Roldan M. de Guia Department of Biochemistry Faculty of Pharmacy University of Santo Tomas2009

Electronic Structure & Covalent Bonding

ORGANIC CHEMISTRYthe study of compounds of carbonthere are over 10 million organic

compounds

(S)-((S)-1-((2S,3S)-3-hexyl-4-oxooxetan-2-yl)tridecan-2-yl) 2-formamido-4-methylpentanoate

ATOMIC STRUCTURE

ELECTRON CONFIGURATIONPauli Exclusion Principle

Orbitals can contain a maximum of two electrons which must be of opposite spin

ms = -½ms = +½

ELECTRON CONFIGURATIONAufbau Principle

Atomic orbitals are filled with electrons from lowest potential energy to highest.

ELECTRON CONFIGURATIONMadelung’s Rule

Orbitals fill with electrons as n + l, where n is the principle quantum number and l is the angular momentum quantum number

explains why the 4s orbital has a lower energy than the 3d orbital

gives the periodic table its characteristic appearance

“Fill up” electrons in lowest energy orbitals (Aufbau principle)

H 1 electron

H 1s1

He 2 electrons

He 1s2

Li 3 electrons

Li 1s22s1

Be 4 electrons

Be 1s22s2

B 5 electrons

B 1s22s22p1

C 6 electrons

? ?

C 6 electrons

The most stable arrangement of electrons in subshells is the one with the greatest number of parallel spins (Hund’s rule).

C 1s22s22p2

N 7 electrons

N 1s22s22p3

O 8 electrons

O 1s22s22p4

F 9 electrons

F 1s22s22p5

Ne 10 electrons

Ne 1s22s22p6

CHEMICAL BONDING

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

CHEMICAL BONDING – Octet Rule

F -Li+ + Li+ F -

IONIC BONDINGNa Na + 1 e

Cl + 1 e Cl

COVALENT BONDING

1s1s

*

E

E*

COVALENT BONDING

F2

HF

A nonpolar covalent bond

A polar covalent bondH F

F F

CLASSIFICATION OF CHEMICAL BONDS

CLASSIFICATION OF CHEMICAL BONDS

Difference in Electronegativity between Bonded

Atoms

Type of BondMost Likely Formed

Between

<0.5 Non-polar covalent 2 non-metals or a non-metal & a

metalloid0.5 – 1.9 Polar covalent

>1.9 IonicA metal & a non-

metal

Care must be taken to distinguish between polar bonds and polar molecules!!

MOLECULAR GEOMETRY - VSEPR

POLARITY OF BONDS

H C lO

H HO

CH

3C C H

3 Care must be taken to distinguish between polar bonds

and polar molecules!! Carbon dioxide:

O C O

LEWIS STRUCTURE

F N F

F

O C O

O

HC O

H

formal charge on

an atom in a Lewis

structure

=1

2

total number of bonding electrons

( )total

number of valence

electrons in the free

atom

-total

number of nonbonding

electrons

-

H C O H

RESONANCE STRUCTURE one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure.

O O O+ -

OOO+-

FORMAL CHARGE = 0

Diazomethane, CH2N2

C N

H

H

NC N

H

H

N or

• Nitromethane, CH3NO2

or

H

CH

H

N

O

O

H

CH

H

N

O

O

STRUCTURES & FORMULAE

MOLECULAR MODELS

HYBRIDIZATION OF ORBITALS

CH4

HYBRIDIZATION OF ORBITALS

sp2 HYBRID ORBITALS

Ethene C C

H

HH

H

CC

H

H

H

H

sp2 HYBRID ORBITALS

sp2 HYBRID ORBITALS

sp2 HYBRID ORBITALS

sp HYBRID ORBITALS

Acetylene

sp HYBRID ORBITALS

sp HYBRID ORBITALS: CO2

O C O

C OO

CO O

Bond Type

Bond Length

(pm)

C-C 154

CC 133

CC 120

C-N 143

CN 138

CN 116

Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond

BOND LENGTH & STRENGTH

Effect of Hybridization on Bond Length & Strength

BOND TYPEC –C

# of BONDSBOND

LENGTH, pm

BOND ENERGY, kcal/mole

sp3 – sp3 0 154 82.76

sp3 – sp2 0 153 85.48

sp3 – sp 0 146 91.58

sp2 – sp2 1 147 91.42

sp2 – sp 0 142 96.48

sp – sp 2 138 103.60

s character = bond length = bond strength

Comparison of Bond Length of C to C Bonds involving Hybrid Orbitals

BOND TYPE BOND LENGTH, pm

Ethane CH3 – CH3 sp3 – sp3 153.8

Propylene CH3 – CH = CH2 sp3 – sp2 150.1

Methylacetylene CH3 – C CH sp3 – sp 145.9

Ethylene CH2 = CH2 sp2 – sp2 133.9

Acetylene CH CH sp – sp 120.7

FUNCTIONAL GROUPS

FUNCTIONAL GROUPS