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objectives
objectives
Understandings:
- Benzene is an aromatic, unsaturated hydrocarbon.
- Benzene does not readily undergo addition reactions but does undergo electrophilic substitution reactions.
Applications and Skills:
- Discussion of the structure of benzene using physical and chemical evidence.
Kekule and Benzene
Kekule and Benzene
Primary Analysis of Benzene revealed a molecule with:
- an empirical formula of CH
- a molecular mass of 78
- a molecular formula of C6H6
Kekulé suggested that benzene was...
- PLANAR
- CYCLIC and
- HAD ALTERNATING DOUBLE AND SINGLE BONDS
HOWEVER...
- it did not readily undergo electrophilic addition - no true C=C bond
- only one 1,2 disubstituted product existed
- all six C—C bond lengths were similar; C=C bonds are shorter than C-C
- the ring was thermodynamically more stable than expected
To explain the above, it was suggested that the structure oscillated (Resonance) between the two Kekulé forms but was represented by neither of them.
- It was a RESONANCE HYBRID.
Thermodynamic Evidence for Stability
Thermodynamic Evidence for Stability
- When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released.
- The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
- When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
- Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
- Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
- It is 152kJ per mole more stable than expected.
- This value is known as the RESONANCE ENERGY.
Hybridisation of the Orbitals in Benzene
Hybridisation of the Orbitals in Benzene
A recap of Hybridisation:
The electronic configuration of a carbon atom is 1s22s22p2
If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3
The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable
The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.
- As these new orbitals comprise of 1 s electron and 3 electrons we call it sp3
In Benzene, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.
- As these new orbitals comprise of 1 s electron and 2 p electrons we call it sp2 - We essentially ignore the unchanged p electron in the naming of the hybrid orbitals.
Bonding in Benzene
Bonding in Benzene
In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.
In ALKENES, the three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them
Covalent bonds are formed by overlap of orbitals.
An sp2 orbital from each carbon overlaps to form a single C-C bond.
The resulting bond is called a SIGMA (δ) bond.
- The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond.
- For maximum overlap and hence the strongest bond, the 2p orbitals are in line.
- This gives rise to the planar arrangement around C=C bonds.
Delocalisation in Benzene
Delocalisation in Benzene
- The theory suggested that instead of three localised (in one position) double bonds,
- the six p (p) electrons making up those bonds were delocalised (not in any one
- particular position) around the ring by overlapping the p orbitals. There would be no
- double bonds and all bond lengths would be equal. It also gave a planar structure.
- This final structure was particularly stable and resisted attempts to break it down through normal electrophilic addition.
- However, substitution of any hydrogen atoms would not affect the delocalisation
benzene past paper questions
benzene past paper questions
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