Polar Covalent Bonds;Acids and Bases

Bond Moments and Dipole Moments

Formal Charge

Resonance

Bronsted-Lowry Acid/Base

Lewis Acid/Base

Pauling Electronegativity Scale

Electronegativity Trends Ability to Attract the Electrons in a Covalent Bond

Electrostatic Potential Map Cholormethane

Opposite polarity in CH3Li

Methanol

Dipole Moment () is sum of the Bond Moments

Nonpolar CompoundsBond Moments Cancel Out

NCl3 and BCl3

NClCl

Cl B

Cl

= 0

Cl Cl

N-Cl bond momentsdo not cancel N

sp3 sp2

pyramidal trigonal planar

Nitromethane

Nitromethane has 2 Formal Charges

CH3NO2 C N

O

O

H

HH

Formal Charge = [Group #] - [# bonds] - [# non-bonding electrons]

N = 5-4-0 = +1

O = 6-1-6 = -1

Bonding Pattens for C, N, and Osp3 sp2 sp

C CC

C

N N N

O O

tetrahedral geometry trigonal planar linear

Common Cationic, Neutral and Anionic Forms

Both Resonance Structures Contribute to the Actual Structure

CH3NO2

C N

O

O

H

H

HHH

H

N

O

O

C

2 Equivalent Resonance Structures

Dipole Moment reflects Both Resonance Structures

C N

O

O

H

H

HHH

H

N

O

O

C

Resonance Hybrid

C

H

HH

O

O

N

Curved Arrow Formalism Shows flow of electrons

C N

O

O

H

H

HHH

H

N

O

O

C

Arrows depict electron pairs moving

Resonance Rules

• Cannot break single (sigma) bonds

• Only electrons move, not atoms

3 possibilities:– Lone pair of e- to adjacent bond position

• Forms bond

bond to adjacent atom

bond to adjacent bond position

Resonance Stabilization of IonsPos. charge is “delocalized”

CH

HC

C

H

H

H

C

C

H

H

H

H

HC

H

HC

H

H

H

CC

resonance hybrid

Two Nonequivalent Resonance Structures in Formaldehyde

Resonance in Acetate Anion

Resonance in [CH2NH2]+

Definitions of Acids/Bases

Arrhenius acid - forms H3O+ in H2O

Bronsted-Lowry acid - donates a H+ (proton)

Lewis acid - accepts an electron pair to form a new bond

Arrhenius base - forms OH - in H2O

Bronsted-Lowry base - accepts a H + (proton)

Lewis base - donates an electron pair to form a new bond

Dissociation in H2OArrhenius Acid forms H3O+

Bronsted-Lowry Acid donates a H+

Resonance Stabilization

C C

O

OH

H

HH

pKa 4.7

+ NaHCO3 HH

H

C

O

OC

HH

H

C

O

OC

H2CO3 + Na

HH

H

C C

O

O

resonance hybrid

Na

Measuring Acid StrengthKa

Acid Strength defined by pKa

HCl + H2O H3O + Cl

Keq = [H3O ][Cl ]

[HCl][H2O]

Ka = Keq[H2O] =[H3O ][Cl ]

[HCl]= 10

7

pKa = -log(Ka) = -7

Stronger Acid Controls Equilibrium

HCl + H2O H3O + Clacid base conjugate conjugate

acid base

pKa = -7 -1.7 stronger weaker

Reaction Described with Arrows

H Cl + OHH O

H

HH+ Cl

Equilibrium Reactions

Acids Can React with Water

The Effect of Resonance on pKa

Identify the Acid and Base

CH3OH + OCOH

O

CH3O + HOCOH

O

Equilibrium Favors Reactants

CH3OH + OCOH

O

CH3O + HOCOH

O

acid base conj. base conj. acid

pKa 15.5 6.5

Draw the Conjugate Acid and Base

CH3CH2CH2OH + H2SO4

Propanol is a Base

base acid+ HSO4

H

CH3CH2CH2OHH2SO4+CH3CH2CH2OH

Draw the Conjugate Acid and Base

CH3CH2CH2OH + Na NH2

Propanol is an Acid

CH3CH2CH2O + Na NH2H CH3CH2CH2O Na

+ NH3

acid base

c.a.

c.b.

Some Acid-Base Reactions

Carboxylic Acids can be Separated from non-acidic

Compounds OCCH3

O

C

O

OH

Aspirin

+ Na OH

OCCH3

O

C

O

O Na

+ H2O

NaOCCH3

O

C

O

O

acid base conj. base conj. acid

HCl

Lewis Acids and Bases

B H B H

base acid complex

arrow always goes from the base to the acid always originate your arrow at the e- donor

Lewis Acid and Base Reactions

B

H

HH+ N

HH

H

Lewis BaseLewis Acid

e- pair acceptor e- pair donor

B N

H

HH

H

HH

acid-base complex

Lewis Base = NucleophileLewis Acid = Electrophile

Draw the Acid-Base Complexes

CH3OCH3 + FeCl3

CH3+ + H2O

HOC(CH3)3 + AlCl3