Single and Multiple Bonds in Carbon Compounds

1
Single and Multiple Bonds in Carbon Compounds
sp3 hybridization on C leads to 4 bonds. CH4 is
a good example. CH3(CH3) is another example
1 C-C sp3/sp3 bond
CH3
C
H
H
H
2
Carbon can also exhibit sp2 hybridization
C2H4 (ethylene)
H
H
H
H
3
2 of sp2 orbitals on each C form C-H bonds (
total of 4 )
Remaining sp2 on each c overlap each other ?
gives C-C bond
This uses 3 valence e - from each C to form 5 sp2
bonds (? bonds)
H
H
120
120
H
H
(Each C contributes 1 valence e - to C-C bond.)
4
Up to this point, need not be planar because ?
bond symmetric under rotation.
2p


C
C


? bond
5
? bond
H
H


sp2
sp2
120
-
-
H
H
6
H
H
sp2
sp2
H
H
Planar Ethylene
7
H
H




H
H
Nodal Line
? ? ? transitions in ethylene occur around 1700
Å (58,500 cm-1) Corresponds to an energy of about
600 kJoules/mole ?
8
? States in Ethylene
?
E
2py
2py
C (Atom)
C (Atom)
?
Molecule
E h? hc/? ? ?1700 Å?
?
Photon E h?
?
9
Saturated carbons form only ? bonds, and ?
higher in energy than p (start to absorb 1600
Å)
? bond formed from two sp3 hybridized orbitals
Cylindrically symmetrical about C-C bond.
10
Carbon sp hybrids Acetylene and the Triple bond
C2H2 is H-CC-H

H
H

-
-
11
H-C-C-H
2py
2py
2px
sp
H
H
C
C
2px
12
Short Comparison of Bond Order, Bond Length, Bond
Energy
C-C C-C C-C Molecule Bond
Order Bond Length Bond E, kcal/mole
Ethane, C2H6 1 (1 ?) 1.54Å 83 Ethylene,
C2H4 2 (1?, 1p) 1.35Å 125 Acetylene, C2H2 3
(1?, 2p) 1.21Å 230
13
Although energy of p in ethylene lt ?,
conjugated polylenes have even lower energy p
levels. These absorb light at longer
wavelength- sometimes even in visible (human
eyes light perception). Conjugated polyenes
CC-CC-CC polyene
14
?MO (Const)2py(1) 2py(2) 2py(3) 2py(4)
2py(5) 2py(6) . Add 2py Atomic Orbitals
on each C
This gives delocalized structure
15
Energy
? antibonding levels
H2CCH-HCCH2
Eh? (photon)
2p orbitals
(one each on 4 carbons)
? bonding levels
Eh? for butadiene ltlt Eh? for ethylene
Molecular Orbital Energies
16
H
H
Electron Delocalization in Carbon Ring
Compounds
120
sp2
H
H
H
H
17
or
?
This is localized picture!
18
Delocalized Picture
H
H
H
H
Can form ? bonds with these p orbitals.
H
H
?MO (Const)2py(1) 2py(2) 2py(3) 2py(4)
2py(5) 2py(6) Add 6 2py Atomic Orbitals on
each C
There are 6 such combinations!
19
?MO (Const)2py(1) 2py(2) 2py(3) 2py(4)
2py(5) 2py(6) Add 6 2py Atomic Orbitals on
each C
20
Can actually form a total of 6 delocalized M.O.s
for benzene (6 2p Atomic Orbitals ? 6 M.O.s). ?
Energy

Antibonding orbitals
0 (isolated C 2p)
p3b
p2b
Bonding orbitals
_
p1b