminum trifluoride (AlF₃), aluminum trichloride (AlCl₃), aluminum tribromide (AlBr₃), and aluminum triiodide (AlI₃).
  • These compounds are Lewis acids due to the electron deficiency on the aluminum atom.
  • Examples:
    • AlF₃ + :NH₃ → AlF₃:NH₃ (Ammonia donates a lone pair of electrons to aluminum trifluoride)
    • AlCl₃ + :NH₃ → AlCl₃:NH₃ (Ammonia donates a lone pair of electrons to aluminum trichloride)
    • AlBr₃ + :NH₃ → AlBr₃:NH₃ (Ammonia donates a lone pair of electrons to aluminum tribromide)
    • AlI₃ + :NH₃ → AlI₃:NH₃ (Ammonia donates a lone pair of electrons to aluminum triiodide)

Slide 5: Gallium Trihalides (GaX₃)

  • Gallium trihalides include gallium trifluoride (GaF₃), gallium trichloride (GaCl₃), gallium tribromide (GaBr₃), and gallium triiodide (GaI₃).
  • These compounds are Lewis acids due to the electron deficiency on the gallium atom.
  • Examples:
    • GaF₃ + :NH₃ → GaF₃:NH₃ (Ammonia donates a lone pair of electrons to gallium trifluoride)
    • GaCl₃ + :NH₃ → GaCl₃:NH₃ (Ammonia donates a lone pair of electrons to gallium trichloride)
    • GaBr₃ + :NH₃ → GaBr₃:NH₃ (Ammonia donates a lone pair of electrons to gallium tribromide)
    • GaI₃ + :NH₃ → GaI₃:NH₃ (Ammonia donates a lone pair of electrons to gallium triiodide)

Slide 6: Indium Trihalides (InX₃)

