2. Polar Covalent Bonds: Acids and Bases Why this chapter? Description of basic ways chemists...

2. Polar Covalent Bonds: 2. Polar Covalent Bonds: Acids and BasesAcids and Bases

Why this chapter?Why this chapter?Description of basic ways chemists account for chemical reactivity.

Establish foundation for understanding specific reactions discussed in subsequent chapters.

2

2.1 Polar Covalent Bonds: 2.1 Polar Covalent Bonds: ElectronegativityElectronegativity

Covalent bonds can have ionic characterThese are polar covalent bonds

◦ Bonding electrons attracted more strongly by one atom than by the other

◦ Electron distribution between atoms is not symmetrical

3

Bond Polarity and ElectronegativityBond Polarity and Electronegativity

Electronegativity (EN): intrinsic ability of an atom to attract the shared electrons in a covalent bond

Differences in EN produce bond polarityArbitrary scale. As shown in Figure 2.2,

electronegativities are based on an arbitrary scale

F is most electronegative (EN = 4.0), Cs is least (EN = 0.7)

Metals on left side of periodic table attract electrons weakly, lower EN

Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly, higher electronegativities

EN of C = 2.5

4

The Periodic Table and ElectronegativityThe Periodic Table and Electronegativity

5

Bond Polarity and Inductive EffectBond Polarity and Inductive EffectNonpolar Covalent Bonds: atoms with similar ENPolar 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

Bonding electrons toward electronegative atom◦ C acquires partial positive charge, +◦ Electronegative atom acquires partial negative

charge, - Inductive effect: shifting of electrons in a bond in

response to EN of nearby atoms

CO O

Polarity, ShapePolarity, Shape

3.53.5 3.53.52.52.5

O=C=O++ ----

Polar Covalent Bonds

Linear Shape (180Linear Shape (180oo))Linear Shape (180Linear Shape (180oo))

Electronegativity Electronegativity DifferenceDifference

< 0.5< 0.5 Nonpolar Nonpolar

0.5-1.70.5-1.7 Polar Polar

>>1.81.8 Ionic Ionic

Nonpolar CompoundNonpolar Compound

COCO22

CH H

O


O=C=O++ ----

e-’s in 2 directions = 180o

LinearLinear

e-’s in 3 directions = 120o

++

-- Trigonal planarTrigonal planar


Polar CompoundPolar Compound

FF B


4.04.0

4.04.0

2.02.0 ++-- --

Polar Covalent Bonds

(120(120oo))Trigonal PlanarTrigonal Planar

(120(120oo))Trigonal PlanarTrigonal Planar


BFBF33

F4.04.0F

B

F

F--


CH HH

Cl

CH H

H

ClH

C

H

ClH

OH H

H-O-H

O

H

H

++

++--

++--

e-’s in 4 directions = 109.5o

TetrahedralTetrahedral

BentBent

4 directions = 109.5o


NH HH ++

--e-’s in 4 directions = 109.5o

PyramidalPyramidal

NH H

HH

NHH++

++

(109.5(109.5oo))Tetrahedral Configuration of ElectronsTetrahedral Configuration of Electrons

Trigonal Pyramid Trigonal Pyramid Configuration of AtomsConfiguration of Atoms

(109.5(109.5oo))Tetrahedral Configuration of ElectronsTetrahedral Configuration of Electrons

Trigonal Pyramid Trigonal Pyramid Configuration of AtomsConfiguration of Atoms

Tetrahedral electron-pair Tetrahedral electron-pair GeometriesGeometries

TetrahedralTetrahedral

PyramidalPyramidalBentBent

15

Electrostatic Potential MapsElectrostatic Potential MapsElectrostatic

potential maps show calculated charge distributions

Colors indicate electron-rich (red) and electron-poor (blue) regions

Arrows indicate direction of bond polarity

16

2.2 Polar Covalent Bonds: Dipole Moments2.2 Polar Covalent Bonds: Dipole Moments Molecules as a whole are often polar from vector summation of individual

bond polarities and lone-pair contributions Strongly polar substances soluble in polar solvents like water; nonpolar

substances are insoluble in water. Dipole moment () - Net molecular polarity, due to difference in

summed charges◦ - magnitude of charge Q at end of molecular dipole times distance r

between charges = Q r, in debyes (D), 1 D = 3.336 1030 coulomb meter length of an average covalent bond, the dipole moment would be 1.60 1029

Cm, or 4.80 D.

