organic bonding - molecular orbital theory Flashcards
(24 cards)
how are σ MOs formed?
from the head-on overlap of s/s, p/p or s/p orbitals
distinguish between σ and σ* orbitals
σ = bonding orbital, formed from in-phase orbital interaction
σ* = antibonding orbital, formed from out-of-phase orbital interaction
how are π MOs formed?
from side-on overlap of p orbitals, specifically px and py
compare + explain the strength of σ and π bonds
σ bonds are stronger than π bonds, this is because head-on overlap is more efficient than side-on overlap
how is organic MO notation different to inorganic MO notation?
in organic chemistry u/g labels are generally omitted
hybridisation definition
when atoms in molecules change the shapes and energies of their atomic orbitals to provide the best overlap and maximise bonding - involves interactions between atomic orbitals on the same atom during bond formation
what are the 3 main types of hybridisation in organic chemistry + give example functional groups
sp3 = 1s+3p orbitals, forms 4 hybrid orbitals, found in alkanes
sp2 = 1s+2pxz orbitals, forms 3 hybrid orbitals, found in alkenes/carbonyls
sp = 1s+1pz orbitals, formed 2 hybrid orbitals, found in alkynes, nitriles
give the structural elements + geometries associated with each type of hybridisation
sp3 = single bonds, sometimes lone pairs, tetrahedral
sp2 = double bonds, trigonal planar
sp = triple bonds, linear
understanding - do orbitals really exist?
no - all maths based to help us understand electron movement + behaviour + bonding of atoms/molecules
* as orbitals don’t physically exist, an unoccupied orbital = a region where we could put electrons, but none are present
how does hybridisation follow conservation of energy?
total energy of hybridised orbitals = total energy of orbitals used to form them
why do molecules with hybridisation display specific geometries?
NOT BECAUSE OF HYBRIDISATION - ORBITALS DON’T EXIST
because it is the lowest energy/most stable arrangement of those atoms - consider electrons repulsion, etc
conformation definition
the spatial arrangement of atoms in a molecule, shape essentially
conformational isomer/conformer definition
molecules that can be interconverted simply by rotation of covalent bonds
what is the difference between conformers and stereoisomers?
for conformers, only rotation is required to interconvert between conformers, and this is permitted by σ bonds, whereas to convert to another stereoisomer bonds must be broken, as typically they include π bonds which don’t allow rotation
what are the 2 main types of conformers?
eclipsed and staggered conformations
- imagine an alkane, C-C bond
eclipsed conformers are higher in energy, because bonds are closer together, as angle between bonds attached to the 2 carbons is 0 (imagine newmann projection) therefore the bonds eclipse each other - this causes strain and repulsion
staggered conformations are when the bonds are as far apart as possible, so angle is maximised (= 60), these are more favourable
this happens because as bonds rotate the energy of the structure changes
give 6 rules for MO formation
- AOs must have correct symmetry to cmobine to give MOs
- no. MOs must = no. AOs combined
- bonding MOs are lower in energy than AOs combined, antibonding MOs are higher in energy
- MOs are filled using the same rules as filling AOs (aufbau, pauli, hund)
- for best interaction AOs combined should be the same size (same principle n?)
- only AOs of sufficiently similar energy interact to form MOs (same principle n)
outline + explain the energetic differences between sp3, sp2 and sp hybrid orbitals
s character sp3 < sp2 < sp
this is reflected in energy/distance from nucleus: sp3 > sp2 > sp
why are double bonds stronger than single bonds if σ bonds are stronger than π bonds?
because double bonds have both σ and π bonds
why do allenes have a twisted structure?
allenes: H2C=C=CH2 (e.g.)
the central carbon is sp hybridised, whereas the surrounding 2 carbons are sp2 hybridised
the central carbon formed a double bond with each surrounding carbon, using orbital px for on and py for the other, since these orbitals point in different directions, and the double bonds are formed from sideways overlap, the structure is twisted
how does electronegativity affect energy of orbtials?
more electronegative elements orbitals are lower in energy
how does electronegativity affect shape of MOs?
an electronegativity difference between 2 atoms forms a polar bond, this means the MOs are distorted, the more electronegative atom pulls bonding MOs towards itself
antibonding orbitals are the opposite, most of the volume is pushed towards the more electropositive atom
what will the hybridisation be for a molecule where the central atom has 3 valence electrons (or 6 electrons total including contributions from bonds)?
could be sp2 or sp3
sp2 leaves an empty p orbital, sp3 leaves an empty sp3 orbital
- if an orbital must be left empty we want it to be the highest energy orbital, as we want to populate the lowest energy orbitals for greater stability
p is higher in energy than sp3 so the atom will be sp2 hybridised, shape will be trigonal planar
- this is the case for any atom with 3 valence electrons, whether naturally or due to +/- ionisation
what will the hybridisation be for a molecule where the central atom has 3 valence electrons and a lone pair (or 8 electrons total, including contributions from 3 bonds)?
could be sp2 or sp3
sp2 leaves lone pair in p orbital, sp3 leaves lone pair in sp3 orbital
- the lone pair is very repulsive to bonds and wants to be as far away as possible, so consider geometries:
sp2 = trigonal planar, with lone pair in axial p orbital, angle involving lone pair = 90
sp3 = trigonal planar (tetrahedral pseudostructure), angle involving lone pair >109.5
so the atom will be sp3 hybridised
when is non-integer hybridisation seen?
when bonding in molecules cannot be rationalised as simply as sp3/sp2/sp - this is mostly seen in very strained molecules e.g. cyclopropane or cubane, where angles are forced to be unfavourably tight - this changes chemical properties
