Molecular orbitals

What are bonding molecular orbitals?

occupy the region of space between 2 nuclei and are formed when atomic orbitals merge

each bonding molecular orbital can hold max 2 electrons

What is the bonding continuum?

describes the position of the molecular orbitals in non-polar covalent and polar covalent bonds as well as ionic bonding

What is hybridisation?

the process of mixing atomic orbitals within an atom to generate a new set of atomic orbitals called hybrid orbitals

What are sigma bonds?

covalent bond formed from end-on overlap of the hybrid orbitals

What are pi bonds?

form side on overlap of unhybridised 2p orbitals

sp3 hybridised carbon (alkanes)

involves a 2s electron being **promoted** to an empty 2p orbital followed by **mixing** of the 2s orbital and the ==3 2p orbitals to produce a new set of 4 sp3 hybrid== orbitals which are degenerate

* all carbons in alkanes are sp3 hybridised (sigma bonds)

sp2 hybridised carbon (alkenes)

electron from 2s orbital is **promoted** to the empty 2p orbital then **mixing** of the 2s orbital and ==2 2p orbitals produces 3 sp2== hybrid orbitals which are degenerate

Benzene

* C6H6
* ring of carbon atoms
* bonding described in terms of sp2 hybridisation
* carbon atoms joined together by sigma bonds

What are alkynes?

carbon to carbon triple bond

sp hybridised carbon (alkynes)

after promotion of an s electron to an empty p orbital, the 2s orbital and one of the 2p orbitals of carbon hybridise to form twi degenerate hybrid orbitals

What is the highest occupied molecular orbital (HOMO)?

the orbital containing the electrons with the highest energy

What is the lowest unoccupied molecular orbital (LUMO)?

the lowest energy antibonding molecular orbital that is empty

Colour in single C-C bonds and no non-bonding electrons

HOMO is the sigma bonding molecular orbital and the LUMO is the sigma\* antibonding molecular orbital

The energy gap between HOMO and LUMO is large and corresponds to UV light so these compounds are **colourless**

What is a conjugated system?

* electrons can be delocalised across a number of carbon atoms
* system adjacent unhybridised p orbitals that overlap side on to form a molecular orbital across a number of carbon atoms

Coloured organic compound

* needs a chain of alternating sigma and pi bonds which provides large degree of conjugation
* conjugated system contains alternating single and double bonds or benzene rings

What is the effect of the number of carbon-carbon double bonds within conjugated system?

affects the colour of the compound which arises as electrons move from HOMO to the LUMO the colour being complementary to the wavelength of light being absorbed

What is the chromophore?

a group of atoms within a molecule that is responsible for the absorption of light in the __**visible**__ region of the electromagnetic spectrum

* exist in molecules containing a conjugated system

Effect of more atoms in conjugated system?

the smaller the HOMO - LUMO energy gap so light of a lower frequency is absorbed

Why do most organic molecules appear colourless?

the energy difference between HOMO and LUMO is relatively large resulting in the absorption of light in the UV region of the spectrum

What is conjugation?

alternating double and single bonds in a long chain