Hybrid AO's and polyatomic MO's - Boston Universityquantum.bu.edu/courses/ch101-fall-2015/quantum...
Transcript of Hybrid AO's and polyatomic MO's - Boston Universityquantum.bu.edu/courses/ch101-fall-2015/quantum...
Hybrid AO's and polyatomic MO's
CH101 Fall 2015Boston University
PermissionsFigures on slides 4-9 are used with permission from Clayden et al., Organic Chemistry (Oxford University Press, 2000), © 2007 Oxford University Press.
Figures on slides 23, 25 and 39 are used with permission from Mahaffy et al., Chemistry: Human Activity, Chemical Reactivity (Nelson, 2011), © 2011 Nelson Education Ltd.
Hybrid AO's and polyatomic MO's
2
Copyright © 2015 Dan Dill [email protected]
Hybridized AO’s account for central atomelectron-pair geometry
Hybrid AO's and polyatomic MO's
3
Copyright © 2012 Dan Dill [email protected]
Central atom AO mixing: Hybrid AO’sUnmixed AO’s have the wrong central atom geometry
Hybrid AO's and polyatomic MO's
4
Copyright © 2012 Dan Dill [email protected]
An s and a p AO make two sp hybrid AO’s
180o angle, for SN = 2Two p’s are unchanged on each atom
Hybrid AO's and polyatomic MO's
5
Copyright © 2012 Dan Dill [email protected]
sp hybrids account for linear geometry
180o angle, for SN = 2Two p’s are unchanged on each atom
Hybrid AO's and polyatomic MO's
6
Copyright © 2012 Dan Dill [email protected]
An s and two p AO’s make three sp2 hybrid AO’s
120o angle, for SN = 3One p is unchanged on each atom
Hybrid AO's and polyatomic MO's
7
Copyright © 2012 Dan Dill [email protected]
sp2 hybrids account for trigonal planar geometry
120o angle, for SN = 3One p is unchanged on each atom
Hybrid AO's and polyatomic MO's
8
Copyright © 2012 Dan Dill [email protected]
An s and three p AO’s make four sp3 hybrid AO’s
109o angle, for SN = 4sp3 hybrids account for tetrahedral geometry
Hybrid AO's and polyatomic MO's
9
Copyright © 2012 Dan Dill [email protected]
Which hybridization to use?steric number electron-pair geometry hybridization
steric number = attached atoms + lone pairs
Hybrid AO's and polyatomic MO's
10
Copyright © 2012 Dan Dill [email protected]
steric numbe
r
electron-pairgeometry
hybridization
moleculargeometry
H2O 4 tetrahedral sp3 bent at 109˚NH3 4 tetrahedral sp3 trigonal pyramidalCH4 4 tetrahedral sp3 tetrahedralSO2 3 trigonal
planarsp2 bent at 120˚
BH3 3 trigonal planar
sp2 trigonal planar
CO2 2 linear sp linear at 180˚
Examples
CO2, carbon dioxideH2CO, formaldehyde
HCO2-, formate
SO2, sulfur dioxide
Hybrid AO's and polyatomic MO's
11
Copyright © 2012 Dan Dill [email protected]
Polyatomic MO recipe
Overview• σ framework of single bonds and lone
pairs• π framework of additional bond pairs• π framework can be delocalized (spread)
over more than two atoms.
Hybrid AO's and polyatomic MO's
12
Copyright © 2012 Dan Dill [email protected]
Polyatomic MO recipe1. Use the Lewis structure to get
• the number of electron pairs• make hybrid AO’s on each atom (except H)
2. Sketch the σ framework and place pairs• in each bonding σ MO• in each nonbonding hybrid AO
3. Sketch the π framework MO’s, • mark as bonding, nonbonding,
antibonding• place remaining pairs (Auf Bau)• get the π bond order
Hybrid AO's and polyatomic MO's
13
Copyright © 2012 Dan Dill [email protected]
σ framework recipe
• Hybridization of terminal atoms the same as their central atom
• Terminal H never hybridized• One pair in each hybrid AO σ bonding
MO• One pair in each non-bonded hybrid AO
Hybrid AO's and polyatomic MO's
14
Copyright © 2012 Dan Dill [email protected]
CO2 sp σ frameworkHybrid AO's and polyatomic MO's
15
Copyright © 2012 Dan Dill [email protected]
sp hybrids overlap to make two sp σ bonding MO’s,
leaving two sp nonbonding AO’s. These can hold …
CO2 sp σ frameworkHybrid AO's and polyatomic MO's
16
Copyright © 2012 Dan Dill [email protected]
sp hybrids overlap to make two sp σ bonding MO’s,
leaving two sp nonbonding AO’s. These can hold …
CO2 sp σ frameworkHybrid AO's and polyatomic MO's
17
Copyright © 2012 Dan Dill [email protected]
sp hybrids overlap to maketwo sp σ bonding MO’s,
leaving two sp nonbonding AO’s. These can hold 4 pairs of electrons.
CO2 sp σ frameworkHybrid AO's and polyatomic MO's
18
Copyright © 2012 Dan Dill [email protected]
sp hybrids overlap to make two sp σ bonding MO’s,
leaving two sp nonbonding AO’s. These can hold 4 pairs of electrons.The remaining 4 pairs are in the …
π framework recipe
• Unused p AO’s form same number of πMO’s
• Number of loops and AO overlap determine whether π MO is …• bonding (π)• nonbonding (πn)• antibonding (π*)
Hybrid AO's and polyatomic MO's
20
Copyright © 2012 Dan Dill [email protected]
CO2 π frameworkHybrid AO's and polyatomic MO's
21
Copyright © 2012 Dan Dill [email protected]
2 pairs in π (bonding) and 2 pairs in πn
(nonbonding); bond order 2
4 pairs are in the (delocalized) π framework
H2CO sp2 σ frameworkHybrid AO's and polyatomic MO's
22
Copyright © 2012 Dan Dill [email protected]
6 pairs in Lewis structure, 5 pairs in σ framework, and so 1 pair in (localized) π framework.