17

Dipole Moments in Water and AmmoniaDipole Moments in Water and AmmoniaLarge dipole moments

◦ EN of O and N > H◦ Both O and N have lone-pair electrons oriented away

from all nuclei

18

Absence of Dipole MomentsAbsence of Dipole Moments

In symmetrical molecules, the dipole moments of each bond has one in the opposite direction

The effects of the local dipoles cancel each other

Which of the above molecules is the Which of the above molecules is the most polar?most polar?1. A2. B3. C4. D5. It cannot be

determined

F

H

H

F

F

H

F

H

H

H

F

F

F

F

F

F

A B C D

Learning Check:Learning Check:

Which of the above molecules is the Which of the above molecules is the most polar?most polar?1. A2. B3. C4. D5. It cannot be

determined

F

H

H

F

F

H

F

H

H

H

F

F

F

F

F

F

A B C D

Solution:Solution:


21

Which of the following molecules is Which of the following molecules is nonpolar?nonpolar?

1. H2O

2. CH3CN

3. CHCl34. CH2Cl25. CO2

Solution:Solution:

22

Which of the following molecules is Which of the following molecules is nonpolar?nonpolar?

1. H2O

2. CH3CN

3. CHCl34. CH2Cl25. CO2

Which of the above molecules has (have) Which of the above molecules has (have) a dipole moment?a dipole moment?

1. A only2. B only3. C only4. A and B5. B and C

F

F

H

HC

F

H

H

FC C

F

H

A B C

H

F


Which of the above molecules has (have) Which of the above molecules has (have) a dipole moment?a dipole moment?

1. A only2. B only3. C only4. A and B5. B and C

F

F

H

HC

F

H

H

FC C

F

H

A B C

H

F

Solution:Solution:

25

2.3 Formal Charges2.3 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

26

• Atomic sulfur has 6 valence electrons.

Dimethyl suloxide sulfur has only 5.

• It has lost an electron and has positive charge.

• Oxygen atom in DMSO has gained electron and has (-) charge.

Formal Charge for Dimethyl Sulfoxide

What are the hybridization and a What are the hybridization and a formal charge, respectively, on boron formal charge, respectively, on boron in BHin BH44

––??

1. sp3, 02. sp3, –13. sp3, +14. sp2, –15. sp2, +1


What are the hybridization and a What are the hybridization and a formal charge, respectively, on boron formal charge, respectively, on boron in BHin BH44

––??

1. sp3, 02. sp3, –13. sp3, +14. sp2, –15. sp2, +1

Solution:Solution:

30

2.4 Resonance2.4 Resonance Some molecules are 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

31

Resonance HybridsResonance Hybrids A structure with resonance forms does not alternate

between the forms Instead, it is a hybrid of the two resonance forms, so the

structure is called a resonance hybrid For example, benzene (C6H6) 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

32

2.5 Rules for Resonance Forms2.5 Rules for Resonance Forms 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

Different resonance forms of a substance don’t have to be equivalent

Resonance forms must be valid Lewis structures: the octet rule applies

The resonance hybrid is more stable than any individual resonance form would be

33

Curved Arrows and Resonance FormsCurved Arrows and Resonance FormsWe can imagine that electrons move in pairs to

convert from one resonance form to anotherA 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

34

2.6 Drawing Resonance Forms2.6 Drawing Resonance Forms

Any three-atom grouping with a multiple bond has two resonance forms

35

Different Atoms in Resonance FormsDifferent Atoms in Resonance Forms 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

36

2,4-Pentanedione2,4-PentanedioneThe 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

Which of the following statements is false Which of the following statements is false concerning resonance structures?concerning resonance structures?

