2012 Orbital Hybrization, Sigma and Pi Bonds

LESSON

Hybridization(video tutorial links included)

IB Chemistry Power Points

Topic 14 (AHL)

Bonding

www.pedagogics.ca

Carbon has an electron arrangement 1s22s22p2

There is a contradiction here . . . . . .

Recall : methane, CH4 – tetrahedral shape (4 equal covalent bonds)

Recall: the s orbital shape

ONE s orbital in each energy level

Recall: the p orbital shape

THREE p orbitals in each energy level (beginning n = 2)

Carbon’s valence shell - 2nd energy level orbitals The 4 orbitals are oriented like so:

Problem #1

This is not a tetrahedral shape. The 3 p orbitals are at 900 to each other and the s orbital is “non-directional”

What is the expected orbital notation of carbonin its ground state?

Carbon ground state electron configuration

Problem #2?

Only 2 electrons appear to be available for bonding

Perhaps carbon “promotes” one of its 2s electrons…

…to the empty 2p orbital.

Solution – require 4 unpaired electrons

BUT. . . this would mean that three of the bonds in a methanemolecule would be identical,

But what about the fourth bond…?

Must have a lower bond energy

This was not observed by chemists

“hybridization” theory - What if the s orbital is combined with three p orbitals to create four equal hybrid orbitals?

These new orbitals have slightly MORE energy thanthe 2s orbital…… and slightly LESS energy than the 2p

orbitals.

But all 4 hybrid orbitals have equal energy.

“hybridized”

Since the 4 hybrid orbitals result from a combination of 1 s and 3 p orbitals, these hybrids are termed sp3 orbitals.

2s 2px 2pz2py1s sp3 sp3 sp3 sp3

109.5o

Hybridize

x

z

y

Summary of sp3 hybridization

sp3 hybridization

in methane

CH4

Summary of sp3 hybridization

Sigma (σ) bondsThe bonds formed in methane between carbon and hydrogen atoms are called sigma bonds.

As shown in the diagram, a sigma bond is formed by the head on overlap of two orbitals. THE OVERLAP REGION LIES DIRECTLY BETWEEN THE TWO NUCLEI. Nuclei can rotate about a sigma bond. Video tutorial link to ethane

sp2 hybridization

When boron bonds with hydrogen, VSEPR predicts a trigonal planar geometry.

Consider the electron configuration of boron:

One s orbital hybridizes with 2 p orbitals to form 3 sp2 orbitals

sp2 hybridization also occurs in molecules that contain double covalent bonds. For example, ethene C2H4

Each carbon : 3 hybridized sp2 orbitals and 1 p orbital

Sigma bond – single bond C-H

Sigma bond – sp2 orbitals: one half of double bond C=C

Pi bond – p orbitals: other half of double bond C=C

A pi bond consists of two p orbitals that overlap above and below a sigma bond. A double bond always consists of ONE sigma bond and ONE pi bond.

The molecule cannot be twisted without breaking the pi bond. Video tutorial ethene

Pi bonds

Practice

Draw a diagram to show the orbital overlap in the sigma and pi bonds in a molecule of formaldehyde CH2O. State the shape of the molecule and the arrangement of negative charge centers around the central atom.

Formaldehyde

Sigma bond

2 Lone Pairs2 Lone Pairs

bond

trigonal planar (3 regions of electron density)

sp hybridization

When beryllium bonds with chlorine, VSEPR predicts a linear geometry.

Consider the electron configuration of beryllium:

One s orbital hybridizes with 1 p orbitals to form 2 sp orbitals

sp hybridization

sp hybridization also occurs in molecules that contain triple covalent bonds. For example, acetylene C2H2

In ethyne C2H2 each carbon has 2 sp orbitals and 2 un-hybridized p orbitals.

The single bond between carbon and hydrogen is a sigma bond.

The triple bond between the two carbons consists of a sigma bond between two sp2 orbitals and two pi bonds between the two sets of p orbitals.

The two pi bonds are in different planes 900 to each other . A triple bond always consists of ONE sigma bond and TWO pi bonds. The molecule cannot be twisted without breaking the pi bonds.

Show video tutorial acetylene