CHE2201, Chapter 2 Learn, 1 Polar Covalent Bonds: Acids and Bases Chapter 2 Suggested Problems ,25-6,31,34,36,39- 40,51-2

CHE2201, Chapter 2 Learn, 2 Polar Covalent Bonds: Electronegativity Covalent bonds can have ionic character These are polar covalent bonds –Bonding electrons attracted more strongly by one atom than by the other –Electron distribution between atoms is not symmetrical

CHE2201, Chapter 2 Learn, 3 The Periodic Table and Electronegativity Electronegativity (EN): intrinsic ability of an atom to attract the shared electrons in a covalent bond Differences in EN produce bond polarity

CHE2201, Chapter 2 Learn, 4 Bond Polarity and Inductive Effect Nonpolar Covalent Bonds: atoms with similar EN Polar Covalent Bonds: Difference in EN of atoms < 2 Ionic Bonds: Difference in EN > 2 –C–H bonds, relatively nonpolar C-O, C-X bonds (more electronegative elements) are polar Inductive effect: shifting of electrons in a bond in response to EN of nearby atoms

CHE2201, Chapter 2 Learn, 5 Electrostatic Potential Maps Electrostatic potential maps show calculated charge distributions EN atom acquires partial negative charge,  - Colors indicate electron-rich (red) and electron-poor (blue) regions Arrows indicate direction of bond polarity

CHE2201, Chapter 2 Learn, 6 Dipole Moments of Molecules Molecules as a whole are often polar from vector summation of individual bond polarities and lone-pair contributions Strongly polar substances are soluble in polar solvents like water; nonpolar substances are insoluble in water.

CHE2201, Chapter 2 Learn, 7 Dipole Moments of Molecules

CHE2201, Chapter 2 Learn, 8 In symmetrical molecules, the dipole moments of each bond have one in the opposite direction The effects of the local dipoles cancel each other Absence of Dipole Moments

CHE2201, Chapter 2 Learn, 9 Large dipole moments –EN of O and N > H –Both O and N have lone-pair electrons oriented away from all nuclei Dipole Moments: O and N

CHE2201, Chapter 2 Learn, 10 Formal Charges Sometimes it is necessary to have structures with formal charges on individual atoms We compare the bonding of the atom in the molecule to the valence electron structure If the atom has one more electron in the molecule, it is shown with a “-” charge If the atom has one less electron, it is shown with a “+” charge Neutral molecules with both a “+” and a “-” are dipolar

CHE2201, Chapter 2 Learn, 11 Formal Charge for Dimethyl Sulfoxide Atomic sulfur has 6 valence electrons. Dimethyl sulfoxide sulfur has only 5. It has lost an electron and has positive charge. Oxygen atom in DMSO has gained electron and has negative charge.

CHE2201, Chapter 2 Learn, 12 Formal Charges Formal Charge = # of valence electrons – (# of bonds + # of lone-pair electrons)

CHE2201, Chapter 2 Learn, 13 Common Formal Charges

CHE2201, Chapter 2 Learn, 14 Some molecules have structures that cannot be shown with a single representation In these cases we draw structures that contribute to the final structure but which differ in the position of the  bond(s) or lone pair(s) Such a structure is delocalized and is represented by resonance forms The resonance forms are connected by a double-headed arrow Resonance

CHE2201, Chapter 2 Learn, 15 A structure with resonance forms does not alternate between the forms Instead, it is a hybrid of the resonance forms, so the structure is called a resonance hybrid For example, benzene (C 6 H 6 ) has two resonance forms with alternating double and single bonds – In the resonance hybrid, the actual structure, all its C-C bonds are equivalent, midway between double and single Resonance Hybrids

CHE2201, Chapter 2 Learn, 16 Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure) Resonance forms differ only in the placement of their  or nonbonding electrons Resonance forms must be valid Lewis structures: the octet rule generally applies The resonance hybrid is more stable than any individual resonance form would be –The more resonance forms that can be drawn for a molecule, the more stable the molecule Different resonance forms of a substance do not have to be equivalent Rules for Resonance Forms

CHE2201, Chapter 2 Learn, 17 We can imagine that electrons move in pairs to convert from one resonance form to another A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow Curved Arrows and Resonance Forms

CHE2201, Chapter 2 Learn, 18 Any three-atom grouping with a p orbital on each atom has two resonance forms Drawing Resonance Forms

CHE2201, Chapter 2 Learn, 19 Sometimes resonance forms involve different atom types as well as locations The resulting resonance hybrid has properties associated with both types of contributors The types may contribute unequally The “enolate” derived from acetone is a good illustration, with delocalization between carbon and oxygen Different Atoms in Resonance Forms

CHE2201, Chapter 2 Learn, 20 The anion derived from 2,4-pentanedione –Lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O bond on the left and on the right –Three resonance structures result 2,4-Pentanedione

CHE2201, Chapter 2 Learn, 21 An Acid Loses a Proton A Base Gains a Proton

CHE2201, Chapter 2 Learn, 22 Most Acid–Base Reactions are Reversible

CHE2201, Chapter 2 Learn, 23 An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid.

CHE2201, Chapter 2 Learn, 24 When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid. The stronger the acid, the weaker its conjugate base. An Acid and its Conjugate Base A Base and its Conjugate Acid

CHE2201, Chapter 2 Learn, 25 Acids Have Different Strengths

CHE2201, Chapter 2 Learn, 26 The Acid Dissociation Constant (K a ) is a Measure of the Extent of Dissociation of an Acid Put the constants on the same side of the equation.

