CHEM 227 1st Edition Lecture 3 Outline of Last Lecture I. 1.12 Drawing Chemical StructuresII. 2.1 Bond Polarity and ElectronegativityIII. 2.2 Polar Covalent bonds: dipole momentsIV. 2.3 Formal ChargesV. 2.4 ResonanceVI. 2.5 Rules for Resonance FormsVII. 2.6 Drawing Resonance FormsOutline of Current Lecture I. 2.7 Acids and Bases: The Bronsted-Lowry DefinitionII. 2.8 Acid and Base strengthIII. 2.9 Predicting Acid-Base Reactions from pKa valuesIV. 2.10 Organic Acids and Organic basesV. 2.11 Acids and Bases: The Lewis DefinitionVI. 2.12 Noncovalent interactions between moleculesCurrent Lecture2.7 Acids and Bases: The Bronsted-Lowry Definition- Bronsted-Lowry defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors.- A Brønsted acid is a substance that donates a hydrogen ion (H+) - A Brønsted base is a substance that accepts the H+- Conjugate base: the product that results when the acid loses a proton- Conjugate acid: the product that results when the base gains a protonThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.2.8 Acid and Base Strength- Acidity constant: Ka: the exact strength of a given acid HA in water solution- pKa = -logKao The stronger the acid, the smaller the pKao The stronger the base, the larger the pKao Examples of some pKa values for acids and their conjugate bases:- pKa = –log Ka- The free energy in an equilibrium is related to –log of Keq (∆G = –RT log Keq)- A smaller value of pKa indicates a stronger acid and is proportional to the energy difference between products and reactants- The pKa of water is 15.742.9 Predicting Acid-Base Reactions from pKa values- pKa values are proportional to equilibrium constantso Useful for predicting whether a given acid-base reaction will take place- The difference in two pKa values is the log of the ratio of equilibrium constants, and can be used to calculate the extent of transfer- The stronger base holds the proton more tightly.- Another way to predict is to remember that the product conjugate acid in an acid-base reaction must be weaker and less reactive than the starting acid and the product conjugate base must be weaker and less reactive than the starting base. 2.10 Organic Acids and Organic Bases- Organic Acids: Characterized by the presence of a positively polarized hydrogen atomo Two kinds: 1. Those such as methanol and acetic acid that have hydrogen atom bonded to an electronegative oxygen atom (O-H) 2. Those such as acetone that contain hydrogen atoms bonded to a carbon atome next to a C=O bond (O=C-C-H)- These are acidic due to the fact that the conjugate base resulting from loss of H+ is stabilized by resonance- Organic Bases: characterized by the presence of an atom with a lone pair of electrons that can bond to H+ Nitrogen-containing compounds such as methylamine – these are the most common organic bases2.11 Acids and Bases: the Lewis Definition- Lewis Acid: substance that accepts an electron pairo A proton would be a lewis acid!- Lewis Base: substance that donates an electron pair- The Lewis definition of acidity includes metal cations, such as Mg2+ - They accept a pair of electrons when they form a bond to a base - Group 3A elements, such as BF3 and AlCl3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases- Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4, are Lewis acids- The direction of electron pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown with curved arrows- Lewis Bases: can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted baseso Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons2.12 Noncovalent interactions between molecules- Noncovalent interactions: there are a variety of interactions between molecules that strongly affect molecular properties- Dipole-Dipole forces: occur between polar molecules as a result of electrostatic interactions among dipoles. The forces can be either attractive or repulsive depending on the orientation of the molecules - Dispersion forces: occur between all neighboring molecules and arise because the electron distribution within molecules is constantly changing. Although uniform on a time-averaged basis, the electron distribution even in nonpolar molecules is likely to be non-uniform at any given instant- Hydrogen bond: Most important noncovalent interaction in biological molecules. An attractive interaction between hydrogen bonded to an electronegative O or N atom and an unshared electron pair on another O or N