Hybridisation of Orbitals Flashcards
How do Molecular orbitals form
Through the combining of atomic orbitals
Organic molecules usually possess tens of atoms
Therefore when considering combing atomic orbitals to make molecular orbitals, how do we overcome this
We use an approach called hybridisation
This simplifies the picture and gives a good ‘day-to-day’ approximation that works particularly well for organic molecules
How does sp3 hybridisation work?
In sp3 hybridisation we combine the s atomic orbitals with the three p atomic orbitals
For example in carbon we combine the 2s atomic orbitals with the three 2p atomic orbitals
How many bonds does carbon usually form? Hence how does this affect the hybridisation?
Carbon usually forms 4 bonds, hence four sp3 on one carbon atom point come together in a tetrahedral like shape .
Therefore the 4 identical, evenly distributed orbitals gives a 109.5 degree angle and tetrahedral geometry
Describe the simplifed molecular orbital diagram for methane
On the LHS, you have the usual electronic configuration for carbon: 1s², 2s², 3p²
To show the hybridisation process which mixes the 2s and three 2p orbitals to produce four sp³ hybrid orbitals, each contains 1 electron
We combine the four sp³ orbitals with four 1s orbitals for the 4 hydrogen atoms to produce a total of 8 molecular orbitals with:
Four sigma bonds (C-H bonds) and four sigma* antibonding orbitals
There are 8 electrons within the sigma bonding orbitals.
Why don’t we the hybridise the atomic orbital in hydrogen, but we do in carbon
Hydrogen only has 1 atomic orbital (1s) hence no hybridisation is required
Compared to methane, what is the difference is hybridisation in ethane
We also need to form another C-C bond by combining sp³ orbital from each carbon atom. This allows the tetrahedral shape to be maintained
Describe the Molecular orbital diagram for ethane
On the LHS and RHS, we have the two unhybridized carbon atoms with their usual electronic configuration
Next to that there is the hybridised version where we have 4 sp³ orbitals from each carbon atom
Two of the sp3 orbitals combine to make the carbon-carbon sigma bonding orbital and antibonding orbital
The two electrons from each orbital both go into the sigma bonding orbital to produce a stable bond
The remaining 3 sp³ orbitals from the carbon atom are used for bonding to hydrogens
How does sp² hybridisation occur
We combine the 2s atomic orbitals with only two 2p² orbitals
Left over we have an unhybridized p orbital
Therefore in carbon you have 3 identical, evenly distributed orbitals with a bond angle of 120°, making it trigonal planar
Therefore when considering sp², how does the bonding work in an ethene molecule
On the LHS, there are 2 carbons, each can bond to 2 hydrogens by overlap of sp2 atomic orbitals with 1s orbital from each of the hydrogen
This leaves one sp² orbital on each carbon atom, which can combine to make the central sigma bond between the two carbon atoms
Left over in green we have the unhybridsed p orbital which can undergo edge-on overlap to produce the pi bond
Describe the Molecular orbital diagram for ethene (where the C-H bonds are not show)
On the LHS and RHS, there is the electronic configuration (for valance electrons) in two carbon molecules
Next to that is the hybridised versions, where each carbon atom has three sp² and an unhybridized p orbital
Two of the sp² orbitals on carbon to produce the C-C sigma bonding orbital and sigma* antibonding orbital
The other two sp² orbitals on carbon are used to bond to hydrogen
The unhybridized p orbital on carbon overlaps to form a pi bonding orbital and a P* antibonding orbital
You cannot rotate around a double bond. In the context of Trans-but-2-ene and its cis isomer, explain in terms of hybridisation of why this is the case
Because the Pi bonding would be broken through the rotation
How does sp hybridisation occur?
We combine the 2s atomic orbitals with one sp atomic orbital. The px (green) and py (orange) orbitals are left unhybridized. The two sp atomic orbitals (red) are identical and evenly distributed
This forms two identical and even distributed atomic orbitals with a bond angle of 180 degrees and a linear shape
How does the bonding occur in ethyne
On the LHS, there is two carbon atoms with sp hybridisation
In each case they can bond to 1 hydrogen through the overlap of one of their sp orbitals and the 1s orbital of hydrogen
The other sp orbitals on the carbon can combine together to form a sigma bond (red)
The unhybridized p orbital can then combine with edge on overlap to produce two pi bonds
This form a triple bond between the two carbons
The atomic s and p orbital hybridise to maximise distance between repulsive electron pairs
Methylamine exhibits sp³ hybridisation, explain how?
Methylamine is pyramidal around the nitrogen – one bonding pair is replaced with a lone pair
Methanol exhibits sp3 hybridisation around the oxygen, explain how
Methanol bent around oxygen - 2 bonding pars are replaced by 2 lone pairs
Methylimine has sp2 hybridisation, explain why
One of the bonds on the nitrogen has been replaced with a lone pair
Hence has a similar shape to ethene
The C=O bond in methanal is sp2 hybridised, explain how
In methanal two of the bonds of the nitrogen have been replaced with lone pairs which occupy two of the sp2 orbitals
Hydrogen cyanide is sp hybridised, explain how
Hydrogen cyanide where one of the carbons is replaced with a nitrogen atom and a lone pair on the nitrogen which replaces the bonded pair seen on ethyne
What hybridisation occurs in carbonyl group
On the left there is a carbon atom with sp2 hybridisation - bonded to 2 hydrogen atoms
Next to it is an oxygen atom with sp2 hybridisation - two lone pairs occupy two sp2 orbitals. The remaining sp2 orbital (on oxygen) is able to interact with the remaining sp2 orbitals on the carbon atom to form sigma bond
The remaining sp2 orbital is able to interact with one of the sp2 orbitals on the carbon to form a sigma bond
The unhybridised p orbitals on carbon and oxygen are able to overlap to form a pi bond
Describe the Molecular orbital diagram for carbonyl group
- Oxygen on the right hand side of the diagram, has two more electrons than carbon
- The energies from the orbitals of oxygen are lower than they are for carbon - diagram isn’t perfectly symmetrical
- This means that if we look at the sigma and pi bonding orbitals in the centre of the diagram, they are closer in energy to the atomic orbitals coming from oxygen, than carbon
- In contrast the sigma and sigma * bonding orbital are closer in energy to carbon than to oxygen
This means the bond is polarised as electrons are distored more towards the oxygen atom