1. The arrangement of nuclei in all contributing structures must be the same.

2. The arrangement of electrons in each contributing structure is different.

3. Each atom in a contributing structure must have a completed valence shell.

4. The contributing structures may have different energies.

5. All contributing structures must have the correct number of valence electrons.


Which of the following statements is false Which of the following statements is false concerning resonance structures?concerning resonance structures?

1. The arrangement of nuclei in all contributing structures must be the same.

2. The arrangement of electrons in each contributing structure is different.

3. Each atom in a contributing structure must have a completed valence shell.

4. The contributing structures may have different energies.

5. All contributing structures must have the correct number of valence electrons.

Solution:Solution:

Consider all equivalent resonance structures for the Consider all equivalent resonance structures for the carbonate anion shown below. If the bond order is carbonate anion shown below. If the bond order is defined as the number of electron pairs shared defined as the number of electron pairs shared between two atoms, what is the average bond order between two atoms, what is the average bond order for the C-O1 bond?for the C-O1 bond?

1. 22. 3/23. 4/3 4. 15. 1/4

O

O O

1


Consider all equivalent resonance structures for the Consider all equivalent resonance structures for the carbonate anion shown below. If the bond order is carbonate anion shown below. If the bond order is defined as the number of electron pairs shared defined as the number of electron pairs shared between two atoms, what is the average bond order between two atoms, what is the average bond order for the C-O1 bond?for the C-O1 bond?

1. 22. 3/23. 4/34. 15. 1/4

O

O O

1

Solution:Solution:

Consider all equivalent resonance Consider all equivalent resonance structures for carbonate anion shown structures for carbonate anion shown below. What is the formal charge on O1?below. What is the formal charge on O1?

1. –12. –2/33. –1/24. –1/35. 0

O

O O

1


Consider all equivalent resonance Consider all equivalent resonance structures for carbonate anion shown structures for carbonate anion shown below. What is the formal charge on O1?below. What is the formal charge on O1?

1. –12. –2/33. –1/24. –1/35. 0

O

O O

1

Solution:Solution:

Anions with the negative charge residing on Anions with the negative charge residing on carbons are called carbanions. In the methyl carbons are called carbanions. In the methyl anion the carbon is anion the carbon is spsp33 hybridized. What is the hybridized. What is the hybridization of carbon 1 in the allyl anion?hybridization of carbon 1 in the allyl anion?

H

H

H H

H

allyl anion (a carbanion)1

1. sp3

2. sp2

3. sp4. sp3d5. carbon 1 is not hybridized


Anions with the negative charge residing on Anions with the negative charge residing on carbons are called carbanions. In the methyl carbons are called carbanions. In the methyl anion the carbon is anion the carbon is spsp33 hybridized. What is the hybridized. What is the hybridization of carbon 1 in the allyl anion?hybridization of carbon 1 in the allyl anion?

H

H

H H

H

allyl anion (a carbanion)1

1. sp3

2. sp2

3. sp4. sp3d5. carbon 1 is not hybridized

Solution:Solution:

How many uncharged resonance How many uncharged resonance structures exist for the following structures exist for the following molecule?molecule?

1. Only the one shown above2. 23. 34. 45. 5


How many uncharged resonance How many uncharged resonance structures exist for the following structures exist for the following molecule?molecule?

1. Only the one shown above2. 23. 34. 45. 5

Solution:Solution:

47

2.7 Acids and Bases: The Brønsted–2.7 Acids and Bases: The Brønsted–Lowry DefinitionLowry Definition

The terms “acid” and “base” can have different meanings in different contexts

For that reason, we specify the usage with more complete terminology

The idea that acids are solutions containing a lot of “H+” and bases are solutions containing a lot of “OH-” is not very useful in organic chemistry

Instead, Brønsted–Lowry theory defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors

Bronsted-Lowry theoryBronsted-Lowry theoryAcidAcid =

a Proton donor donates a proton, H+, to solvent

Example: HClHCl

HCl + H2O H3O+ + Cl- hydronium ion

Example: HNO3

HNO3 is the acid: it donates the proton.

