CHEM 2020 Fall 2011 Course Kit part 1.pdf

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5 Oct 2013
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Course
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Lewis Structures
used to symbolize covalent bonding with
paired dots (:) or dashes (—) between atoms to represent covalent bonds
2 shared e¯ = single bond
4 shared e¯ = double bond
6 shared e¯ = triple bond
: or
:: or
::: or
and paired dots (:) for non-bonding electron pairs (lone pairs )
examples
atoms are arranged to have noble gas valence electron totals
2 for H [He ]
8 for row 2 elements (B, C, N, O, F) [Ne ]
18 for row 3 elements (Al, Si, P, S, Cl) [Ar ]
number of bonds to neutral elements = element’s valence
univalent alkyl and aryl groups are frequently symbolized by R & Ar
Lewis structures only relate connectivity , not structure
sorbitals are spherical & porbitals are octahedrally disposed
but bonds in molecules are not spherically nor octahedrally disposed
Atomic Orbitals
by Valence Shell Electron Pair Repulsion (VSEPR) Theory, bonding & non-bonding e¯ pairs repel
each other & stay as far away as possible from each other
to do this, orbitals can hybridize to give hybrid orbitals as linear combinations of
atomic orbitals (LCAO)
one sorbital and one porbital can form 2equivalent sp orbitals
one sorbital and two porbitals can form 3equivalent sp2orbitals
one sorbital and three porbitals can form 4equivalent sp3orbitals
Hybridization
Structure, Bonding & Properties
Structure, Bonding & Properties Wade 6th/7th ed. Chapters 1 & 2
Atomic Hybridization
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p
s
Be
hybridize
p
sp
Be
Be
HH
Be HHdigonal (linear)
H-Be-H 180°
p
s
B
hybridize
p
sp2
B
B
H
H
H
H
B
H
HHB
HHtrigonal
all H-B-H 120°
p
s
C
hybridize
sp3
C
C
H
H
H
H
tetragonal
(tetrahedral)
all H-C-H 109.4°
HC
H
H
HH
C
H
H
H
when two atomic orbitals from separate atoms mix in bonding, they form two molecular orbitals
(MOs) one by constructive overlap (bonding MO)
one by destructive overlap (anti-bonding MO)
bonding MOs are lower in energy (by the bond energy) and stabilizing; antibonding MOs are
destabilizing if populated
by reference to the mixing of satomic orbitals, these MOs are called σand σ* MOs and the bonds are
called σbonds
σ
σ*
any s-type of orbital can enter into σbonds: s, sp, sp2or sp3
the first bond between atoms is always a σbond
Sigma (σ) Bonding
Molecular Orbitals
sp sp
σ
σ
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Pi (π) Bonding
p
s
p
sp2
OO
hybridize
OOOO
σ
σ
π
π
when two patomic orbitals from separate atoms mix in bonding, they form two πmolecular orbitals
(MOs) one by constructive overlap (bonding πMO)
one by destructive overlap (anti-bonding π*MO)
πbonding MOs are lower in energy than are porbitals (by the πbond energy) and stabilizing;
antibonding π* MOs are destabilizing if populated
by reference to the mixing of patomic orbitals, these MOs are called πand π* MOs and the bonds
are called πbonds
only p-type orbitals can enter into πbonds
the second & third bonds between atoms are always πbonds
π
π
p
p
Conjugation & Resonance
CC
C
C
CC
CC
pp
π
π
p
π2
π1
π2
π1
π
π
p
when two πbonds are adjacent (from four patomic orbitals on four consecutive atoms in
parallel alignment), they can mix and form additional MOs
one additional bonding πMO by constructive overlap between the double bonds, providing
additional stabilization
one additional anti-bonding π* MO by destructive overlap between them, providing additional
destabilization
so long as the antibonding π* MOs are not fully populated, the mixing of adjacent πbonds is
stabilizing; in neutral or cationic πsystems, only the bonding MOs are ever occupied
this is the basis of conjugation and applies to any number of consecutive and parallel πbonds
(from even numbers of consecutive and parallel porbitals); multiple bonds are always
more stable when adjacent (conjugated) than when not
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