organic chemistry · vsepr electron domain: a region in space (around a particular atom) that...
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Organic Chemistry
Review Information for Unit 1VSEPRHybrid OrbitalsPolar Molecules
VSEPR
The valence shell electron pair repulsion model (VSEPR) can be used to predict the geometry around a particular atom in an organic compound.
VSEPR assumes that the electron domainsaround a particular atom will be arranged in a way that minimizes repulsion between the negatively charged regions containing electrons.
VSEPR
Electron domain: a region in space (around a particular atom) that contains electrons
Electron domain geometry: the arrangement of electron domains around a particular atom
2 electron domains linear 3 electron domains trigonal planar 4 electron domains tetrahedral
Remember: Double and triple bonds count
as one electron domain.
VSEPR
You must be able to determine the electron domain geometry around a specified atom in an organic compound. Draw the Lewis structure Count the number of electron domains Assign the electron domain geometry
VSEPR
Example: What is the electron domain geometry around C in the following molecule? Around N?
H C N
H
H
H
H
VSEPR
Example: What is the electron domain geometry around C in the following molecule? Around N?
H C N
H
H
H
H
C: 4 electron domains tetrahedral
N: 4 electron domains tetrahedral
Answer:
VSEPR
Example: Identify the electron domain geometry for C, H, and N for the following compound.
HCNH2
C
O
H N C
O
VSEPR
Example: Identify the electron domain geometry for C, H, and N for the following compound.
Trigonal planartetrahedral
HCNH2
C
O
H N C
O
HCNH2
C
O
H N C
O
Answer:
Hybrid Orbitals
Simple overlap of atomic orbitals containing unpaired electrons generally does not adequately explain the geometries of organic compounds.
Valence bond theory uses hybrid orbitals to explain and predict structures and geometries of organic compounds atomic orbitals formed when two or more orbitals on the same atom are mixed
Hybrid Orbitals
Atomic OrbitalsCombined Hybrid Orbitals Produced
s, p two sp hybrid orbitalss, p, p three sp2 hybrid orbitalss, p, p, p four sp3 hybrid orbitalss, p, p, p, d five sp3d hybrid orbitalss, p, p, p, d, d six sp3d2 hybrid orbitals
Note: Only sp, sp2, and sp3 hybrid orbitals are important in organic compounds.
Hybrid Orbitals
Each type of hybrid orbital is associated with a particular type of electron domain geometry. the same geometry that would be predicted by VSEPR
Hybrid # of Electron Electron DomainOrbital Set Domains Geometry
sp 2 linearsp2 3 trigonal planarsp3 4 tetrahedralsp3d 5 trigonal bipyramidalsp3d2 6 octahedral
Hybrid Orbitals
Example: Identify the hybrid orbitals used by each C in the following compound.
CH CO2HC CH
2
Step 1: Draw the Lewis structure:
C CH C C O H
OH
H
Hybrid Orbitals
Step 2: Count the number of electron domains around each carbon and use that to identify the hybrid orbitals:
C CH C C O H
OH
H2
2
4 3
C CH C C O H
OH
Hsp
sp
sp3sp2
Answer:
Polarity
The polarity of a bond is measured by its dipole moment.
The polarity of a molecule is measured by its molecular dipole moment. Dipole moment of a molecule as a whole indicator of a molecule’s overall polarity vector sum of individual bond dipole moments
reflects magnitude and direction of individual bond dipole moments
Polarity
Compare formaldehyde and CO2:
Formaldehyde has 1 polar C=O bondmolecular dipole moment = 2.3
CO2 has 2 polar C=O bondsmolecular dipole moment = 0
WHY??
O
H
C O
CH
O
O
H
C O
CH
O
Polarity
In some molecules like CO2, the bond dipole moments cancel out due to the molecule’s electron domain geometry.
O
H
C O
CH
O
O
H
C O
CH
O
m = 0
m = 2.3 D
Polarity
In order to determine if a molecule is polar or not, you must: draw a 3-D structure for the compound
Use VSEPR to determine the electron domain geometries around each atom
identify the direction of the dipole moments for each polar bond
determine if the vector sum of the individual dipole moments leads to a nonzero molecular dipole moment (i.e. a polar molecule)
Polarity
Example: Determine if each of the following compounds is polar or nonpolar. For those that are polar, determine the direction of the molecular dipole moment.O
CH3
CH3CCH
3
C O
CH
O
CCH
3
O
O
O
CH3
CH3CCH
3
CCl4
C O
CH
O
CCH
3
O
O
Polarity
Draw Lewis structures and draw 3-D structure showing geometry of polar bonds in the molecule:
CH3
CCH
3
O
CH3
CCH
3
O
C
Cl
ClCl
Cl
Polarity
Show dipole moments for each polar bond:
CH3
CCH
3
O
CH3
CCH
3
O
C
Cl
ClCl
Cl
Polarity
Determine if molecule has a dipole moment.
CH3
CCH
3
O
Dipole moments
cancel out.
nonpolar
polar
Molecular dipole moment
C
Cl
ClCl
Cl