Topic 18 : Organic III Flashcards
Describe the bonding between orbitals in benzene?
Orbital hybridisation produces 3 sp2 orbitals and 1 p orbital on each carbon, sigma bonding between 2 sp2 orbitals or between sp2 and s of hydrogens. Leftover p orbitals form a delocalised pi bond with the molecular orbital spread over the whole carbon ring.
What does it mean if benzene is a resonance hybrid?
The structure is not actually resonating between the two forms of benzene (with the double and single bonds alternating), it has a single structure in which all the bonds are the same - between a single and double bond.
How does the enthalpy of hydrogenation of benzene provide evidence for the structure?
The enthalpy of hydrogenation of benzene is lower than what would be expected ( roughly 3 x enthalpy of hydration of cyclohexene) which shows that the structure of benzene is not equivalent to cyclohexatriene.
What roughly is the stabilising effect of the pi delocalisation?
~152kJ/mol
How does the bond enthalpy of benzene compare to cyclohexatriene?
The bond enthalpy of benzene (worked out experimentally) is roughly 160kJ/mol more than the bond enthalpy of cyclohexatriene (worked out using average bond enthalpies). This suggests that more energy is needed to disrupt the bonding in benzene than in cyclohexatriene (this extra energy is referred to as the stabilising effect).
How does IR data support the structure of benzene?
The IR spectrum from benzene is simpler because of greater symmetry in benzene than in cyclohexane.
How does the delocalisation of pi-electrons in the benzene ring make it more stable than expected?
Delocalisation of the pi system means that the 6 electrons are evenly distributed through the benzene ring. This means that there are no areas of high electron density for electrophilic attack to occur.
If the benzene ring is stable, why do arenes still undergo reactions such as halogenation and nitration?
Benzene is not electron rich enough to be attacked by a partially positive species, nevertheless species with a full positive charge will potentially attack the ring.
What are the physical properties of benzene?
- Aromatic smell
- Non polar
- Van der waals between molecules are very strong - liquid at room temp
- Not soluble in water (immiscible)
- Carcinogenic
Why does benzene undergo substitution reactions rather than addition reactions?
Benzene is thermodynamically very stable because of the delocalisation of the pi molecular orbital. Addition reactions would disrupt the delocalisation and hence reduce the stability of the ring. Substitution reactions only involve a temporary disruption of the delocalisation.
Why is benzene resistant to attack by bromine (Br-Br)?
Pi electrons are delocalised so electron density between two carbon atoms is lower compared with alkenes. The electron density is insufficient to polarise a bromine molecule. Even distribution of the electrons in the ring makes the ring more stable and hence more energy is needed to disrupt the delocalised pi cloud of electrons.
A halogen carrier is needed to generate a more powerful electrophile, Br+. The charge is large enough to attract the pi electrons in benzene and disrupt the delocalised pi cloud of electrons.
What are the conditions and reagents needed for the hydrogenation of cyclohexene?
Hydrogen gas, nickel catalyst, heat
Why does the bond lengths in benzene suggest it is not equivalent to cyclohexatriene?
Electron diffraction measurements show that all the carbon-carbon bonds are of equal length (intermediate between C-C and C=C). In cyclohexatriene, the double bonds would be shorter than the C-C bonds.
How many electrons are found in the pi bonding system above and below the plane of the atoms in the benzene molecule?
6
Give 4 pieces of evidence benzene doesn’t have alternating single and double bonds.
- Benzene doesn’t undergo electrophilic addition reactions like an alkene to produce disubstituted isomers
- Electron diffraction measurements show that all the carbon-carbon bonds are of equal length/ intermediate between C-C and C=C
- The enthalpy of hydrogenation of benzene is less exothermic than expected
- The enthalpy of formation of benzene is less endothermic than expected
Describe the combustion of benzene.
- Produces a characteristic sooty/smoky yellow flame in normal conditions (due to insufficient oxygen)
- requires a large volume of oxygen for complete combustion
How does a halogen carrier allow substitution reactions of benzene with halogens to occur?
The halogen carries are covalent molecules and soluble in benzene.
The halogen carrier polarises the halogen molecule so that it behaves as an electrophile.
Br2 - FeBr3 or anhydrous AlBr3
Cl2 - FeCl3, anhydrous AlCl3
What is the reaction for the formation of the electrophilie in halogenation reactions of benze?
Cl-Cl + FeCl3 -> [FeCl4]- + Cl+
Or with anhydrous AlCl3
What is needed for the nitration of benzene?
Concentrated sulfuric acid
AND concentrated nitric acid
Reflux at 50 degrees C
What is the overall equation for the nitration of benzene?
Benzene + nitric acid -> nitro-benzene + water
Concentration sulfuric acid cat. at 50 C
