Introductory Chemistry II Acids and Bases Lecture 6 PDF

CHEM 1820 INTRODUCTORY CHEMISTRY II ACIDS AND BASES DR. JOYANN MARKS LECTURE 6 EMAIL: [email protected] Last time... Protic and Aprotic solvents Dependence of acid or base character of substances on the solvent Lux Flood definition  Lewis acids When the oxo...

CHEM 1820 INTRODUCTORY CHEMISTRY II ACIDS AND BASES DR. JOYANN MARKS LECTURE 6 EMAIL: [email protected] Last time... Protic and Aprotic solvents Dependence of acid or base character of substances on the solvent Lux Flood definition  Lewis acids When the oxoacid B(OH)3 (Boric acid) is added to water it does not dissociate and donate a proton like H2SO4 ❑Instead boric acid accepts electrons from water and generates H3O+ in solution ❑B(OH)3 is a Lewis Acid Lewis acid and base definition- most general G. N. Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. This definition covers many systems where protons are not involved. This definition includes the Brönsted-Lowry and Solvent system definitions as special cases. Lewis acids and bases This definition includes the ‘standard” Brønsted-Lowry acid- base reactions: H+ (aq) + ∶NH3 (aq) → NH4+(aq) A + ∶B → A⎼B Lewis Acid + Lewis Base → Complex Lewis acids and bases an acid as an electron pair acceptor A base is an electron pair donor. The product of the reaction is called an adduct or a complex. A dative covalent bond is formed between nitrogen and boron BF3 (boron trifluoride) accepts the nitrogen lone pair electrons into a vacant orbital on boron, so it acts as a Lewis acid. Ammonia donates the lone pair into the boron orbital, so it acts as a Lewis base. Are BF3 and NH3 Bronsted Lowry acid and base?  Are BF3 and NH3 Bronsted Lowry acid and base? No! There is no proton transfer. Lewis definition also works with molecules that neither give up nor accept a hydrogen ion Lewis acids and bases What makes Boron a good Lewis acid for this reaction? What makes Ammonia a good base for the reaction? Where else have you seen Boron behave in this way?  Lewis acids and bases This definition also includes the reaction of metal ions with ligands to form coordination compounds: Ag+ (aq) + 2∶NH3 (aq) ⇌ Ag(NH3)2+(aq) Lewis Acid-Base adducts in which the Lewis acid is a metal ion are typically called coordination compounds. The anion or molecule interacting in this way with the metal ion is called a ligand.  Lewis acids and bases ❖ Metal cations are Lewis acids and ligands are Lewis bases ❖The terms Lewis acid and Lewis base are thermodynamic terms and relate to equilibria and the products of the reactions. When considering kinetics and reaction mechanisms it is more normal to refer to an electron pair donor as a nucleophile and an electron pair acceptor as an electrophile. Let’s test your skills... Identify the Lewis acids and bases in the following reactions: a) AsF5 + F- → AsF6- b) Cu2+ + 6NH3 → [Cu(NH3)6]2+ Let’s test your skills... Identify the Lewis acids and bases in the following reactions: a) AsF5 + F- → AsF6- Acid base b) Cu2+ + 6NH3 → [Cu(NH3)6]2+ Acid base STRATEGY: Identify the reactant that can donate an electron pair/acts as a Lewis base and the reactant that can interact with an electron pair and act as a Lewis acid Lewis acids and bases an acid as an electron pair acceptor A base is an electron pair donor. Cations more acidic than neutral molecules; Anions more basic than neutral molecules Lewis acidity or basicity is affected by the nature of substituents (electronic and steric effects) Lewis acids are electron deficient species or contain electron deficient sites Effect of electronic and steric effects Electron withdrawing substituents tend to decrease the Lewis basicity of basic sites while electron donating substituents tend to increase Lewis basicity by making them more electron rich. Electron withdrawing substituents tend to increase the Lewis acidity of acidic sites by making those sites more electron deficient while electron donating substituents tend decrease Lewis acidity by making sites less electron deficient. Lewis acids and bases an acid as an electron pair acceptor A base is an electron pair donor. Lewis acidity or basicity is affected by the nature of substituents (electronic and steric effects) Class Exercise: Arrange (CH3)3N, F3N and H3N in order of decreasing base strength. Explain. Arrange (CH3)3N, F3N and H3N in order of decreasing base strength. Explain. Most basic Least basic  Steric effects can influence the ability to form adducts https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Miessler_Fischer_Tarr)/06%3A_Acid-Base_and_Donor- Acceptor_Chemistry/6.04%3A_Lewis_Concept_and_Frontier_Orbitals/6.4.07%3A_Bulky_groups_weaken_the_strength_of_Lewis_acids_and_bases_because_they_introduce_st eric_strain_into_the_resulting_acid-base_adduct.#:~:text=Steric%20effects%20can%20influence%20the,centers%20to%20approach%20and%20interact. The orbital approach to Lewis acid base reactions ❑The frontier orbital concept conceptualizes chemical bonding and reactivity in terms of the interactions between frontier orbitals on the chemical species as they interact to form a bond or undergo a reaction. ❑Frontier orbitals are those at the frontier between occupied and unoccupied. They are often taken to be the highest energy occupied and lowest energy unoccupied molecular orbitals, called the HOMO and LUMO levels. ❑Lewis acid-base interaction involves an interaction between some of the frontier orbitals of the Lewis acid and base, specifically the donation of electrons from the bases' HOMO level into the acid's LUMO level.  BH3 and NH3 The formation of adduct between NH3 and BH3 involves the donation of electrons from ammonia's a1 HOMO into BH3's a2'' LUMO level. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Miessler_Fischer_Tarr)/06%3A_Acid-Base_and_Donor- Acceptor_Chemistry/6.04%3A_Lewis_Concept_and_Frontier_Orbitals/6.4.01%3A_The_frontier_orbital_approach_considers_Lewis_acid_base_reactions_in_terms_of_the_donation_of_electrons_from_the_base's_ highest_occupied_orbital_into_the_acid's_lowest_unoccupied_orbital.  acid adduct base ❑ A HOMO and a LUMO of appropriate symmetry combine to produce a bonding molecular orbital of lower energy in the adduct. ❑If there is a net lowering of energy, the adduct is stable.