1.3 Covalent Bonding - Electrons Shared

1.2-1.3 Bonding

Atoms trying to attain the stable configuration of a noble (inert) gas - often referred to as the octet rule

1.2 Ionic Bonding - Electrons Transferred

type of bond that is formed is dictated by the

relative electronegativities of the elements involved

Electronegativity

the attraction of an atom for electrons

1.2 Ionic bonding

Electrons Transferred

Big differences in E.N. values

Metals reacting with non-metals

Important Electronegativity Values

H

2.1

Li Be B C N O F

1.0 2.0 2.5 3.0 3.5 4.0

Cl

3.0

Br

2.8

I

2.5

1.3 Covalent Bonding - Similar electronegativities

H . + H . H : H

Hydrogen atoms Hydrogen molecule

C + 4 H C

H

H

H

H

Lewis dot representations of molecules

B.D.E

+104 kcal/mol

B.D.E

+104 kcal/mol

B.D.E. = bond dissociation energy

1.3 Lewis Dot Structures of Molecules

1.4 Double bonds and triple bonds

H C C HH : C : : : C : H

C : : C

H H

H H

C CH

H

H

H

Double bonds - alkenes

Triple bonds - alkynes

1.5 Polar covalent bonds and electronegativity

H2 HF H2O

CH4 CH3Cl

Based on electronegativity

LiLiHH

FF::........HH

1.6 Structural Formula - Shorthand in Organic Chemistry

CH3CH2CH2CH3

H H

HH H

H HH

HH

CH3CH2CH2CH2OH OH

H ClH

H

HH

H HH

H

HH

Cl

1.6 Constitutional Isomers

H

CH

H

O C H

H

H

H C C O H

H

H

H

H

Same molecular formula, completely different chemical and physical properties

1.7 Formal Charge

Formal charge = group number

- number of bonds

- number of unshared electrons

O NO

OH

OO

O

1.8 Resonance Structures - Electron Delocalization

OO

O OO

O

CH3C

O

O

CH3C

O

O

Table 1.6 – formal rules for resonance

1.9 Shapes of Molecules

Shapes of molecules are predicted using VSEPR theory

1.9 Shape of a molecule in terms of its atoms

Figure 1.9

Table 1.7 – VSEPR and molecular geometry

Trigonal planar geometry of bonds to carbon in H2C=O

Linear geometry of carbon dioxide

1.10 Molecular dipole moments

Figure 1.7

• Curved arrows are used to track the flow of

electrons in chemical reactions.

• Consider the reaction shown below which shows

the dissociation of AB:

1.11 Curved Arrows – Extremely Important

A B A+ + B-

Many reactions involve both bond breaking

and bond formation. More than one arrow

may be required.

Curved Arrows to Describe a Reaction

H O + C

H

H

H

Br C

H

O

H

HH + Br-

1.12 Acids and Bases - Definitions

Arrhenius

An acid ionizes in water to give protons. A base

ionizes in water to give hydroxide ions.

Brønsted-Lowry

An acid is a proton donor. A base is a proton

acceptor.

Lewis

An acid is an electron pair acceptor. A base is an

electron pair donor.

H AB .. B H A–..

+

1.13 A Brønsted-Lowry Acid-Base Reaction

A proton is transferred from the acid to the base. + +

base acid conjugate

acid

conjugate

base

hydronium ion (Hhydronium ion (H33OO++))

HH BrBrOO

HH

HH

.... ....

HH

HH

.... OO HH BrBr––

.... ........

........

........

++

Proton Transfer from HBr to Water

basebase acidacid conjugate conjugate conjugate conjugate

acid acid base base

++ ++

[H3O+][Br–]

[HBr]Ka =

H BrO

H

H

.. ..

H

H

.. O H Br–

.. ....

....

....

++ +

pKa = – log10 Ka

Equilibrium Constant for Proton Transfer

H O H + H Br H O H + Br

H?

Acids and Bases: Arrow Pushing

H O H + H Br H O H + Br

H

H O H + H Br H O H + Br

H

[H3O+][Br–]

[HBr]Ka = ~ 106 for HBr, pKa = - 5.8

Need to know by next class:

pKa = -log10Ka

STRONG ACID = LOW pKa WEAK ACID = HIGH pKa

HI, HCl, HNO3, H3PO4 pKa -10 to -5 Super strong acids

H3O+ pKa – 1.7

RCO2H pKa ~ 5 acids

PhOH pKa ~ 10 get

H2O, ROH pKa ~ 16 weaker

RCCH (alkynes) pKa ~ 26

RNH2 pKa ~ 36 Extremely weak acid

RCH3 pKa ~ 60 Not acidic at all

1.14 What happened to pKb?

• A separate “basicity constant” Kb is not necessary.

• Because of the conjugate relationships in the

Brønsted-Lowry approach, we can examine acid-base reactions by relying exclusively on pKa values.

C

H

H

H

H C

H

H

H

pKa ~60

Essentially not acidic

Corresponding base

Extremely strong

1.15 How Structure Affects Acid/Base Strength

Bond Strength

• Acidity of HX increases (HI>HBr>HCl>HF) down the periodic table as

H-X bond strength decreases and conjugate base (X:- anion) size increases (basic strength of anion decreases).

strongest H—X bond weakest H—X bond

ElectronegativityAcidity increases across periodic table as the atom attached to H gets more electronegative (HF>H2O>H2N>CH4).

least electronegative most electronegative

Inductive EffectsElectronegative groups/atoms remote from the acidic H can effect the pKa of the acid.

pKa = 16 pKa = 11.3

CH3CH2O H CF3CH2O H

• O – H bond in CF3CH2OH is more polarized

• CF3CH2O- is stabilized by EW fluorine atoms

Resonance Stabilization in AnionDelocalization of charge in anion (resonance) makes the anion more stable and thus the conjugate acid more acidic e.g. (CH3CO2H > CH3CH2OH).

CH3C

O

O

CH3C

O

O

CH3C

OH

O

CH3 CH2 OH CH3 CH2 O

pKa ~16

pKa ~5

1.16 Acid-base reactions - equilibria

H Cl NaOH NaCl + H2O+

H3C

O

OHNaOH

H3C

O

ONaH2O+ +

H2OCH3ONaNaOHCH3OH + +

The equilibrium will lie to the side of the

weaker conjugate base

1.17 Lewis acids and Lewis bases

FF33BB ++ OO

CHCH22CHCH33

CHCH22CHCH33

•••• ••••–– ++

FF33BB OO

CHCH22CHCH33

CHCH22CHCH33

••••

Lewis acidLewis acid Lewis baseLewis base

Product is a stable substance. It is a liquid with a boiling point of 126°C. Of the two reactants, BF3 is a gas and CH3CH2OCH2CH3 has a boiling point of 34°C.