27
SCH 102 Dr. Solomon Derese 136 sp 2 sp 2 d sp 2 sp 2 p z p z p p Hybridization
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
