1.3 covalent bonding - electrons shared 1.2-1.3 bonding 1.2 ionic bonding - electrons transferred...

1.3 Covalent Bonding - Electrons Shared

1.2-1.3 Bonding

1.2 Ionic Bonding - Electrons Transferred

type of bond that is formed is dictated by the

relative electronegativities of the elements involved

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

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Electronegativity

the attraction of an atom for electrons

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1.2 Ionic bonding

Electrons TransferredBig differences in E.N. values

Metals reacting with non-metals

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Important Electronegativity Values

H2.1Li Be B C N O F1.0 2.0 2.5 3.0 3.5 4.0

Cl3.0Br 2.8I2.5

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1.3 Covalent Bonding - Similar electronegativities

H . + H . H : H

Hydrogen atoms Hydrogen molecule

C + 4 H C

H

H

H

H

B.D.E

+104 kcal/mol

B.D.E+104 kcal/mol

B.D.E. = bond dissociation energy YSUYSU

1.3 Lewis Dot Structures of Molecules

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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

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1.5 Polar covalent bonds and electronegativity

H2 HF H2O

CH4 CH3Cl

Based on electronegativity

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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

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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

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1.7 Formal Charge

Formal charge = group number

- number of bonds

- number of unshared electrons

O NO

OH

OO

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1.8 Resonance Structures - Electron Delocalization

OO

O OO

O

CH3C

O

O

CH3C

O

O

Table 1.6 – formal rules for resonance YSUYSU

1.9 Shapes of Molecules

Shapes of molecules are predicted using VSEPR theory

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1.9 Shape of a molecule in terms of its atoms

Figure 1.9

Table 1.7 – VSEPR and molecular geometry

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Trigonal planar geometry of bonds to carbon in H2C=O

Linear geometry of carbon dioxide

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1.10 Molecular dipole moments

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Figure 1.7

• Curved arrows used to track flow of electrons in chemical reactions.

• Consider reaction shown below which shows the dissociation of AB:

1.11 Curved Arrows – Extremely Important

A B A+ + B-

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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-

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1.12 Acids and Bases - Definitions

ArrheniusAn acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions.

Brønsted-LowryAn acid is a proton donor. A base is a proton acceptor.

LewisAn acid is an electron pair acceptor. A base is an electron pair donor.

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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

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Proton Transfer from HBr to Water

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Equilibrium Constant for Proton Transfer

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Acids and Bases: Arrow Pushing

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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

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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

HpKa ~60

Essentially not acidic

Corresponding base

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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.

HFHF HClHCl HBrHBr HIHI

ppKKaa3.13.1 -3.9-3.9 -5.8-5.8 -10.4-10.4

weakest acid strongest acid

strongest H—X bond weakest H—X bond

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

ppKKaa6600

3636 1616 3.13.1 weakest acid strongest acid least electronegative most electronegative

CHCH44 NHNH33 HH22OO HFHF

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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

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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 OpKa ~16

pKa ~5

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1.16 Acid-base reactions - equilibria

The equilibrium will lie to the side of the weaker conjugate base

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H Cl NaOH NaCl + H2O+

H3C

O

OHNaOH

H3C

O

ONaH2O+ +

CH3ONaCH3OH NaOH H2O+ +

1.17 Lewis acids and Lewis bases

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.

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