3.2 Molecular Orbitals Flashcards
Unit 3: Organic Chemistry & Instrumental Analysis
What is an Atomic Orbital?
An area in space in which there is a 95% chance of finding an electron in an atom at any point in time
What are the two different types of molecular orbitals?
-Sigma molecular orbital
-Pi molecular orbital
How does a sigma molecular orbital form?
They are formed by the end-on overlaps of two atomic orbitals along the axis of a covalent bond
How does a pi molecular orbital form?
They form by side-on overlaps of two parallel atomic orbitals that lie perpendicular to the axis of the covalent bond
Which are strong; Sigma or pi bonds?
Pi bonds are weaker than sigma bonds
What is a bond between atoms based upon?
The basis of the bond between atoms is the attraction between the positively charged nuclei, and the negatively charged electrons in the bonding molecular orbital
What must be true of the numbers of atomic and molecular orbitals?
The number of molecular orbitals formed is equal to the number of atomic orbitals that combine
Why is a molecular orbital stable?
The bonding molecular orbital is in a lower energy state because the two nuclei are held within the area of electron density
What happens when 2 atomic orbitals combine and it leads to constructive interference between two waves?
It reinforces there intensity, the inter-nuclear electron probability density is increased
What happens when 2 atomic orbitals combine and it leads to destructive interference between two wave functions?
This reduces their intensity and causes a decrease in the inter-nuclear electron probability density
Why is an antibonding molecular orbital less stable?
It is in a higher energy state, the electron density is concentrated outside the bonding area leading to increased repulsion between the nuclei pushing them apart. This raises the energy of the molecule weakening the bond so the molecule is less stable
When is an antibonding molecular orbital used?
They are used when excited electrons are promoted. These arrangements are often unstable
What is Electronegativitey?
The attraction of an atom involved in a bond, for the bonding pair of electrons
What is Hybridisation?
This is the process in which atomic orbitals mix within an atom to generate a set of new atomic orbitals - hybrid orbitals
Where does the energy of the hybrid orbitals lie?
Between that of the orbitals that make them up
What kind of hybridization occurs in alkanes?
sp3
How do 4 orbitals combine to form 4 degenerate sp3 hybrid orbitals? (alkanes)
-In the carbon atom, one of the 2s electrons is promoted to the third 2p orbital. The 2s orbital mixes with the three 2p orbitals, this forms four hybrid sp3 orbitals arranged in a tetrahedral geometry
Are the four sp3 orbitals degenerate or not?
They are degenerate and their energy is between that of the 2s and 2p orbitals
What is the maximum angle between orbitals in sp3
109.5
What type of molecular orbital forms between each sp3 orbital and the 1s orbital of each hydrogen atom?
Sigma bond
Describe the bonding in ethane (C2H6)
-Each carbon atom forms three sigma bonds with the three hydrogen atoms using three of its hybrid sp3 orbitals
-The fourth sp3 orbital forms a sigma bond with the fourth sp3 orbital of the second carbon atom
What are key features of alkenes?
Unsaturated, contain carbon to carbon double bonds
What kind of hybridization occurs in alkenes and why?
sp2
One of the 2s electrons is promoted into the empty 2p orbital. Two of the 2p orbitals hybridise with the 2s orbital, forming three degenerate sp2 orbitals
The result is three degenerate sp2 orbitals and a single 2p orbital in each carbon atom
Why does a double bond not require double as much energy to break then a single bond?
It suggests that the pi bond is weaker than the sigma bond
What type of hybridization occurs in a benzene ring?
Sp2
How does the bonding in benzene allow for it to be so stable?
Each of the six carbon atoms in benzene undergoes sp2 hybridization, so can form three sigma bonds
-One with each of the 2 neighboring carbon atoms
- One with a hydrogen atom
What is an alkyne?
An organic molecule with a carbon to carbon triple bond
How many Sigma and Pi bonds are there in a alkyne?
1 sigma bond and 2 pi bonds
Why are molecules containing double and triple bonds coloured?
They absorb visible light
What does HOMO mean?
The highest occupied molecular orbital
What does LUMO mean?
The lowest occupied molecular orbital
Where does an electron transition occur?
In the HOMO and LUMO gap
Transitions only including sigma bonds or compounds including sigma bonds and simple pi bonds are colourless, why?
They require much more energy, so they correspond to the UV part of the spectrum (no colour)
What is a chromophore?
A group of atoms in an organic molecules structure that absorbs photons of light in the visible part of the EM spectrum
What is a conjugated system?
A system of alternating single and double bonds
What occurs with the bonding in a conjugated system?
The bonding in a conjugated system is a system of unhybridized p-orbitals on adjacent atoms that overlap side on to form a pi molecular orbital that lies across a number of carbon atoms
What occurs with the electrons in a conjugated system?
The electrons are delocalized so move through the whole molecular orbital making the conjugated system very stable
As the number of double bond carbons involved in the conjugated system increases what occurs to the HOMO LUMO gap?
It decreases, this means that less energy is required to promote an electron so a lower wavelength is absorbed
