unit 2b: molecular orbitals Flashcards
molecular orbitals
formed when atoms approach each other to form bonds and their atomic orbitals combine e.g. two hydrogen atoms- the 1s orbitals from each atom combine
the number of molecular orbitals that form
is equal to the number of atomic orbitals that combine
bonding orbital
the highest occupied molecular orbital (HOMO)
antibonding orbital
lowest unoccupied molecular orbital (LUMO)
molecular orbitals can hold
a maximum of two electrons
electrons fill
bonding MOs and leave the higher energy antibonding MOs unfilled
in the bonding MO
the attraction of the positive nuclei and the negative electrons is the basis of bonding between atoms
electrons fill from the
lowest energy orbital up
non polar covalent bond
electrons are shared equally so the bonding MO is spread evenly over both nuclei, the bonding MO is symmetrical around the midpoint between the two atoms
polar covalent bond
the two nuclei are sharing electrons but not evenly, the bonding MO is asymmetrical around the midpoint between the two atoms, more of the MO appears over the more electronegative atom
ionic bond
an electron has transferred to the more electronegative atom, the bonding MO will sit almost fully over the negative ion, this is an extreme case of asymmetry
sigma molecular orbitals
formed by the end-on overlap of atomic orbitals
carbons ability to form 4 bonds
can be explained by a theory called hybridisation
2s and 2p orbitals
are close in energy allowing one of the 2s electrons to be promoted to the empty 2p orbital
bonding in alkanes
the three 2p orbitals and the 2s orbital mix with each other (hybridise) and create four new hybrid atomic orbitals
sp3 orbitals
they are made of one s and three p orbitals and are degenerate
sp3 hybridisation
one electron occupies each sp3 orbital resulting in 4 unpaired electrons in identical, degenerate orbitals
four sp3 orbitals
take up a tetrahedral arrangement around the C atom
forming methane
the s orbitals of four hydrogen atoms approach the four sp3 orbitals, end on overlap occurs between them, four new molecular orbitals are created
bonds in methane
sigma molecular orbitals are formed therefore sigma bonds form between C and each H, one molecule of methane contains four sigma bonds
bonding in ethane
end on overlap occurs between three of the sp3 orbitals of each carbon
ethane has … bonds
seven sigma bonds, six C-H sigma bonds and one C-C sigma bond, both carbons are sp3 hybridised
sideways overlap compared to end on overlap
weaker
pi orbitals
formed from sideways overlap of orbitals, the bonding pi orbital exists in two parts from the top and bottom lobes
bonding in alkene
only two of the p orbitals then mix with the 2s orbital
sp2 hybridisation
occurs in alkenes where three degenerate sp2 hybrid orbitals are formed, the other p orbital remains unhybridised
three sp2 hybrid orbitals
adopt a trigonal planar arrangement, the unhybridised p orbital is perpendicular to the sp2 hybrid orbitals
ethene bonding
4 sigma bonds form between the sp2 orbitals and the s orbitals of the hydrogens, a sigma bond forms between both carbons, sideways overlap occurs between the two unhybridised p orbitals forming a pi bond
a C=C double bond consists of
one sigma bond and one pi bond
molecules that contain a benzene ring are known as
aromatics
bonding in aromatics
the carbon atoms in benzene undergo sp2 hybridisation, end on overlap occurs between the carbon atoms to form sigma bonds and one hydrogen is bonded to each carbon with a sigma bond
pi bonds in benzene
each carbon has an unhybridised p orbital, sideways overlap occurs forming a donut shaped pi bonding system above and below the carbon ring
alkynes
unsaturated hydrocarbons which all contain a carbon to carbon triple bond
sp hybridisation
takes place to allow a triple bond to form
bonding in alkynes
an electron from the 2s is promoted to the empty 2p, the 2s orbital and one of the 2p orbitals hybridise to form two degenerate sp hybrid orbitals leaving two unhybridised p orbitals
sp hybrid orbitals
are linear around the carbon, the two unhybridised p orbitals lie perpendicular to the sp hybrid orbitals and to each other
C-C
sp3 hybridisation, 1 sigma bond
C=C
sp2 hybridisation, 1 sigma bond and 1 pi bond
C=-C
sp hybridisation, 1 sigma bond and 2 pi bonds
molecular orbital theory
can be used to explain why organic molecules are colourless or coloured
deltaE
the absorption of energy can cause electrons to be promoted from HOMO to LUMO
in most organic molecules
deltaE is large meaning the wavelength of light absorbed lies outwith the visible spectrum so no colour is observed
chromophores
a group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum
coloured organic molecules contain
chromophores
chromophore example
beta carotene
chromophores exist in
molecules with a conjugated system
conjugated system
adjacent unhybridised p orbitals that overlap sideways to form a molecular orbital across a number of carbon atoms where the electrons are delocalised
a conjugated system often consists of
alternating double and single bonds (or multiple connected benzene rings)
a chromophore/ conjugated system always starts and ends with
a double bond
as the conjugated system gets larger
the energy gap between HOMO and LUMO decreases therefore lower energy light is required to promote an electron so the light absorbed is part of the visible spectrum
the colour of the compound is
the complementary colour of the light absorbed
different colours of organic compounds are the result of
differing sizes of the conjugated system
explain fully how colour arises in molecules
electrons move from HOMO to LUMO, absorption of light means the light of the complementary colour is seen
explain why a molecule with a shorted chromophore will absorb a shorter wavelength of light
there is less conjugation, larger energy gap
explain what is meant by sp2 hybridisation
mixing an s orbital with two p orbitals
explain how a conjugated system gives rise to the colour red
electrons promoted from HOMO to LUMO, the complementary colour is absorbed
