3.1 molecular orbitals Flashcards
can VSEPR explain the bonding in all compounds
no
what can molecular theory provide an explanation for
more complex molecules
when do molecular orbitals form
when atomic orbitals combine
what is the number of molecular orbitals formed equal to
the number of atomic orbitals that combine
what does the combination of two atomic orbitals result in
the formation of a bonding molecular orbital and an antibonding orbital
what encompasses both nuclei in a molecular orbital
the bonding molecular orbital
what forms the basis of bonding between atoms
the attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital
how many electrons max can each molecular orbital hold
2
how are bonding molecular orbitals for non polar covalent bonds described
symmetrical about the mid point between two atoms
what do polar covalent bonds result from
bonding molecular orbitals that are asymmetric about the midpoint between the two atoms
which atom in an orbital has the greater share of bonding electrons
the one with the greater value for electronegativity
how are ionic compounds described in relation to molecular orbitals
an extreme case of asymmetry, with the bonding molecular orbital being almost entirely located around just one atom, resulting in the formation of ions
what are sigma bonds
the molecular orbitals that form by end on overlap of atomic orbitals along the axis of the covalent bonds
what are pi bonds
the molecular orbitals that form by side on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond
what can electronic configuration of an isolated carbon atom not explain
the number of bonds formed by carbon atoms in molecules
how can the bonding and shape of molecules of carbon be explained
by hybridisation
what can hybridisation explain
the bonding and shape of molecules of carbon
what is hybridisation
the processs of mixing atomic orbitals within an atom to generate a set of new atomic orbitals
what are hybrid orbitals
the degenerate orbitals produced by hybridisation
in alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form…
four degenerate sp3 hybrid orbitals
what do degenerate sp3 hybrid orbitals arrange in
a tetrahedral arrangement
how do sp3 hybrid orbitals form sigma bonds
by overlapping end on with other atomic orbitals
how can bonding in alkenes be explained
in terms of sp2 hybridisation
how does hybridisation occur in alkenes
the 2s orbital and two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals
what arrangement is adopted by sp2 hybrid orbitals
trigonal planar
which bond type is formed by hybrid sp2 orbitals
sigma
which bond type is formed by hybrid sp3 orbitals
sigma
what happens to the remaining 2p orbital on each carbon atom of the double bond in alkenes when hybridisation occurs
remains unhybridised and lies perpendicular to the axis of the sigma bond
which bond type is formed by the unhybridised p orbitals
pi
how can the bonding in benzene and other aromatic systems be described in terms of
sp2 hybridisation
how are the carbon atoms in benzene arranged
a cyclic structure with sigma bonds between the carbon atoms
which bonds hold the carbon atoms together in a benzene ring
sigma
what do the unhybridised p orbitals on each carbon atom in a benzene ring do
they overlap side on to form a pi molecular system, perpendicular to the plane of the sigma bonds
how can the electrons in a benzene ring be described
delocalised
how can the bonding in alkynes be explained
in terms of sp hybridisation
what arrangement do the 2s and 2p orbitals of carbon adopt when they hydridise to form two degenerate hybrid orbitals
linear
which bonds are formed when the hybrid sp orbitals overlap
sigma
which bonds are formed when the unhybridised p orbitals overlap
pi
what can molecular orbital theory be used to explain
why organic molecules are colourless or coloured
what is the HOMO
the highest occupied molecular orbital
what is the LUMO
the lowest unoccupied molecular orbital
what is the highest occupied molecular orbital
the highest bonding molecular orbital containing electrons
what do electrons leave when they fill bonding molecular orbitals
they leave higher energy antibonding orbitals unfilled
what can cause electrons to be promoted from HOMO to LUMO
absorption of electromagnetic energy
why do most organic molecules appear colourless
because the energy difference between
why do most organic molecules appear colourless
because the energy difference between HOMO and LUMO is relatively large, resutling in absorption of light from the UV region of the spectrum
what are chromophores
a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
when can light be absorbed
when electrons in a chromophore are promoted from the HOMO to the LUMO
in which molecules do chromophores exist
those containing a conjugated system
what is a conjugated system
a system of adjacent unhybridised p orbitals that overlap side on to form a molecular orbital across a number of carbon atoms
what state are electrons in when they are in a conjugated system
delocalised
what are examples of molecules with conjugated systems
molecules with alternating single and double bonds
aromatic systems
what is the relationship between the number of atoms in the conjugated system and the energy gap between HOMO and LUMO
the more atoms in the conjugated system, the smaller the energy gap between HOMO and LUMO.
what is the relationship between the number of atoms in the conjugated system and the energy gap between HOMO and LUMO
the more atoms in the conjugated system, the smaller the energy gap between HOMO and LUMO.
what happens when a wavelength of light is absorbed in the visible region
the compound will exhibit the complementary colour
what happens to the bonds in reactant molecules when an organic reaction takes place
they are broken down and the bonds in the product are made
what is bond fission
the process of bond breaking
what are the different types of bond fission
homolytic and heterolytic
what does homolytic fission result in
the formation of two neutral radical
when does homolytic fission occur
when each atom retains one electron from the sigma covalent bond and the bond breaks evenly
when does homolytic fission normally occur (in relation to bond types)
when non polar covalent bonds are broken
what do reactions involving homolytic fission tend to result in the formation of
a very complex mixture of products, making them unsuitable for organic synthesis
what does heterolytic fission result in
the formation of two oppositely charged ions
when does heterolytic fission occur
when one atom retains both electrons from the sigma covalent bond and the bond breaks unevenly.
when does heterolytic fission usually occur in relation to bonds
when polar covalent bonds are broken
why are reactions involving heterolytic fission better suited for organic synthesis
they tend to result in far few products than reactions involving homolytic fission
how can the movement of electrons during bond fission and bond making be represented
using curly arrow notation
what is a conjugated system
molecules containing delocalised electrons spread over a number of atoms
what must be present for the bonds to be conjugated in long carbon chains
alternating single and double bonds
