Module 6: Organic Chemistry and Analysis Flashcards
What was Kekulé’s model of benzene?
Alternating single and double bonds.
What was the evidence against Kekulé’s model of benzene?
- Benzene isn’t as reactive as other alkenes. It doesn’t decolourise bromine water and can’t undergo electrophilic addition.
- All the carbon-carbon bond have a length of 0.139 nm, which is between the length of carbon-carbon single and double bonds.
- It’s enthalpy of hydrogenation is only -208 kJ mol⁻¹, and it should be -360 kJ mol⁻¹ (3x the enthalpy of hydrogenation of cyclohexene) if Kekulé’s model is correct.
Describe the delocalised model of benzene?
- Benzene is a planar cyclic hydrocarbon.
- Three out of four of the valence electrons of the carbons are bonded to two other carbons. This leaves one electron.
- The 6 p-orbitals overlap sideways in both directions above and below the plane.
This forms a ring of delocalised electrons.
When naming aromatic compounds, when is benzene not the prefix?
When the benzene ring is attached to alkyl groups with a carbon chain of less than 7, halogens and nitro groups.
When naming aromatic compounds, when is benzene the prefix?
The prefix used is phenyl.
When benzene is bonded to an alkyl chain with a functional group or an alkyl chain with 7 or more carbons.
What are the three exceptions when naming aromatic compound?
- Benzoic acid
- Phenylamine
- Benzaldehyde
How are aromatic compounds named when benzene has more than one substituent group?
In alphabetical order using the smallest numbers possible.
What are the conditions for nitration of benzene?
Catalysed using concentrated sulfuric acid at 50℃ and refluxed
What is the difference between how alkenes and benzene react?
Alkenes react by electrophilic addition and benzene reacts by electrophilic substitution.
If a group is electron-withdrawing…
It deactivates the ring, as it pulls the electron ring towards it. This makes it 3,5 (meta)-directing.
An example of this is nitro groups, halogens and carbonyls.
If a group is electron-donating…
It activates the ring, as it adds to the electron density of the ring. This makes it 2,4,6 (ortho-para)-directing.
An example of this is amine groups, hydroxyl groups and alkyl groups.
Is a nitro group electron-withdrawing or electron-donating?
Electron-withdrawing.
Is an amine group electron-withdrawing or electron-donating?
Electron-donating.
What is a phenol?
A compound where a hydroxyl group is directly bonded to an aromatic ring. If the hydroxyl is primary, phenol forms the root of the name.
Phenol + Bromine → …
… 2,4,6 - tribromophenol + hydrogen bromide
Observations: Bromine decolourises and a white precipitate is formed.
