Organic Chemistry - 3)a) Molecular Orbitals Flashcards
2)a) Molecular Orbitals (27 cards)
where do electrons of different spin spend most of their time between the two nuclei?
the bonding orbital
what is the bonding orbital?
where it is most likely to find electrons
what determines the shape of a molecular orbital?
quantum mechanics
why are antibonding orbitals formed?
the number of combining atomic orbitals is the same as the number of molecular orbitals formed, so bonding and antibonding orbitals are formed
state three properties of antibonding orbitals:
they have higher energy than bonding and atomic orbitals
they are a different shape
they do not contain electrons
what type of bonding arises as a result of end-on-end overlapping of atomic orbitals?
sigma bonds
how are Pi bonds formed?
by the side-on-side overlap of p-orbitals
when do Pi bonds occur?
when atoms form multiple bonds
(c=c)
what is weaker out of the Pi and Sigma bonds?
Pi bonds
what is sp3 hybridisation?
the mixing of one s orbital and three p orbitals to form four sp3 hybrid molecules of equal energy
what is sp2 hybridisation?
the mixing of one s and two p orbitals to form three sp2 orbitals of equal energy, leaving on p orbital unchanged
how are pi bonds formed?
the unhybridised p orbitals overlap side-on-side to form a pi bond
why are sigma bonds stronger than pi bonds?
they are very efficient at overlapping
why are pi bonds weaker?
they are formed by the side-on-side overlapping of unhybridised p orbitals which is less efficient and hence form weaker bonds
what is sp hybridisation?
the mixing of one s and one p orbital, leaving two unhybridised p orbitals
alkynes contain a carbon to carbon triple bond, describe in detail, the hybridisation and bonding that occurs in a triple bond:
one s and one p orbital mixes to form two sp orbitals. The sp orbitals overlap end-on-end to form a sigma bond. The two unhybridised p orbitals over lap side-on-side to form two pi bonds.
what type of hybridisation is present in alkanes?
sp3 hybridisation
what type of hybridisation is present in alkenes?
sp2 hybridisation
what type of hybridisation is present in alkynes?
sp hybridisation
what type of compounds appear colourless?
saturated (sigma bonds only)
why do saturated compounds appear colourless?
they only contain sigma bonds. The HOMO is the sigma bonding orbital and LUMO is the sigma antibonding orbital. This energy gap is very large and hence corresponds to the ultraviolet part of the spectrum. No visible light is absorbed and so the compound is colourless.
what can be said about unsaturated compounds like ethene or butene?
they contain simple pi bonds and hence still have a large transition to promote electrons from HOMO to LUMO. They also absorb light from ultraviolet part of the spectrum and appear colourless
what happens when light is absorbed?
the energy from photons is used to promote excited electrons from the HOMO to LUMO
what structures do coloured compounds contain?
conjugated system
