Hybrid AO's and polyatomic MO's CH101 Fall 2012 Boston University
Boston University Slideshow Title Goes HereFigures 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 © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
Hybridized AO’s account for central atomelectron‐pair geometry
Hybrid AO's and polyatomic MO's
3
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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 number
electron‐pairgeometry
hybridization moleculargeometry
H2O 4 tetrahedral sp3 bent at 109˚
NH3 4 tetrahedral sp3 trigonal pyramidal
CH4 4 tetrahedral sp3 tetrahedral
SO2 3 trigonal planar sp2 bent at 120˚
BH3 3 trigonal planar sp2 trigonal planar
CO2 2 linear sp linear at 180˚
Boston University Slideshow Title Goes Here
Examples
CO2, carbon dioxideH2CO, formaldehyde
HCO2‐, formate
SO2, sulfur dioxide
Hybrid AO's and polyatomic MO's
11
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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 σMO 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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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 …
Boston University Slideshow Title Goes Here
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 …
Boston University Slideshow Title Goes Here
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.
Boston University Slideshow Title Goes Here
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 …
Boston University Slideshow Title Goes Here
CO2 π frameworkHybrid AO's and polyatomic MO's
19
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
π framework recipe
Unused p AO’s form same number of πMO’s Number of loops and AO overlap determine
whether πMO is … bonding (π) nonbonding (πn) antibonding (π*)
Hybrid AO's and polyatomic MO's
20
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
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
Boston University Slideshow Title Goes Here
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.
Boston University Slideshow Title Goes Here
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.
Boston University Slideshow Title Goes Here
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
Boston University Slideshow Title Goes Here
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
Boston University Slideshow Title Goes Here
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.
Boston University Slideshow Title Goes Here
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
Boston University Slideshow Title Goes Here
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.
Boston University Slideshow Title Goes Here
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
Boston University Slideshow Title Goes Here
SO2 correlation diagramHybrid AO's and polyatomic MO's
30
Copyright © 2012 Dan Dill [email protected]
10
9
8
Boston University Slideshow Title Goes Here
Do these on your ownHCOOH, formic acid
allyl: [H2C=CH−CH2]−, anion; [H2C=CH−CH2]+, cationH2C=C=CH2, allene
H2C=CH−CH=CH2, 1,3‐butadieneO3, ozone
For 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 πbond order
Hybrid AO's and polyatomic MO's
31
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
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
norbital
What 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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
[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]
Boston University Slideshow Title Goes Here
[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 orbital
If −CH2 is sp2, then 7 pairs in σ framework 1 pair in delocalized bonding π1 orbital 1 pair in delocalized nonbonding π2
n orbital
Increased delocalization makes −CH2 is sp2 more stable (extension to recipe)
Hybrid AO's and polyatomic MO's
35
Copyright © 2012 Dan Dill [email protected]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
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]
Boston University Slideshow Title Goes Here
O3 ozone σ and π frameworksHybrid AO's and polyatomic MO's
39
Copyright © 2012 Dan Dill [email protected]