SCH 102

Dr. Solomon Derese 136

sp2sp2

d

sp2 sp2

pzpzp

p

Hybridization

SCH 102

Dr. Solomon Derese 137

Hybridization of Carbon

SCH 102

Dr. Solomon Derese 138

Carbon has, outside its nucleus, six electrons andtherefore the ground state electronic configuration ofcarbon is:

1S2 2S2 2P2

1S 2S 2P2 2 2

Ground statepx py pz

This, however, represent the ground state of thecarbon atom in which only two unpaired electronsare available for bond formation with other atoms,i.e. at first sight carbon might appear to be onlydivalent.

SCH 102

Dr. Solomon Derese 139

A typical C-H has a bond strength of 100 Kcal/mol

Net-Energy change = (400-97) Kcal/mol = 300 Kcal/mol

In the excited state carbon has four unpaired electrons andcan form four bonds with hydrogen.

SCH 102

Dr. Solomon Derese 140

Methane - sp3 HybridizedWhat is the nature of the four C-H bonds inmethane?Since excited carbon uses two kinds of orbitals(2s and 2p) for bonding purpose, we mightexpect methane to have two kinds of C-Hbonds.

In fact this is not the case.A large amount of evidence show that all four C-Hbonds in methane are identical in terms of theirlength and strength.

SCH 102

Dr. Solomon Derese 141

The valence orbitals of a carbon atom are neither s orp but a hybrid or mixture of orbitals.

Since three p orbitals are mixed with one s-orbital,we call the hybrid orbitals sp3, meaning that eachof them has one-fourth s-character and three-fourth p-character.

Hybridization is the combination of two or moreatomic orbitals to form the same number of hybridorbitals, each having the same shape and energy.

SCH 102

Dr. Solomon Derese 142

Each bond in CH4 is formed by overlap of an sp3

hybrid orbital of carbon with a 1s orbital of hydrogen.These four bonds point to the corners of atetrahedron.

HH

H

HC .. .

. 1.101090

A0

Bond strength = 104 Kcal/mol

SCH 102

Dr. Solomon Derese 143

The tetrahedral shape of an sp3 hybridized carbon

SCH 102

Dr. Solomon Derese 144

All four C – H bonds in methane aresigma (d) bonds, because theelectron density is concentrated onthe axis joining C and H and areformed by head on overlap oforbitals.

SCH 102

Dr. Solomon Derese 145

Ethane (C2H6) – sp3 Hybridization

C CH

H

H

H

H

H

We can picture the ethane molecule by assumingthat the two carbon atoms bond to each other bysigma overlap of an sp3 hybrid orbital from eachcarbon.

SCH 102

Dr. Solomon Derese 146

C CH

H

H

H

H

H

1090

1090

1.54

1.10

0

A

0

A

Structure of Ethane

SCH 102

Dr. Solomon Derese 147

Ethane (C2H6) – sp3 Hybridization

SCH 102

Dr. Solomon Derese 148

Ethylene (C2H4)-sp2 Hybridization

H

H H

H

SCH 102

Dr. Solomon Derese 149

sp2 hybridization of a carbon

ground state

excited state

sp2 hybridized state

2pzsp2

2s 2p

2s 2p

unhybridized p orbital

SCH 102

Dr. Solomon Derese 150sp2 hybridized carbon

120°

sp2

sp2

pz

sp2 x

y

z

SCH 102

Dr. Solomon Derese 151

sp2sp2

d

sp2 sp2

pzpzp

p

d Bond are formed by end-on overlap of two sp2

hybrid orbitals. p bond are formed by side-by-sideoverlap of two 2p orbitals.

The electron density in a p bond is farther from thetwo nuclei, p bonds are usually weaker and thereforemore easily broken than d bonds.

SCH 102

Dr. Solomon Derese 152

Planar shape

SCH 102

Dr. Solomon Derese 153

The carbon-carbon double bond is rigid and bondrotation can not occur.For rotation to occur the p-bond must be broken. Theenergy barrier for bond rotation in ethene (ethylene)is 235 kJ/mol while for ethane is only 12 kJ/mol.

The rigidity of the double bond gives rise to thepossibility of stereoisomerism (geometric isomerism)which is commonly referred to as cis-trans isomerismin alkenes.

Cis isomer cannot become trans without a chemicalreaction occurring.

SCH 102

Dr. Solomon Derese 154

trans = The substituents are onopposite side of the doublebond.

cis = The substituents are onthe same side of the doublebond.

SCH 102

Dr. Solomon Derese 155

Examples

C CH

ClCl

HC C

Cl

HCl

Hcis-1,2-dichloroethene trans-1,2-dichloroethene

SCH 102

Dr. Solomon Derese 156

Ethyne (C2H2)-sp-Hybridization

C C HH

SCH 102

Dr. Solomon Derese 157

sp hybridization of a carbon

ground state

excited state

sp hybridized state2pzsp

2s 2p

2s 2p

unhybridized p orbitals2py

SCH 102

Dr. Solomon Derese 158

x

y

z

sp hybridized carbon

sp

py

sp

pz

SCH 102

Dr. Solomon Derese 159

180o

Alkynes have a linear geometry with C-C bond anglesof 180°.

SCH 102

Dr. Solomon Derese 160

Comparison of Carbon-Carbon and Carbon-Hydrogen bonds in methane, ethane, ethylene andethyne.

Molecule Bond Bond strength (Kcal/mol)

Bond length (Å)

S-character (%)

Methane, CH4 CSP3-H1S 104 1.10 25Ethane, CH3-CH3 CSP3 - CSP3

CSP3-H1S

8898

1.541.10

25

Ethylene, H2C=CH2 CSP2 - CSP2CSP2-H1S

152103

1.331.076

33

Ethyne, HC≡CH CSP - CSPCSP-H1S

200125

1.201.06

50

SCH 102

Dr. Solomon Derese 161

HO

CH3OH

C CH

SP

SP3

SP3

SP3

SP2

SCH 102

Dr. Solomon Derese 162

ProblemI. Locate and identify the functional groups in the

following molecules:

II. Indicate bond angle at each carbon in thefollowing compounds