H2CO sp2 σ frameworkHybrid AO's and polyatomic MO's
23
Copyright © 2012 Dan Dill [email protected]
6 pairs in Lewis structure, 5 pairs in σ framework, and so 1 pair in (localized) π framework.
H2CO π frameworkHybrid AO's and polyatomic MO's
24
Copyright © 2012 Dan Dill [email protected]
1 pair in (localized) π framework
1 pair in π (bonding); bond order 1
H2CO π frameworkHybrid AO's and polyatomic MO's
25
Copyright © 2012 Dan Dill [email protected]
1 pair in (localized) π framework
1 pair in π (bonding); bond order 1
HCOO- sp2 σ frameworkHybrid AO's and polyatomic MO's
26
Copyright © 2012 Dan Dill [email protected]
9 pairs in Lewis structure, 7 pairs in σ framework, and so 2 pairs in (delocalized) π framework.
HCOO- π frameworkHybrid AO's and polyatomic MO's
27
Copyright © 2012 Dan Dill [email protected]
1 pair in π (bonding) and 1 pair in πn (nonbonding); bond order 1
2 pairs in (delocalized) π framework
SO2 sp2 σ frameworkHybrid AO's and polyatomic MO's
28
Copyright © 2012 Dan Dill [email protected]
9 pairs in Lewis structure, 7 pairs in σ framework, and so 2 pairs in (delocalized) π framework.
SO2 π frameworkHybrid AO's and polyatomic MO's
29
Copyright © 2012 Dan Dill [email protected]
2 pairs in (delocalized) π framework
1 pair in π (bonding) and 1 pair in πn
(nonbonding); bond order 1
SO2 correlation diagramHybrid AO's and polyatomic MO's
30
Copyright © 2012 Dan Dill [email protected]
10
9
8
Do these on your ownHCOOH, formic acid
allyl: [H2C=CH−CH2]−, anion; [H2C=CH−CH2]+, cation
H2C=C=CH2, alleneH2C=CH−CH=CH2, 1,3-butadiene
O3, ozoneFor each one,
• Write the Lewis structure• Sketch the σ framework and assign its pairs• Sketch the π framework MO’s, identify
localization or delocalization, bonding, nonbonding, antibonding, and assign its pairs, and get the π bond order
Hybrid AO's and polyatomic MO's
31
Copyright © 2012 Dan Dill [email protected]
HCOOH, formic acidFormate, HCOO− π , is planar and has• 9 pairs• 7 pairs in σ framework• 1 pair in a delocalized bonding π1 orbital• 1 pair in a delocalized nonbonding π2
n orbitalWhat about formic acid?• 9 pairs• 8 pairs in σ framework• 1 pair in localized bonding π1 orbital
Hybrid AO's and polyatomic MO's
32
Copyright © 2012 Dan Dill [email protected]
H2C=CH−CH2, allyl (radical)What about allyl? Use −CH2 sp2, to have increased delocalization:• 8 ½ pairs• 7 pairs in σ framework• 1 pair in delocalized bonding π1 orbital• ½ pair (1 electron) in delocalized nonbonding π2
n
orbital
Hybrid AO's and polyatomic MO's
33
Copyright © 2012 Dan Dill [email protected]
[H2C=CH−CH2]+, allyl cationWhat about allyl cation?• 8 pairs• 7 pairs in σ framework• 1 pair in delocalized bonding π1 orbital
Hybrid AO's and polyatomic MO's
34
Copyright © 2012 Dan Dill [email protected]
[H2C=CH−CH2]−, allyl anionWhat about allyl anion? If −CH2 is sp3, then• 9 pairs• 8 pairs in σ framework• 1 pair in localized bonding π1 orbitalIf −CH2 is sp2, then• 7 pairs in σ framework• 1 pair in delocalized bonding π1 orbital• 1 pair in delocalized nonbonding π2
n orbitalIncreased delocalization makes −CH2 is sp2
more stable (extension to recipe)
Hybrid AO's and polyatomic MO's
35
Copyright © 2012 Dan Dill [email protected]
H2C=C=CH2, alleneWhat about allene?• 8 pairs• 6 pairs in σ framework• 2 pair in two localized bonding π1 orbitals
Hybrid AO's and polyatomic MO's
36
Copyright © 2012 Dan Dill [email protected]
H2C=CH−CH=CH2, 1,3-butadieneWhat about 1,3-butadiene?• 11 pairs• 9 pairs in σ framework• 1 pair in delocalized bonding π1 orbital• 1 pair in delocalized bonding π2 orbital
Hybrid AO's and polyatomic MO's
37
Copyright © 2012 Dan Dill [email protected]
O3, ozoneWhat about ozone?• 9 pairs• 7 pairs in σ framework• 1 pair in delocalized bonding π1 orbital• 1 pair in delocalized nonbonding π2
n orbital
Ozone is polar. Why?The two electrons in the nonbonding MO are each only on the terminal O’s!
Hybrid AO's and polyatomic MO's
38
Copyright © 2012 Dan Dill [email protected]
O3 ozone σ and π frameworksHybrid AO's and polyatomic MO's
39
Copyright © 2012 Dan Dill [email protected]