Bronsted-Lowry theoryBronsted-Lowry theory

HNO3 + H2O HH33OO++ + NO3-

Base Base == a proton acceptor accepts a Proton, H+, from

solvent

NHNH33 + H2O NH4+ + OH-

Accepted H+

Bronsted-Lowry theoryBronsted-Lowry theory

NH3 is the Base: it takes the proton.

Hydroxide ions (OH-) form when water donates the proton.

CommonCommon AcidsAcidsBattery Acid Stomach Acid

Coca Cola

Carbonated Water

Vinegar Citrus fruitsVitamin CGrapesAspirin

H2SO4

HCl

H3PO4

H2CO3

HC2H3O2

H3C6O7H8

HC6O6H7 H2C4O6H4 H2C9O4H8

Sulfuric AcidHydrochloric Acid

Phosphoric Acid

Carbonic AcidAcetic AcidCitric Acid

Ascorbic Acid Tartaric AcidAcetyl Salicylic Acid

Naming AcidsNaming Acids•Binary AcidsBinary Acids = = (-ide ending)

HCl

Hydro- ______-ic Acid

= Hydrochloric Acid

•OxoacidsOxoacids = = H, Nonmetal & O, Nonmetal & O--ateate ending ending (The higher number of O’s)(The higher number of O’s)

= Hydrogen Chloride

____________________________--ic ic AcidAcid

HNOHNO33 = Hydrogen Nitr= Hydrogen Nitrateate = Nitr= Nitric ic AcidAcid

--iteite ending ending (The lower number of O’s)(The lower number of O’s)

____________________________--ous ous AcidAcidHNOHNO22 = Hydrogen Nitr= Hydrogen Nitriteite = Nitr= Nitrous ous AcidAcid

Naming AcidsNaming AcidsNon-Acid name Acid Name

H2SO4

H2SO3

H2S

H3PO4

HBr

H2CO3

HNO2

HNO3

Dihydrogen Sulfate

Hydrogen Bromide

Trihydrogen Phosphate

Dihydrogen carbonate

Acetic Acid

Sulfuric Acid

Hydrobromic Acid

Phosphoric Acid

Dihydrogen Sulfite

Dihydrogen Sulfide

Sulfurous Acid

Hydrosulfuric Acid

Hydrogen Nitrate

Hydrogen Nitrite Nitrous Acid

Nitric Acid

Hydrogen AcetateHC2H3O2

Carbonic Acid

Naming AcidsNaming AcidsNon-Acid name Acid Name

H2SO4

H2SO3

H2SO2

HCl

HClO

HClO2

HClO3

Dihydrogen Sulfate

Hydrogen Chloride

Hydrogen hypochlorite

Perchloric Acid

Sulfuric Acid

Hydrochloric Acid

Dihydrogen Sulfite

Dihydrogen Sulfide

Sulfurous Acid

Hyposulfurous Acid

Hydrogen chlorate

Hydrogen chlorite Chlorous Acid

Chloric Acid

Hydrogen PerchlorateHClO4

Hypochlorous Acid

H2SO4

HCl

H3PO4

H2CO3

HC2H3O2

H3C6O7H8

HC6O6H7 H2C4O6H4 H2C9O4H8

CommonCommon AcidsAcidsBattery Acid Stomach Acid

Coca Cola Carbonated Water

Vinegar Citrus fruitsVitamin CGrapesAspirin

StrongStrong•100% ionization100% ionization•Strong electrolyteStrong electrolyte

WeakWeak•Partial ionizationPartial ionization•Weak electrolyteWeak electrolyte

•Taste sourTaste sour

AcidsAcidsH-Cl + H-O-H + Cl-

HO

H

H+

HO

H

H+

Cl-

H-ClH-Cl

H+

HC2H3O2

C2H3O21-

HC2H3O2

H-C2H3O2 + H-O-HH

OH

H+ + C2H3O2

-

BaseBase = gives hydroxide ions in water.(Arrhenius definition)

