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