Acids and Bases Unit 2

Arrhenius Acids and Bases In 1884, Svante Arrhenius proposed these definitions acid: a substance that produces H3O+ ions aqueous solution base: a substance that produces OH- ions in aqueous solution this definition of an acid is a slight modification of the original Arrhenius definition, which was that an acid produces H+ in aqueous solution today we know that H+ reacts immediately with a water molecule to give a hydronium ion (H3O+)

Brønsted-Lowry Definitions Acid: a proton (H+) donor Base: a proton acceptor

Conjugate Acids & Bases conjugate base: the species formed from an acid when it donates a proton to a base conjugate acid: the species formed from a base when it accepts a proton from an acid acid-base reaction: a proton-transfer reaction conjugate acid-base pair: any pair of molecules or ions that can be interconverted by transfer of a proton

Conjugate Acids & Bases Brønsted-Lowry definitions do not require water as a reactant consider the following reaction between acetic acid and ammonia

Conjugate Acids & Bases we can use curved arrows to show the flow of electrons in an acid-base reaction

Conjugate Acids & Bases Many organic molecules have two or more sites that can act as proton acceptors in these molecules, the favored site of protonation is the one in which the charge is more delocalized question: which oxygen of a carboxylic acid is protonated?

Weak Acids and Bases We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as: water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L we can combine these constants to give a new constant, Ka, called an acid dissociation constant

Weak Acids and Bases

Molecular Structure and Acidity The overriding principle in determining the relative acidities of uncharged organic acids is the stability of the anion, A-, resulting from the loss of a proton the more stable the anion, the greater the acidity of HA Ways to stabilize anions include having the negative charge on a more electronegative atom on a larger atom delocalized through resonance delocalized by the inductive effect in an orbital with more s character

Effect of Electronegativity on pKa As the bond to H becomes more polarized, H becomes more positive and the bond is easier to break.

Effect of Size on pKa As size increases, the H is more loosely held and the bond is easier to break. A larger size also stabilizes the anion.

Effect of Resonance on pKa If the negative charge on an atom can be delocalized over two or more atoms, the acidity of that compound will be greater than when the negative charge cannot be delocalized. The ethoxide anion is less acidic than the acetate ion simply because the acetate ion can delocalize the negative charge. Methanesulfonic acid can delocalize the charge in three different resonance forms, making it more acidic than the acetate ion.

Lewis Acids and Bases Lewis definition Acids accept electrons. (electrophile) Bases donate electrons. (nucleophile) Hint for recognizing the base – look for electrons!!!! Either a lone pair or pi bonded electrons seek out electrophiles.

Lewis Acids and Bases Acid-base reactions can take place with or without a proton. Lewis bases are species that are able to donate a pair of electrons - called nucleophiles (lover of nuclei). Lewis acids are species that can accept this same pair of electrons – called electrophiles (lover of electrons). The reactions are drawn using curved arrow formalism (movement of electrons represented with arrows).

Lewis Acids and Bases Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of electrons Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of electrons - + A + : B A B new covalent bond formed in this Lewis acid-base reaction Lewis acid Lewis base

Lewis Acids and Bases : H H : B r : + : C H - C - H + : B r : C H - C 3 - C - H 3 + : B r - : C H 3 - C - H 3 : H H H H sec- Butyl cation (a carbocation) Bromide ion 2-Bromobutane C H 3 2 C H 3 2 F F + - : O : + B F : O B - F C H 3 2 F C H 3 2 F Diethyl ether (a Lewis base) Boron trifluoride (a Lewis acid) A BF 3 -ether complex