CHE2201, Chapter 2 Learn, 27 The stronger the acid, the larger the K a. The stronger the acid, the smaller the pK a. Acid Strength pK a = –log K a

CHE2201, Chapter 2 Learn, 28 pK a ’s of Some Common Acids

CHE2201, Chapter 2 Learn, 29

CHE2201, Chapter 2 Learn, 30 Which Reactant is the Acid? Water is the base.Water is the acid. The stronger acid behaves as the acid.

CHE2201, Chapter 2 Learn, 31 The Position of Equilibrium The equilibrium favors formation of the weaker acid.

CHE2201, Chapter 2 Learn, 32 pK a values are useful for predicting whether a given acid-base reaction will take place The stronger the acid, the weaker its conjugate base The stronger base holds the proton more tightly Where does this equilibrium lie? Predicting Acid–Base Reactions from pK a Values

CHE2201, Chapter 2 Learn, 33 The Stronger the Acid, the Weaker Its Conjugate Base stable bases are weak bases

CHE2201, Chapter 2 Learn, 34 Organic Acids: -characterized by the presence of positively polarized hydrogen atom Organic Acids and Organic Bases

CHE2201, Chapter 2 Learn, 35 Those that lose a proton from O–H, such as methanol and acetic acid Those that lose a proton from C–H, usually from a carbon atom next to a C=O double bond (O=C–C–H) Organic Acids

CHE2201, Chapter 2 Learn, 36 Have an atom with a lone pair of electrons that can bond to H + Nitrogen-containing compounds derived from ammonia are the most common organic bases Oxygen-containing compounds can react as bases when with a strong acid or as acids with strong bases Organic Bases

CHE2201, Chapter 2 Learn, 37 Lewis Acids and Bases Lewis definitions: acid: a species that accepts an electron pair from another atom base: a species that donates an electron pair to another atom All Brønsted acids are Lewis acids. All Brønsted bases are Lewis bases.

CHE2201, Chapter 2 Learn, 38 Lewis Acids and Bases

CHE2201, Chapter 2 Learn, 39 Some Lewis Acids

CHE2201, Chapter 2 Learn, 40 Some Lewis Bases

CHE2201, Chapter 2 Learn, 41 Organic chemistry is 3-D space Molecular shape is critical in determining the chemistry a compound undergoes in the lab, and in living organisms Molecular Models

CHE2201, Chapter 2 Learn, 42 Several types: -Dipole-dipole forces -Dispersion forces -Hydrogen bonds Noncovalent Interactions When thinking about chemical reactivity, chemists typically focus on bonds, the covalent interactions between atoms within molecules. Also important are the interactions between molecules that affect molecular properties.

CHE2201, Chapter 2 Learn, 43  Occur between polar molecules as a result of electrostatic interactions among dipoles  Forces can be attractive or repulsive depending on orientation of the molecules Dipole-Dipole

CHE2201, Chapter 2 Learn, 44  Occur between all neighboring molecules and arise because the electron distribution within molecules that is constantly changing Dispersion Forces

CHE2201, Chapter 2 Learn, 45  Most important noncovalent interaction in biological molecules  Forces are result of attractive interaction between a hydrogen bonded to an electronegative atom and an unshared electron pair on another O or N atom Hydrogen Bond Forces

CHE2201, Chapter 2 Learn, 46 Hydrogen Bonds in Biological Molecules

CHE2201, Chapter 2 Learn, 47 Electronegativity Affects pK a Values When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom.

CHE2201, Chapter 2 Learn, 48 The Stronger the Acid, the Weaker Its Conjugate Base stable bases are weak bases

CHE2201, Chapter 2 Learn, 49 Why are Alcohols Stronger Acids Than Amines? Oxygen is more electronegative than nitrogen.

CHE2201, Chapter 2 Learn, 50 Why Are Protonated Alcohols Stronger Acids Than Protonated Amines? Oxygen is more electronegative than nitrogen.

CHE2201, Chapter 2 Learn, 51 Hybridization Affects Electronegativity The weakest acid has the strongest conjugate base.

CHE2201, Chapter 2 Learn, 52 Size Affects pK a Values When atoms differ in size, the strongest acid has its hydrogen bonded to the largest atom.

CHE2201, Chapter 2 Learn, 53 Some pK a Values

CHE2201, Chapter 2 Learn, 54 Substituents Affect the Strength of the Acid inductive electron withdrawal

CHE2201, Chapter 2 Learn, 55 A Substituent’s Effect on pK a Depends on Distance

CHE2201, Chapter 2 Learn, 56 Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 1. inductive electron withdrawal

CHE2201, Chapter 2 Learn, 57 Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 2. delocalized electrons (resonance)

CHE2201, Chapter 2 Learn, 58 electronegativity and size Summary of Factors That Affect Acid Strength

CHE2201, Chapter 2 Learn, 59 Summary of Factors That Affect Acid Strength hybridization

CHE2201, Chapter 2 Learn, 60 Summary of Factors That Affect Acid Strength inductive electron withdrawal

CHE2201, Chapter 2 Learn, 61 electron delocalization Summary of Factors That Affect Acid Strength

CHE2201, Chapter 2 Learn, 62 Rank the following substances in order of increasing acidity. Let’s Work a Problem

CHE2201, Chapter 2 Learn, 63 Do not make things too hard if we don’t have to. The lower the value of the pKa, the more acidic the molecule is: Answer