Chapter 2- Polar Covalent Bonds; Acids and Bases Ashley Piekarski, Ph.D. Why do I care, Dr. P? In Chapter 1, we studied valence bond theory which uses hybrid orbitals to account for the observed shapes of organic molecules In Chapter 2, we will study how the electrons are distributed in covalent bonds and how that distribudon affects chemical reacdvity 1
Electronegativity What is electronegadvity? Electronegativity The electronegativity value! indicates the attraction of an atom for shared electrons! increases from left to right going across a period on the periodic table! decreases going down a group on the periodic table! is high for the nonmetals, with fluorine as the highest! is low for the metals 2
Electronegativity values High values Low values Ionic bond An ionic bond! occurs between metal and nonmetal ions! is a result of electron transfer! has a large electronegativity difference (1.8 or more). Examples: Atoms Electronegativity Type of Bond Difference Cl K 3.0 0.8 = 2.2 Ionic N Na 3.0 0.9 = 2.1 Ionic S Cs 2.5 0.7 = 1.8 Ionic 3
Nonpolar covalent bond A nonpolar covalent bond! occurs between nonmetals! has an equal or almost equal sharing of electrons! has almost no electronegativity difference (0.0 to 0.4) Examples: Atoms Electronegativity Type of Bond Difference N N 3.0 3.0 = 0.0 Nonpolar covalent Cl Br 3.0 2.8 = 0.2 Nonpolar covalent H Si 2.1 1.8 = 0.3 Nonpolar covalent Polar covalent bond A polar covalent bond! occurs between nonmetal atoms! has an unequal sharing of electrons! has a moderate electronegativity difference (0.5 to 1.7) Examples: Atoms Electronegativity Type of Bond Difference O Cl 3.5 3.0 = 0.5 Polar covalent Cl C 3.0 2.5 = 0.5 Polar covalent O S 3.5 2.5 = 1.0 Polar covalent 4
Electronegativity and bond types Methanol Draw the structure for methanol. What is the electronegadvity value for oxygen? What is the electronegadvity value for carbon? 5
Methanol- electrostatic map ElectrostaDc potendal maps show calculated charge distribudons Colors indicate electron- rich (red) and electron- poor (blue) regions Arrow indicate direcdon of bond polarity Bond polarity This is the basis of organic chemistry. Understanding the polarity of a molecule helps to know the reacdvity! Note: electrostadc potendal maps in textbook give you a clue to the electron- rich and electron- poor atoms in molecules Red: negadve Blue: posidve 6
Inductive effect InducDve effect: the shixing of electrons in a sigma bond in response to the electronegadvity of nearby atoms Metals inductively donate electrons Non-metals inductively withdraw electrons Learning check Assign δ+/δ- charges to show the direcdon of expected polarity. H 3 C-MgBr H 3 C-SH H 2 N-H 7
Dipole Moments Molecular polarity results from the vector summa,on of all individual bond polarides and lone- pair contribudons. The quantity measured is called a dipole moment, µ µ = Q r Q = charge r = distance Dipole Moments 8
Learning check An aromadc compound, benzene, has a dipole moment of zero. Why? Learning check Chloromethane, CH 3 Cl, has a dipole moment of 1.87. Make a three- dimensional drawing of methyl chloride and show the direcdon of the dipole moment. 9
Formal charges Formal charges are only electron bookkeeping and do NOT imply the presence of actual ionic charges gives clues to the chemical reactivity Formal Charge = (# of VE's in free atom)- # of bonding electrons 2 # of nonbonding electrons ( ) Learning check Calculate the formal charges for all the atoms in the acetate ion: 10
Resonance What is resonance? it is the way we describe electron delocalization in a compound that has pi bonding Resonance Hybrid A structure with resonance forms does not alternate between the forms Instead, it is a hybrid of the two resonance forms 11
Resonance rules! Rule 1: Individual resonance forms are imaginary, not real. Resonance rules! Rule 2: Resonance forms differ only in the placement of their pi or nonbonding electrons 12
Resonance rules! Rule 3: Different resonance forms of a substance don t have to be equivalent. Resonance rules! Rule 4: Resonance forms obey normal rules of valency 13
Learning check Draw the resonance forms for the 2,4- pentanedione axer it has reacted with a strong base. Acids and Bases According to the Brønsted Lowry theory,! acids donate a proton (H + )! bases accept a proton (H + ) 14
Acids and Bases In the reaction of ammonia and water,! NH 3 is the base that accepts H +! H 2 O is the acid that donates H + Learning check Draw the reacdon of acedc acid reacdng with sodium hydroxide. What is the acid and what is the base? What are the products formed? 15
Acid and Base Strength What is the generic reacdon scheme for a weak acid reacdng with water? What is the K a of this reacdon? Based on this expression, if your K a is very large is it a strong or weak acid? pka Do you remember how to find pk a? Would a stronger acid have a smaller or larger pk a? For convenience, acid strengths are expressed using pk a values In organic chemistry it is good to start learning the pk a of acids and their conjugate basesà gives you a clue to their reactivity 16
pk a Table Predicting acid-base reactions pk a values are related as logarithms to equilibrium constants Useful for predicdng whether a given acid- base reacdon will take place The stronger base holds on the proton more Dghtly 17
Learning check Organic acids Organic acids characterized by the presence of a positivelycharged hydrogen atom 18
Organic acids Those that lose a proton from O- H, such as methanol or acedc acid Those that lose a proton from C- H, usually from a carbon atom next to a C=O double bond Conjugate bases- electrostatic maps Which element now has a substandal amount of negadve charge axer deprotonadon? 19
Organic bases 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 a strong acid or as acids with strong bases Lewis Definition Lewis acids are electron pair acceptors and Lewis bases are electron pair donors The Lewis definidon leads to a general descripdon of many reacdon pajerns 20
Lewis acids The Lewis definidon of acidity includes metal cadons, such as Mg 2+ They accept a pair of electrons when they form a bond to a base Group 3A elements, such as BF 3 and AlCl 3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases TransiDon- metal compounds, such as TiCl 4, FeCl 3, ZnCl 2, and SnCl 4, are Lewis acids Organic compounds that undergo addidon reacdons with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids The combinadon of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid Lewis acid-base reaction 21
Note: curved arrows A curved arrow always means that a pair of electrons move from the atom at the tail of the arrow to the atom at the head of the arrow Lewis bases Lewis bases can accept protons as well as Lewis acids, therefore the definidon encompasses that for Brønsted bases Most oxygen- and nitrogen- containing organic compounds are Lewis bases because they have lone pairs of electrons 22
Noncovalent interactions Dipole- dipole forces Dispersion forces Hydrogen bonds Dipole-Dipole Occur between polar molecules as a result of electrostadc interacdons among dipoles Forces can be ajracdve or repulsive depending on orientadon of the molecules 23
Dispersion Forces Occur between all neighboring molecules and arise because the electron distribudon within molecules that are constantly changing Hydrogen Bond Forces Most important noncovalent interacdon in biological molecules Forces are a result of ajracdve interacdon between a hydrogen bonded to an electronegadve O or N atom and an unshared electron pair on another O or N atom 24
Applications Hydrogen bonding in DNA 25