  • Indium trihalides include indium trifluoride (InF₃), indium trichloride (InCl₃), indium tribromide (InBr₃), and indium triiodide (InI₃).
  • These compounds are Lewis acids due to the electron deficiency on the indium atom.
  • Examples:
    • InF₃ + :NH₃ → InF₃:NH₃ (Ammonia donates a lone pair of electrons to indium trifluoride)
    • InCl₃ + :NH₃ → InCl₃:NH₃ (Ammonia donates a lone pair of electrons to indium trichloride)
    • InBr₃ + :NH₃ → InBr₃:NH₃ (Ammonia donates a lone pair of electrons to indium tribromide)
    • InI₃ + :NH₃ → InI₃:NH₃ (Ammonia donates a lone pair of electrons to indium triiodide)
  1. Aluminum Trihalides (AlX₃)
  • Aluminum trihalides include aluminum trifluoride (AlF₃), aluminum trichloride (AlCl₃), aluminum tribromide (AlBr₃), and aluminum triiodide (AlI₃).
  • These compounds are Lewis acids due to the electron deficiency on the aluminum atom.
  • They are commonly used as catalysts in various chemical reactions.
  • AlCl₃ is widely used in Friedel-Crafts reactions to introduce alkyl or acyl groups onto aromatic compounds.
  • Example reaction: AlCl₃ + CH₃Cl → CH₃AlCl₂ + HCl
  • The Lewis acid-base interaction between aluminum trichloride and chloromethane leads to the formation of a compound with a new carbon-aluminum bond.
  1. Gallium Trihalides (GaX₃)
  • Gallium trihalides include gallium trifluoride (GaF₃), gallium trichloride (GaCl₃), gallium tribromide (GaBr₃), and gallium triiodide (GaI₃).
  • These compounds are Lewis acids due to the electron deficiency on the gallium atom.
  • GaCl₃ is used as a catalyst in the production of polyethylene terephthalate (PET) plastics.
  • Example reaction: GaCl₃ + H₂O + CH₃OH → Ga(OCH₃)₃ + 3HCl
  • The Lewis acid-base interaction between gallium trichloride, water, and methanol leads to the formation of gallium methylate and hydrochloric acid.
  1. Indium Trihalides (InX₃)
  • Indium trihalides include indium trifluoride (InF₃), indium trichloride (InCl₃), indium tribromide (InBr₃), and indium triiodide (InI₃).
  • These compounds are Lewis acids due to the electron deficiency on the indium atom.
  • InI₃ is used as a sensitizer in the production of photographic film.
  • Example reaction: InI₃ + Ag → In + AgI₃
  • The Lewis acid-base interaction between indium triiodide and silver leads to the reduction of silver iodide and the formation of indium metal.
  1. Thallium Trihalides (TlX₃)
  • Thallium trihalides include thallium trifluoride (TlF₃), thallium trichloride (TlCl₃), thallium tribromide (TlBr₃), and thallium triiodide (TlI₃).
  • These compounds are Lewis acids due to the electron deficiency on the thallium atom.
  • TlCl₃ and TlBr₃ are used as precursors in the synthesis of organometallic compounds.
  • Example reaction: 3TlCl₃ + 2Na(C₅H₅) → 3Tl(C₅H₅) + 6NaCl
  • The Lewis acid-base interaction between thallium trichloride, sodium cyclopentadienide, and sodium chloride leads to the formation of thallium cyclopentadienide and sodium chloride.
  1. Application of Lewis Acids in Organic Synthesis
  • Lewis acids find extensive application in organic synthesis as catalysts.
  • They can facilitate the activation of certain functional groups and promote desired reactions.
  • The Friedel-Crafts reaction is one such example where aluminum or boron trihalides act as catalysts to introduce alkyl or acyl groups onto aromatic compounds.
  • Lewis acids are also used in the Diels-Alder reaction, where they can coordinate with dienophiles and dienes to facilitate the formation of cyclic products.
  • Examples: AlCl₃, BCl₃, FeCl₃, and ZnCl₂ are commonly used Lewis acids in organic synthesis.
  1. Limitations of Lewis Acids
  • Lewis acids are not suitable for all types of reactions and can sometimes exhibit limitations.
  • They may be too reactive and cause undesired side reactions.
  • The electron-deficient nature of Lewis acids can result in strong Lewis acid-base interactions, leading to the formation of stable complexes that might not readily dissociate.
  • Some Lewis acids may be toxic or hazardous, requiring careful handling.
  • It is essential to consider the specific reaction conditions and the compatibility of Lewis acids with the other reactants to achieve the desired outcome.
  1. Introduction to Lewis Bases
  • Lewis bases are electron pair donors in Lewis acid-base reactions.
  • They have a lone pair of electrons available for donation.
  • Common examples of Lewis bases include ammonia (NH₃), water (H₂O), alcohols (ROH), and amines (RNH₂).
  • The Lewis base donates its lone pair of electrons to the Lewis acid, resulting in the formation of a coordinate covalent bond.
  1. Lewis Acid-Base Reactions
  • Lewis acid-base reactions involve the transfer of a lone pair of electrons from the Lewis base to the Lewis acid.
  • This transfer leads to the formation of a coordinate covalent bond between the two species.
  • The Lewis acid-base interaction is often reversible, allowing the products to dissociate back into their respective reactants.
  • The strength of the Lewis acid-base interaction depends on factors such as electron density, size, and charge of the species involved.
  1. Lewis Acid-Base Addition Mechanism
  • In Lewis acid-base addition reactions, the Lewis acid and the Lewis base directly react with each other.
  • The Lewis acid accepts the lone pair of electrons from the Lewis base, resulting in bond formation.
  • The addition mechanism is often observed in reactions involving boron, aluminum, and other p-block elements.
  • Example reaction: BCl₃ + :NH₃ → BCl₃:NH₃
  • The Lewis base (:NH₃) donates a lone pair of electrons to boron trichloride (BCl₃), forming a coordinate covalent bond.
  1. Summary of Lewis Acids and Bases
  • Lewis acids are electron pair acceptors, while Lewis bases are electron pair donors.
  • Group 13 trihalides, such as boron, aluminum, gallium, indium, and thallium trihalides, are Lewis acids due to their electron deficiency.
  • Lewis acids find extensive application as catalysts in various chemical reactions.
  • Lewis acid-base reactions involve the formation of coordinate covalent bonds.
  • The strength of the Lewis acid-base interaction depends on various factors like electron density, size, and charge.

Slide 21: Lewis Acid-Base Mechanism

  • Lewis acid-base reactions involve the formation of coordinate covalent bonds between the Lewis acid and the Lewis base.
  • The Lewis acid acts as an electron pair acceptor, while the Lewis base acts as an electron pair donor.
  • The reaction can be represented as: Lewis Acid + Lewis Base → Lewis Acid-Base Adduct
  • The adduct is a stable compound where the Lewis acid and the Lewis base are bonded through the shared electron pair.

