Electronic Structure & Covalent Bonding (2gmt)

Electronic Structure & Covalent Bonding Roldan M. de Guia Department of Biochemistry Faculty of Pharmacy University of Santo Tomas 2009

ORGANIC CHEMISTRY

the study of compounds of carbon there are over 10 million organic

compounds

(S)-((S)-1-((2S,3S)-3-hexyl-4-oxooxetan-2-yl)tridecan-2-yl) 2-formamido4-methylpentanoate

ATOMIC STRUCTURE

ELECTRON CONFIGURATION Pauli Exclusion Principle

 Orbitals can contain a maximum of two electrons which

must be of opposite spin

ms = +½ ms = -½

ELECTRON CONFIGURATION Aufbau Principle

 Atomic orbitals are filled with electrons from lowest

potential energy to highest.

ELECTRON CONFIGURATION Madelung’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)

? ? Li Be B C5 6 34electrons electrons 2 22 2 12 1 BBe Li1s1s 1s 2s2s 2s 2p

H He12electron electrons He H 1s 1s12

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

F N O Ne C9 6 7 810 electrons electrons electrons 2 2 2 5 Ne C N O F 1s 1s 1s222s 2s 2s22p 2p 2p2346

CHEMICAL BONDING Atom

Electronic Structure

Electronic Configuration

Lewis Dot Structure

3p 3s

Boron

2

2

1

1s 2s 2p

2p 2s

B

1s

3p 3s

Phosphorus

[Ne] 3s23p3

2p 2s

1s

P

CHEMICAL BONDING – Octet Rule

Li+ +

F

-

Li+ F -

IONIC BONDING Na Cl + 1 e

Na

+ 1e Cl

COVALENT BONDING σ* Eσ* 1s

1s

Eσ σ

COVALENT BONDING  F2

F

 HF

H F δ+

F

δ−

A nonpolar covalent bond

A polar covalent bond

CLASSIFICATION OF CHEMICAL BONDS

CLASSIFICATION OF CHEMICAL BONDS Difference in Electronegativity between Bonded Atoms

Type of Bond

<0.5

Non-polar covalent

0.5 – 1.9

Polar covalent

>1.9

Ionic

Most Likely Formed Between 2 non-metals or a non-metal & a metalloid A metal & a nonmetal

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

MOLECULAR GEOMETRY VSEPR

POLARITY OF BONDS

H

Cl

O H

H

O C

H C CH 3 3  Care must be taken to distinguish between polar bonds and polar molecules!! Carbon dioxide:

O C O

LEWIS STRUCTURE F

N

F

O

C

F

H

O

O

C

O

total formal charge on = number of valence an atom in a electrons in Lewis the free structure atom

H

-

H H

C

total number of nonbonding electrons

O

-

1 2

total number of bonding electrons

(

)

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

O

+ O

O

FORMAL CHARGE = 0

 Diazomethane, CH2N2

H

H

C N N

or

H •

C N N H

Nitromethane, CH3NO2

H O H C N O H

H O or

H C N O H

STRUCTURES & FORMULAE

MOLECULAR MODELS

HYBRIDIZATION OF ORBITALS

CH4

HYBRIDIZATION OF ORBITALS

sp2 HYBRID ORBITALS H

Ethene

H C C

H H

H C

H

H

C H

sp2 HYBRID ORBITALS

sp2 HYBRID ORBITALS

sp2 HYBRID ORBITALS

sp HYBRID ORBITALS

Acetylen e

sp HYBRID ORBITALS

sp HYBRID ORBITALS: CO2

O C O

C

O O

C

O O

Lengths of Covalent Bonds Bond Type

Bond Length (pm)

C-C

154

C= C

133

C≡ C

120

C-N

143

C= N

138

C≡ N

116

Bond Lengths Triple bond < Double Bond < Single Bond

BOND LENGTH & STRENGTH

Effect of Hybridization on Bond Length & Strength # of π BONDS

BOND 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

BOND TYPE C –C

↑ 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