C4.2 Aromaticity Flashcards
Benzene molecular formula
C6H6
Aliphatic
straight chains
Aromatic (and one property)
ring structure
sweet smelling
naming if benzene is the main group
-benzene
If benzene is a side chain
-phenyl
Describe chlorobenzene’s structure
benzene with one chlorine functional group
Describe nitrobenzene’s structure
benzene with one NO2 functional group
Describe methylbenzene’s structure
Benzene with one CH3 functional group
Describe Kekule’s structure of benzene
6 carbons in a ring with alternating C-C and C=C bonds with one hydrogen bonded to each carbon
List 3 three pieces of evidence against Kekule’s model
- The molecule’s carbon to carbon bonds all having the same bond length.
- Lack of electrophilic addition reactions.
- Benzene’s lower than expected enthalpy of hydrogenation.
Describe why The molecule’s carbon to carbon bonds all having the same bond length disproves Kekule’s structure.
Using x-rays we can study the location of atoms in a molecule. A double bond is much shorter than a single bond, but all the bonds are the same length in a benzene molecule. Each bond within the structure is the same length and between the expected length of a single and double bond. If Kekule’s structure was correct, half the bond would be single bond length and the other half at double bond length.
Describe why benzene not undergoing electrophilic addition disproves Kekule’s structure.
the double bonds within alkenes are reactive functional groups and they undergo electrophilic addition reaction to get rid of the double bond.
With benzene, electrophilic substitution occurs. The benzene circle remains unchanged at the end of the reaction, suggesting that the benzene circle is more stable than double bonds in alkenes.
Describe why benzene’s lower than expected enthalpy of hydrogenation disproves Kekule’s structure.
Hydrogenation enthalpy is the energy change as hydrogen is added to an unsaturated molecule (C=C) to change it into a standard molecule (C-C). It’s possible to estimate the hydrogenation enthalpy of benzene from information about the hydrogenation enthalpy of one double bond.
Benzene is +152kJ/mol more stable than Kekule’s structure.
What is meant by delocalisation enthalpy or resonance enthalpy?
The difference between any enthalpy value for Kekule’s structure and any value measured for benzene.
What are the bon angles between the sp2 orbitals?
120
What happens to C third p orbital in benzene?
Each p orbital can overlap side by side with the two p orbitals by its side to create a circle of 6 overlapping orbitals.
The p orbitals combine to form a cloud of…
electron density above and below the carbon circle.
The circle in benzene represents the cloud of electron density called…
π system
Electrophilic substitution reactions maintain the ________ of the benzene ring.
stability
Nitration of benzene reagents, conditions and reaction classification.
Reagents: concentrated sulphuric acid and concentrated nitric acid.
Conditions: 50*C
Classification: Electrophilic substitution reaction (NO2 is an electrophile)
During the nitration of benzene, a hydrogen is substituted for a…
NO2 group
Halogenation of benzene requires a catalyst, usually:
Aluminium chloride or Iron(111) chloride (replaced with the halogen)
Halogenation of benzene conditions and reaction classification:
Conditions: room temperature in darkness
Classification: Electrophilic substitution
During the second step of nitration and halogenation of benzene, a ___________ is produced
Wheland intermediate
Product of benzene nitration
Benzene with NO2 group attached and H+ ion
Product of the halogenation of benzene
Benzene with a halogen group and HX.
X representing the halogen
What occur during the Friedel Crafts Reaction
Forming a C-C bond.
Substitution reaction between halogenoalkane and benzene ring.
What catalyst must be present for Friedel crafts Reaction?
Aluminium chloride
If chlorobenzene and chloroalkanes are heated to 100*C with sodium hydroxide(aq), what would happen and explain.
Chloroalkane substitutes Cl for OH group. No reaction with chlorobenzene.
much more energy is require to break the C-Cl bond in chlorobenzene, which leads to a far higher activation energy.
Explain the acidity of phenol.
Donates protons.
More acidic than an aliphatic alcohol.
One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.
interact with pi system to strengthen the bond.
Explain the basicity of phenylamine
Base accepts protons
Lone pair of electrons from the NH3 interact with pi system.
The compound is less likely to accept H= ions.
Classify the reaction mechanism for benzene nitration.
Substitution reaction.
Why do you need a catalyst in the reaction between chloromethane and benzene?
cause polarity
Describe the structure of, and bonding in benzene and explain why benzene is less ready to undergo addition reactions than alkenes.
- C6H6 ring hexagonally shaped
- each C is covalently bonded to 2 other C ad one H
- planar molecule
- 120* bond angles
- each C has an unbonded p-orbital and they all overlap to form a cloud of electron density above and below the carbon ring
- π system
- alkenes have C=C, attracts electrophiles more readily, causing an addition reaction.
- benzene’s π system makes it more stable that C=C so takes a lot more delocalisation energy to break the bond, so benzene undergoes substitution.