Slide 22: Lewis Acid-Base Complexes

  • Lewis acid-base complexes are formed when the Lewis base donates a lone pair of electrons to the Lewis acid.
  • These complexes have enhanced stability due to the formation of coordinate covalent bonds.
  • The Lewis acid and the Lewis base in the complex are held together by electrostatic forces.
  • The stability of the complexes can be influenced by factors such as the strength of the Lewis acid-base interaction and the polarity of the molecules involved.
  • As we move down the group 13 in the periodic table, the Lewis acidity of the trihalides decreases.
  • Boron trihalides are the strongest Lewis acids in the group, while thallium trihalides are the weakest.
  • This trend is attributed to the increase in atomic size and the decrease in electronegativity as we move down the group.
  • The larger size and lower electronegativity of the lower group elements result in decreased electron deficiency and decreased ability to accept electron pairs.
  • The Lewis acidity of group 13 elements in period 3 (aluminum, gallium, and indium) increases as we move from left to right across the period.
  • This trend is associated with the decreasing atomic size and increasing charge-to-size ratio of the elements.
  • The smaller size and higher charge-to-size ratio make the central atom more electron-deficient, enhancing its ability to accept electron pairs and act as a Lewis acid.

Slide 25: Application of Group 13 Trihalides as Catalysts

  • Group 13 trihalides, particularly aluminum and boron trihalides, find extensive application as Lewis acid catalysts in organic synthesis.
  • They can activate certain functional groups, promote desired reactions, and enhance reaction rates.
  • Examples of their applications include Friedel-Crafts reactions, Diels-Alder reactions, and polymerization reactions.
  • Aluminum trichloride (AlCl₃) is commonly used in the synthesis of dyes and perfumes, while boron trifluoride (BF₃) is used in organic synthesis and as a catalyst for polymerization reactions.

Slide 26: Limitations of Group 13 Trihalides as Catalysts

  • While group 13 trihalides are valuable catalysts, they also have certain limitations.
  • They can exhibit high reactivity and may cause undesired side reactions.
  • Some trihalides, such as boron trifluoride (BF₃), are highly volatile and require careful handling.
  • Overuse of Lewis acid catalysts can lead to overfunctionalization or crosslinking in reaction products.
  • It is crucial to optimize reaction conditions and catalyst concentrations to achieve the desired outcome.

Slide 27: Environmental Impact of Group 13 Trihalides

  • Group 13 trihalides, especially those containing fluorine and chlorine, can have environmental impacts.
  • Fluorinated trihalides, such as boron trifluoride (BF₃) and aluminum trifluoride (AlF₃), are potent greenhouse gases with high global warming potentials.
  • Chlorinated trihalides, such as aluminum trichloride (AlCl₃), can contribute to water pollution if not properly managed.
  • It is important to handle and dispose of these compounds responsibly to minimize their impact on the environment.

Slide 28: Boron Trifluoride - Successful Applications

  • Boron trifluoride (BF₃), one of the group 13 trihalides, has found successful applications in various fields.
  • It is widely used as a Lewis acid catalyst in organic synthesis, particularly in reactions involving alcohols and carboxylic acids.
  • BF₃ is an essential component in the production of high-quality aluminum and magnesium metal.
  • It is also used in the petroleum industry for the refining and purification of hydrocarbon fuels.
  • Additionally, BF₃ has applications in the production of plastics, detergents, and pharmaceutical intermediates.

Slide 29: Aluminum Trichloride - Successful Applications

  • Aluminum trichloride (AlCl₃), an important member of group 13 trihalides, has numerous successful applications.
  • It is widely used as a catalyst in the production of polyethylene terephthalate (PET) plastics.
  • AlCl₃ is an essential ingredient in the Friedel-Crafts reaction, enabling the synthesis of various aromatic compounds.
  • It also finds applications in the synthesis of dyes, fragrances, and pharmaceuticals.
  • Furthermore, aluminum trichloride is used as an electrolyte in the production of aluminum metal.

Slide 30: Summary

  • Group 13 trihalides, including boron, aluminum, gallium, indium, and thallium trihalides, act as Lewis acids due to their electron deficiency.
  • Lewis acid-base reactions involve the formation of coordinate covalent bonds between the Lewis acid and the Lewis base.
  • The strength of the Lewis acid-base interaction depends on factors such as electron density, size, and charge.
  • Group 13 trihalides find extensive application as catalysts in various organic synthesis reactions.
  • It is essential to consider the limitations and environmental impact of these compounds in their applications.
  • Each member of the group has specific successful applications, such as boron trifluoride in organic synthesis and aluminum trichloride in PET plastic production.