lecture 4 - Arrhenius Acids and Bases Acids and Bases...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Acids Acids and and Bases Bases Chapter 4 Arrhenius Acids and Bases Arrhenius Acids and Bases acid: acid: a substance that produces H 3 O + ions aqueous solution base: base: a substance that produces OH - ions in aqueous solution Br ønsted nsted -Lowry Definitions Lowry Definitions ± Acid: Acid: a proton donor ± Base: Base: a proton acceptor Br Br ønsted nsted -Lowry Lowry (acids/bases) (acids/bases) + Proton donor Proton acceptor - OH H O H H H+ + O H H + Proton acceptor Proton donor + H O H H NH H H H H + N H H : :: : : : : Conjugate Acids & Bases conjugate base conjugate base: the species formed from an acid when it donates a proton to a base conjugate acid: conjugate acid: the species formed from a base when it accepts a proton from an acid acid -base reaction: base reaction: a proton-transfer reaction conjugate acid -base pair: base pair: any pair of molecules or ions that can be interconverted by transfer of a proton
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Brønsted-Lowry definitions do not require water as a reactant Consider the reaction between acetic acid and ammonia We use curved arrows to show the We use curved arrows to show the flow of electrons in an acid flow of electrons in an acid -base base reaction reaction In this reaction ethanol is the acid (proton donor), and amide ion (NH 2 - ) is the base (proton acceptor) Conjugate Acids & Bases 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
Background image of page 2
3 Conjugate Acids & Bases Conjugate Acids & Bases ± question: which oxygen of a carboxylic acid is protonated? (There are 2 possibilities) Conjugate Acids & Bases Conjugate Acids & Bases ± for protonation on the carbonyl oxygen, we can write three contributing structures ± two place the positive charge on oxygen, one places it on carbon + CH 3 -C-O-H O H 3 -C O H + -C-O-H 3 -C=O-H O H + A-1 (C and O have complete octets) A-2 (C has incomplete octet) A-3 (C and O have complete octets) Conjugate Acids & Bases ± A-1 and A-3 make the greater contribution because all atoms have complete octets ± the positive charge is delocalized over three atoms with the greater share on the two equivalent oxygens + 3 -C-O-H O H 3 O H + -C-O-H 3 -C=O-H O H + A-1 (C and O have complete octets) A-2 (C has incomplete octet) A-3 (C and O have complete octets) Conjugate Acids & Bases Conjugate Acids & Bases ± for protonation on the hydroxyl oxygen, we can write two contributing structures 3 -C-O-H O H + B-1 3 -C-O-H O H + + B-2 (charge separation and adjacent positive charges) Conjugate Acids & Bases ± B-2 makes only a minor contribution because of charge separation and adjacent positive charges ± therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen 3 -C-O-H O H + B-1 3 -C-O-H O H + + B-2 (charge separation and Conjugate Acids & Bases Conjugate Acids & Bases ± Problem 4.3 Problem 4.3 Does proton transfer to an amide group occur preferentially on the amide oxygen or the amide nitrogen?
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

Page1 / 13

lecture 4 - Arrhenius Acids and Bases Acids and Bases...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online