Introducing Benzene Flashcards
What does a benzene molecule consist of?
6 carbon atoms, with each carbon atom joined to 2 other carbon atoms and 1 hydrogen atom (C6H6).
What is a benzene molecule classed as? (2)?
An aromatic hydrocarbon or an arene.
Why was there confusions with determining the structure of benzene?
Because it molecular formula is C6H6, it suggests that benzene contains many double bonds or triple bonds which makes a compound very reactive but benzene appeared unreactive.
What did the Kekule model suggest about the structure of benzene?
It was a six membered ring of carbon atoms joined by alternate single and double bonds.
What are the 4 problems with Kekule’s model?
- The lack of reactivity of benzene.
- The lengths of the C-C bonds in benzene.
- Hydrogenation enthalpies.
- Isomers.
Why does the lack of reactivity of benzene disprove Kekule’s model?
If benzene contained C=C bonds, it should decolourise bromine in an electrophilic addition reaction but it doesn’t undergo electrophillic addition reactions and does not decolourise bromine under normal conditions (it undergoes electrophilic substitution instead). Therefore, does not contain C=C bonds.
Why does the lengths of the C-C bonds in benzene disprove Kekule’s model?
If benzene has conjugate double and single bonds, there are 2 different bond lengths around the ring:
- An alkene C=C bond is 0.134nm.
- An alkane/alkene C-C bonds is 0.153nm.
X-ray analysis has shown that in a benzene ring, there is only one bond length of 0.139nm in length. The actual structure seems to be between a single (0.134nm) bond and a double (0.153nm) bond.
Why does hydrogenation enthalpies disprove Kekule’s model?
- If benzene did have the Kekule structure, then it would be expected to have an enthalpy change of hydrogenation that is 3X that of cyclohexene.
- When cyclohexene is hydrogenated, one double bond reacts with hydrogen and the enthalpy change of hydrogenation is -120 kjmol-1.
- As the Kekule structure is predicted to contain 3 double bonds, the expected enthalpy change for reacting 3 double bonds with hydrogen would be -360 kjmol-1.
- The actual enthalpy change of hydrogenation of benzene is only -208kjmol-1. This means that 152kjmol-1 less energy is produced than expected. The actual structure of benzene is therefore more stable than the theoretical Kekule model of benzene.
Why was the delocalised model of benzene made?
Because scientists thought the evidence for the Kekule model was too disapproving.
What are the 6 features of the delocalised model of benzene?
- Benzene is a planar, cyclic, hexagonal hydrocarbon with six carbon atoms and six hydrogen atoms.
- Each carbon atom uses 3 of its available 4 electrons in bonding to two other carbons and one hydrogen.
- Each carbon has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms.
- Adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
- This overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure.
- The six electrons occupying this system of pi-bonds are said to be delocalised.
What are aromatic compounds?
Molecules that contain one or more benzene rings.
When naming aromatic compounds, when is benzene used as the suffix?
When only one substituent is attached to the benzene.
What are 4 types of examples where benzene is used to name aromatic compounds?
- When a halogen is attached (chlorobenzene).
- When a nitro group is attached (nitrobenzene).
- When a methyl/alkyl group is attached (methylbenzene).
- (Chloromethyl)benzene.
What is the molecular formula of phenyl?
C6H5 (1 less hydrogen than benzene).
When naming aromatic compounds, when is phenyl used as the prefix?
When attached to a functional group, still only one substituent.