= takes hydrogen ions in water.(Bronsted-Lowry definition)

BasesBases

NaOH

Na+ + OH-OH-

OH-

Na+

Na+

NaOH

58

Brønsted Acids and BasesBrønsted Acids and Bases“Brønsted-Lowry” is usually

shortened to “Brønsted”A Brønsted acid is a substance

that donates a hydrogen ion (H+)A Brønsted base is a substance

that accepts the H+

◦ “proton” is a synonym for H+ - loss of an electron from H leaving the bare nucleus—a proton

59

The Reaction of Acid with BaseThe Reaction of Acid with Base Hydronium ion, product when base H2O gains a proton HCl donates a proton to water molecule, yielding hydronium

ion (H3O+) [conjugate acid] and Cl [conjugate base] The reverse is also a Brønsted acid–base reaction of the

conjugate acid and conjugate base

60

2.8 Acid and Base Strength2.8 Acid and Base Strength

The equilibrium constant (Keq) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A-) is a measure related to the strength of the acid

Stronger acids have larger Keq

Note that brackets [ ] indicate concentration, moles per liter, M.

61

KKaa – the Acidity Constant – the Acidity Constant The concentration of water as a solvent does not change

significantly when it is protonated The molecular weight of H2O is 18 and one liter weighs 1000

grams, so the concentration is ~ 55.4 M at 25° The acidity constant, Ka for HA Keq times 55.6 M (leaving

[water] out of the expression) Ka ranges from 1015 for the strongest acids to very small values

(10-60) for the weakest

63

ppKKaa – the Acid Strength Scale – the Acid Strength Scale

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 15.74

64

2.9 Predicting Acid–Base Reactions 2.9 Predicting Acid–Base Reactions from pfrom pKKaa Values Values

pKa values are related as logarithms to equilibrium constants 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

65

2.10 Organic Acids and Organic Bases2.10 Organic Acids and Organic Bases Organic Acids:- characterized by the presence of

positively polarized hydrogen atom

66

Organic AcidsOrganic AcidsThose that lose a proton from O–H, such as

methanol and acetic acidThose that lose a proton from C–H, usually from a

carbon atom next to a C=O double bond (O=C–C–H)

67

Organic BasesOrganic BasesHave 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

68

2.11 Acids and Bases: The Lewis Definition2.11 Acids and Bases: The Lewis Definition

Lewis acids are electron pair acceptors and Lewis bases are electron pair donors

Brønsted acids are not Lewis acids because they cannot accept an electron pair directly (only a proton would be a Lewis acid)

The Lewis definition leads to a general description of many reaction patterns but there is no scale of strengths as in the Brønsted definition of pKa

69

Lewis Acids and the Curved Arrow Lewis Acids and the Curved Arrow FormalismFormalism

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

Organic compounds that undergo addition reactions with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids

The combination of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid

70

Illustration of Curved Arrows in Following Illustration of Curved Arrows in Following Lewis Acid-Base ReactionsLewis Acid-Base Reactions

71

Lewis BasesLewis Bases Lewis bases can accept protons as well as Lewis acids,

therefore the definition encompasses that for Brønsted bases Most oxygen- and nitrogen-containing organic compounds

are Lewis bases because they have lone pairs of electrons Some compounds can act as both acids and bases,

depending on the reaction

All Lewis acids can also be All Lewis acids can also be classified as Brønsted-Lowry acids.classified as Brønsted-Lowry acids.

1. True2. False


All Lewis acids can also be All Lewis acids can also be classified as Brønsted-Lowry acids.classified as Brønsted-Lowry acids.

1. True2. False

Solution:Solution:

Which statement best describes the Which statement best describes the following reaction?following reaction?

NH3 + BH3 H3N BH3

1. an acid-base reaction where NH3 acts as a Brønsted-Lowry acid

2. an acid-base reaction where NH3 acts as a Brønsted-Lowry base

3. an acid-base reaction where NH3 acts as a Lewis acid

4. an acid-base reaction where NH3 acts as a Lewis base

5. None of these adequately describes the reaction.


Which statement best describes the Which statement best describes the following reaction?following reaction?

