Presentation on theme: "Molecular Orbitals and Hybridisation"— Presentation transcript:
1Molecular Orbitals and Hybridisation Organic ChemistryMolecular Orbitals and Hybridisation
2Molecular orbitalsOrbitals can be used to explain bonding between atoms. Atomic orbitals are the volume of space that the electrons of an atom are likely to be found in.HH1s atomic orbitals of hydrogenThe atomic orbitals containing the valence electrons (outer electrons) are the ones that are important to us.
3When atomic orbitals overlap, they combine to form molecular orbitals. In the case of hydrogen, the overlap of two 1s atomic orbitals results in the formation of a σ (sigma) molecular orbital.HHH1s atomic orbitals of hydrogenσ molecular orbitalσ bonds are covalent bonds formed between atoms when end-on overlap of orbitals occurs.This molecular orbital is more stable than each of the separate atomic orbitals and contributes to the shape of the molecule.
4The molecular orbital formed is a lower energy arrangement than the separate atomic orbitals. Molecular orbitals encompass the whole molecule and are not simply found between atoms inside a molecule.Increasing energyσ1s1sHHH
5Bonding continuumThe shape of the molecular orbital formed from overlapping atomic orbitals will govern the type of intermolecular bonding that is observed.
6Non-polar (pure) covalent bonds Non-polar covalent bonds or pure covalent bonds are formed between two atoms of the same element, or two atoms with a very low difference in electronegativity.The molecular orbital formed from overlapping atomic orbitals is symmetrical around a mid-point where the bonding electrons are most likely to be found.
7Non-polar (pure) covalent bonds Example: fluorineThe overlap of two 2p orbitals results in the formation of a σ orbital.F+F2p atomic orbital2p atomic orbitalσ molecular orbital
8Non-polar (pure) covalent bonds Example: fluorineThe overlap of two 2p orbitals results in the formation of a σ orbital.F+F2p atomic orbital2p atomic orbitalFσ molecular orbitalIn a fluorine molecule, or any non-polar covalent bond, the σ bonding orbital is symmetrical.
9Polar covalent bondsWhen there is a large difference between the electronegativities of the two elements involved in the bond, the bonding molecular orbital will be asymmetrical.Since oxygen is more electronegative than hydrogen, the molecular orbital formed will be asymmetrical, with the bonding electrons more likely to be found around the δ– oxygen atom.Example: waterδ–Oδ+δ+HH
10Ionic bondsWhen ionic bonds form, there is extreme asymmetry and the bonding molecular orbital is almost entirely around one atom.
11HybridisationIn its ground state, an isolated atom of carbon has the electron arrangement 1s2 2s2 2p2.1s2s2pCHWhy then, if there are only two unpaired electrons, do carbon atoms form four covalent bonds?
12HybridisationThe shapes of the atomic orbitals involved cannot explain the bonding observed in compounds such as alkanes.1s2s2pxzyxzy1s orbital2s orbitalxzyxzyxzy2px orbital2py orbital2pz orbital
13Hybrid theory assumes that the 2s and 2p orbitals of carbon atoms combine (or mix) to form four degenerate orbitals (i.e. orbitals of equal energy)Increasing energy2s2phybridised orbitalsThe hybrid orbitals formed from one s orbital and three p orbitals are called sp3 orbitals.
14an sp3 hybridised orbital The sp3 orbitals formed are all half-filled, with the electron far more likely to be found in the larger lobe.Since electrons repel each other, the four sp3 hybridised orbitals surrounding a central carbon atom result in a familiar tetrahedral shape, with a maximum possible angle between each orbital of 109.5°.
15AlkanesIn methane, all four hybrid orbitals are used to form σ bonds between the central carbon atom and hydrogen atoms.CH
16AlkanesCarbon-to-carbon single bonds in alkanes result from overlapping sp3 orbitals forming σ bonds.σ bonds are covalent bonds formed by end-on overlap of two atomic orbitals and since σ bonds must lie along the line joining both atoms, there will be free rotation around these orbitals.HHCCHHσ bondHH
17AlkenesHow can we explain the existence of double bonds as observed in alkenes?C CHC CHHAs with alkanes, bonding in alkenes is due to hybridisation.
18single unhybridised 2p orbital As with alkanes, an electron from the 2s shell is promoted to the empty 2p orbital.This results in the formation of three hybrid orbitals, with one remaining unhybridised 2p orbital.single unhybridised 2p orbitalIncreasing energy2s2phybridised orbitalsThe hybrid orbitals formed from one s orbital and two p orbitals are called sp2 orbitals.
19The three sp2 orbitals repel each other, resulting in a bond angle of 120° between them. The hybrid orbitals are responsible for overlapping to form σ bonds joining their central carbon atoms to both carbon and hydrogen.CHsp2 orbitals
20unhybridised 2p orbitals The unhybridised p orbitals are perpendicular to the plane of the molecule.unhybridised 2p orbitalsCHσ bondsσ bondsσ bondCHThe p orbitals of the carbon atoms are parallel and close enough to overlap sideways.
21This new orbital is called a pi (π) orbital or more commonly a π bond. This sideways overlap between the 2p orbitals produces a new molecular orbital between the two carbon atoms.CHA π bond is a covalent bond formed by the sideways overlap of two parallel atomic orbitals.This new orbital is called a pi (π) orbital or more commonly a π bond.
22HCσ and π bondsLooking at information comparing σ and π bonds, we can see that double bonds are stronger than single bonds, but not twice as strong. This is because the sideways overlap (π bond) is weaker than the end-on overlap (σ bond).Bond typeBondingorbitals presentBond lengthMeanbond enthalpy1 σ154 pm370 kJ mol–11 σ + 1 π134 pm602 kJ mol–11 σ + 2 π121 pm835 kJ mol–1C CC CC C
23Aromatic hydrocarbons Aromatic compounds differ to other hydrocarbons as they contain delocalised electrons.Example: benzene (C6H6)Chemists initially represented a molecule of benzene as shown here.CHHowever, contrary to what might be expected from this structure, benzene is a very stable, saturated structure that does not undergo addition reactions.
24This model does not explain why all the bonds in benzene can be observed to be the same length, not three longer single bonds and three shorter double bonds.In benzene, each carbon atom has used three of its four valence electrons to form σ bonds. The fourth electron of each carbon atom is delocalised over the entire ring, not involved in π bonding.CH
25The σ bonding can be described as existing between six sp2 hybridised orbitals. There are six C–C σ bonds in the molecule and so each carbon atom has two σ bonds to adjacent carbon atoms.CCCHHCCCHHHHEvery carbon atom also has a σ bond to a hydrogen.
26This results in a planar molecule with the unfilled 2p orbital of each carbon atom above and below the plane of the molecule.CHCH
27These 2p orbitals all combine to form a set of delocalised π molecular orbitals above and below the plane of the molecule.CHThe structure of benzene is drawn as shown to represent the delocalised electron clouds.
28A substituted benzene ring is called a phenyl group (C6H5) and can be represented: Many medicines, antiseptics, drugs and other useful products contain aromatic rings.CH3NO2O2NOHClTrinitrotoluene(TNT)Trichlorophenol(TCP)