Geometry and Hybridization 2 PDF

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Geometry and Hybridization in Organic Chemistry sp2 hybridization

How to check Lewis Structures To determine the geometry of a molecule, you must first have its correct Lewis structure drawn. This means connecting the atoms correctly and placing any lone pairs where they belong.

In sp2 hybridization the s orbital is mixed with only two out of three p orbitals. The resulting 3 sp2 orbitals are then arranged in a trigonal planar geometry (120o).

The chart below summarizes the most common Electron and Molecular Geometries you will encounter in Organic Chemistry:

Remember, C, N, O, F and the other halogens follow the octet rule.

p orbital

3 sp2 orbitals + 1 p

2p

If the atom does not have an octet, it may have a formal charge.

hybridize

C

2s sp2 hybrid orbitals

The chart below shows standard valency, lone pairs and charges for drawing correct Lewis structures. 3 orbitals mixed

3 orbitals formed

Two such carbon atoms make a sigma bond by overlapping one of the three sp2 orbitals. Two hydrogens make bonds with each carbon by overlapping their s orbitals with the other two sp2 orbitals. The two p orbitals of the carbons overlap to make one bond. bond forms Key points: * All the atoms are in one plane.

p orbitals H H

107o

H C

C H H

+

104.5o

C H

H

* The angle between atoms is 120o.

C H H

sp orbitals

* The angle between the plane and p

H C

2

C

H

orbitals is 90o.

H

Note: Lone pairs repel stronger so the angels can vary. For example, the angle between hydrogens in H2O is 104.5o and is 107o in NH3. 120o

The Lewis structures below show some examples of having wrong number of electrons or a charges: H3C O H H H3C O H H3C O

should have a positive charge on the oxygen the oxygen should have another lone pair should be a negative charge on the oxygen

sp hybridization

Hybridization and Hybrid Orbitals

In sp hybridization the s orbital is mixed with only one out of the three p orbitals. The resulting two sp hybrid orbitals are then arranged in a linear geometry (180o).

Hybridization is a theory that is used to explain certain molecular geometries that would have not been possible otherwise.

H3C O H H H H3C O

sp3 hybridization:

2p

The classical example is the tetrahedral geometry of methane (CH4). The ground state electron configuration of carbon is 1s22s22P2. According to this, carbon should only form two bonds and have a lone pair of electrons because it has only two unpaired valence electrons. The strucuture of

H3C O

Predicting Electron and Molecular Geometry (VSEPR Theory) possible. The optimal orientation/angle between these atoms varies depending on how many

C sp hybrid orbitals

the first hydrocarbon, however, is CH4 and hybridization explains how that happens.

The principle of VSEPR theory is that the atoms and lone pairs repel each other as far away as

2p orbitals

2 sp orbitals + 2p

hybridize

2s

2 orbitals formed

2 orbitals mixed

Step 1) One electron jumps from the 2s to the 2p orbital. This leads to the excited state of the carbon. Step 2) The s and p orbitals of the excited state carbon are hybridized to form four identical in size, shape and energy sp3 orbitals. The formation of these degenerate hybrid orbitals compensates the energy uphill of the s-p transition as they have lower energy than the p orbitals.

Two carbon atoms make a sigma bond by overlapping the sp orbitals. One hydrogen binds to each carbon atom by overlapping its s orbital with the other sp orbital. The two p orbitals of each carbon overlap to make two bonds. bond form

atoms and lone pairs are around the central atom.

Key points:

To determine the angel, simply locate the central atom and sum up the number of atoms and E Steric Number = Atoms + Lone Pairs

H

4 sp3 orbitals

2p

2p

lone pairs around it. This is called Steric Number (SN).

C

+

C

H

H

hybridize

H C

2s

2s

C

C

H

* All the atoms have linear geometry. * The angle between atoms is 180o.

C H

1s22s22P2 In organic chemistry, you will only need to know up to SN = 4 for the most part.

Examples:

* Cl Be Cl

F

H * H C H H

* H N H H

SN = 3

SN = 3

SN = 4

SN = 4

F SN = 2

SN = 2

F * B

C

4

H H

For Electron Geometry, we treat the atoms and electrons equally. For naming the Molecular Geometry, we ignore the lone pairs. So, for the first molecule (CH4), the electron and molecular geometries are tetrahedral. However,

sp3

hybrid orbitals

H

1s orbitals

s-p Hybridization types do not refer to carbon atoms only. The theory applies to any atom. To determine the hybridization of a given atom in any molecule, determine its steric number : This method works flawlessly - almost no exceptions. If the steric number is 4, it is sp3 If the steric number is 3 - sp2

109.5o

C

H H

Notice that the last two molecules have the same steric number (4) but different number of atoms and lone pairs. This is where the difference of Electron and Molecular Geometry comes into play.

4 orbitals formed

A rule for any hybridization: the number of hybrid orbitals is equal to the number of mixing orbitlas. The four sp 3-hybridized orbitals arrange in a tetrahedral geometry and make bonds by overlapping with the s orbitals of four hydrogens: This explains the symmetrical geometry of methane (CH4) where all the bonds have the same length and bond angle.

Notice that multiple bonds do not matter, it is atoms + lone pairs for any bond type.

bond

CH HC SN = 3,

Trends relating bond type and hybridization: All four C – H bonds in methane are single bonds that are formed by head on overlapping of and s orbitals C H bond Remember that single bonds are sigma (

SN = 4, sp3 hybridized NH2

If the steric number is 2 - sp

sp3 orbitals sp3

bonds. H

for the second molecule (NH3), the electron geometry is tetrahedral, while the molecular geometry is trigonal pyramidal.

How to Quickly Determine the Hybridization of Any Atom

4 orbitals mixed

O C* H H

* H C N

H C

Carbon atoms in all the alkanes are sp 3 hybridized.

H H

C H H

c - c bond by overlapping of orbitals of the two carbons

sp3

sp2 hybridized

If the atom has a double bond - most often sp2 If it only has single bonds - most often sp3 If it has a triple bond - most often sp SN = 4, (Remember these are patterns but do not rely on them sp3 hybridized solely as they do have exceptions)

SN = 3, sp2 hybridized

C O HC H C N H O SN = 4, sp3 hybridized C C SN = 2, sp hybridized C Cl

CH2

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