NH3 + BH3 H3N BH3

1. an acid-base reaction where NH3 acts as a Brønsted-Lowry acid

2. an acid-base reaction where NH3 acts as a Brønsted-Lowry base

3. an acid-base reaction where NH3 acts as a Lewis acid

4. an acid-base reaction where NH3 acts as a Lewis base

5. None of these adequately describes the reaction.

Solution:Solution:

What is the main reason why molecule What is the main reason why molecule AA is less acidic than molecule is less acidic than molecule BB??AA: CH: CH33CHCH22CHCH22-H -H BB: CH: CH33CHCH22O-HO-H

1. There are more hydrogens in A.2. The C-H bond is weaker than the O-H bond.3. The conjugate base of A is more stable than the

conjugate base of B.4. In the conjugate base of B, the negative charge

resides on a more electronegative atom. 5. There is resonance and inductive stabilization of

conjugate base of B.


What is the main reason why molecule What is the main reason why molecule AA is less acidic than molecule is less acidic than molecule BB??AA: CH: CH33CHCH22CHCH22-H -H BB: CH: CH33CHCH22O-HO-H

1. There are more hydrogens in A.2. The C-H bond is weaker than the O-H bond.3. The conjugate base of A is more stable than the

conjugate base of B.4. In the conjugate base of B, the negative charge

resides on a more electronegative atom. 5. There is resonance and inductive stabilization of

conjugate base of B.

Solution:Solution:

Ammonia can act as all four classifications Ammonia can act as all four classifications of acids or bases (Lewis acid, Lewis base, of acids or bases (Lewis acid, Lewis base, Brønsted-Lowry acid, and Brønsted-Lowry Brønsted-Lowry acid, and Brønsted-Lowry base.)base.)

1. True2. False


Ammonia can act as all four classifications Ammonia can act as all four classifications of acids or bases (Lewis acid, Lewis base, of acids or bases (Lewis acid, Lewis base, Brønsted-Lowry acid, and Brønsted-Lowry Brønsted-Lowry acid, and Brønsted-Lowry base.)base.)

1. True2. False

Solution:Solution:

Van der Waals interactions refer to the weak Van der Waals interactions refer to the weak attractive forces that occur between individual attractive forces that occur between individual molecules and are very important in determining molecules and are very important in determining the shape and properties of biological molecules the shape and properties of biological molecules such as proteins. Which of the following play no such as proteins. Which of the following play no role in van der Waals interactions?role in van der Waals interactions?

1. hydrogen bonding2. dispersion forces 3. dipole-dipole forces4. magnetic forces5. All of these are important in van der

Waals interactions.


Van der Waals interactions refer to the weak Van der Waals interactions refer to the weak attractive forces that occur between individual attractive forces that occur between individual molecules and are very important in determining molecules and are very important in determining the shape and properties of biological molecules the shape and properties of biological molecules such as proteins. Which of the following play no such as proteins. Which of the following play no role in van der Waals interactions?role in van der Waals interactions?

1. hydrogen bonding2. dispersion forces 3. dipole-dipole forces4. magnetic forces5. All of these are important in van der

Waals interactions.

Solution:Solution:

Which is the correct order of the Which is the correct order of the ppKKaa values? values?

1. H2O > HO– > H3O+

2. HO– > H3O+ > H2O

3. H3O+ > HO – > H2O

4. HO– > H2O > H3O+

5. H3O+ > H2O > HO–


Which is the correct order of the Which is the correct order of the ppKKaa values? values?

1. H2O > HO– > H3O+

2. HO– > H3O+ > H2O

3. H3O+ > HO – > H2O

4. HO– > H2O > H3O+

5. H3O+ > H2O > HO–

Solution:Solution:

84

2.12 Molecular Models2.12 Molecular Models Organic chemistry is 3-D spaceMolecular shape is critical in determining the chemistry a

compound undergoes in the lab, and in living organisms

85

2.13 Noncovalent Interactions2.13 Noncovalent InteractionsSeveral types:- Dipole-dipole forces- Dispersion forces- Hydrogen bonds

86

Dipole-DipoleDipole-Dipole

• Occur between polar molecules as a result of electrostatic interactions among dipoles

• Forces can be attractive of repulsive depending on orientation of the molecules

87

Dispersion ForcesDispersion Forces• Occur between all neighboring molecules and arise because the electron distribution within molecules that are constantly changing

88

Hydrogen Bond ForcesHydrogen Bond Forces• Most important noncovalent interaction in biological molecules

• Forces are result of attractive interaction between a hydrogen bonded to an electronegative O or N atom and an unshared electron pair on another O or N atom

90

SummarySummaryOrganic molecules often have polar covalent

bonds as a result of unsymmetrical electron sharing caused by differences in the electronegativity of atoms

The polarity of a molecule is measured by its dipole moment, .

(+) and () indicate formal charges on atoms in molecules to keep track of valence electrons around an atom

Some substances must be shown as a resonance hybrid of two or more resonance forms that differ by the location of electrons.

A Brønsted(–Lowry) acid donates a protonA Brønsted(–Lowry) base accepts a protonThe strength Brønsted acid is related to the -1

times the logarithm of the acidity constant, pKa. Weaker acids have higher pKa’s

91

Summary (cont’d)Summary (cont’d)A Lewis acid has an empty orbital that can

accept an electron pairA Lewis base can donate an unshared

electron pair In condensed structures C-C and C-H are

impliedSkeletal structures show bonds and not C

or H (C is shown as a junction of two lines) – other atoms are shown

Molecular models are useful for representing structures for study

Noncovalent interactions have several types: dipole-dipole, dispersion, and hydrogen bond forces

Which statement about the acid-base Which statement about the acid-base equilibrium shown above is equilibrium shown above is incorrectincorrect??1. the equilibrium favors the products2. water is a conjugate acid in this equilibrium3. hydroxide is the strongest base present4. phenol is 6 times more acidic than water5. the negative charge in the phenoxide is

resonance stabilized

OH

+ OH

O

+ H2O

pKa = 10 pKa = 16

phenol hydroxide phenoxide


Which statement about the acid-base Which statement about the acid-base equilibrium shown above is equilibrium shown above is incorrectincorrect??1. the equilibrium favors the products2. water is a conjugate acid in this equilibrium3. hydroxide is the strongest base present4. phenol is 6 times more acidic than water5. the negative charge in the phenoxide is

resonance stabilized

OH

+ OH

O

+ H2O

pKa = 10 pKa = 16

phenol hydroxide phenoxide

Solution:Solution:

What is the most likely product of the following What is the most likely product of the following Lewis acid-base reaction?Lewis acid-base reaction?

1. A2. B3. C4. D5. E

O

O

+ BF3 product

O

O

O

O

O

O

O

OBF3 BF3

BF3 BF3

O

O BF3

A B C D E


What is the most likely product of the following What is the most likely product of the following Lewis acid-base reaction?Lewis acid-base reaction?

1. A2. B3. C4. D5. E

O

O

+ BF3 product

O

O

O

O

O

O

O

OBF3 BF3

BF3 BF3

O

O BF3

A B C D E

Solution:Solution:

Which of these structures is not a resonance Which of these structures is not a resonance form of the others? (The lone pairs are not form of the others? (The lone pairs are not shown explicitly.)shown explicitly.)

1. a2. b3. c4. d5. e

NH

NH

N

HN N

a) b) c) d) e)

H H


Which of these structures is not a resonance Which of these structures is not a resonance form of the others? (The lone pairs are not form of the others? (The lone pairs are not shown explicitly.)shown explicitly.)

1. a2. b3. c4. d5. e

NH

NH

N

HN N

a) b) c) d) e)

H H

Solution:Solution:

2. Polar Covalent Bonds: Acids and Bases Why this chapter? Description of basic ways chemists...

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Transcript of 2. Polar Covalent Bonds: Acids and Bases Why this chapter? Description of basic ways chemists...