Aromatic Chemistry Chp25 Flashcards
What did the bonds of benzene turn out to be?
Turned out to be the same length; 0.140nm, a perfect hexagon
Why is benzene more stable than an alkene?
The lone electrons in the p orbitals overlap above and below the ring to form pi bonds, the pi bonds are then delocalised so charge is spread making it more stable therefore more energy is required to break bonds in benzene.
Why doesn’t benzene undergo electrophilic addition?
This would disrupt the stable ring of electrons.
How is acylation useful?
Adding a acyl group weakens benzene structure and makes it easier to modify further to make useful products.
Why are halogen carriers used?
Halogen carrier act a catalyst (AlCl3) which will produce a much stronger electrophile with a stronger positive charge so it can pass through the benzene ring.
Conditions reagents and catalysts for Nitration
Conditions - 50•c
Reagents - Conc HNO3/ Conc H2SO4
Catalyst - conc H2SO4
Formation of electrophile for nitration (equation)
HNO3 reacts with H2SO4 and makes the electrophile and water.
HNO3 + H2SO4 -> (NO2+) + H2O + (HSO4-)
Conditions, reagents and catalyst for Halogenation.
Reagents - FeBr/AlBr3 (conditions)
Conditions - Room temperature
Formation of electrophile Halogenation (equation)
Br2 + FeBr3 —> FeBr4- + Br+
Friedel Crafts Alkylation
C2H5Cl + AlCl3 —> C2H5 + HCl
Friedel Crafts Acylation
C2H3ClO + AlCl3 —> COCH3 + HCl
Describe the bonding in a benzene molecule
Each carbon atom is bonded to two other carbon atoms and one hydrogen. The 4th bond pair of electrons of each carbon atom is delocalised creating a ring of delocalised electrons above and below the plane.
How is the kekulé model different from the accepted model?
Ring of carbon atom joined by alternate single and double bonds.
Reactivity of phenol
Phenol is more reactive than benzene, phenol reacts instantly with bromine at room temp to form a white precipitate.
How does the aromatic ring become activated?
The lone pair of electrons on the oxygen atom in the phenol group delocalised into benzene ring, lone pair becomes part of the delocalised ring increasing the electron density in the ring, this makes make aromatic ring more nucleophilic and more susceptible.
What do phenols react with
Strong bases but no weak bases
Group attached to second carbon
Ortho
Group attached to third carbon
Meta
Group attached to fourth carbon
Para
What can substituted groups be classified as?
-Electron withdrawing (deactivating groups)
- Electron donating (activating groups)
What do electron donating groups do?
- Increase electron density
- Allow electrophile to react faster with benzene
- 2,4 directing (ortho, para)
What are some electron donating groups?
OH - O lone pair orbitals overlap with ring
NH2 - N lone pair orbitals overlap with ring
What do electron withdrawing groups do?
- Decreasing electron density
- Slow electrophile reaction in comparison benzene
- 3-directing, meta
What are some electron withdrawing group?
- NO2 and COOH oxygen is more electronegative drawing electrophiles away
What are some evidence that doubts kekule’s model
-Enthalpy of hydrogenation is less exothermic than expected when compared to enthalpy of hydrogenation of cyclohexene.
- Bond length was different
- benzene only reacts with bromine at high temperature or with a presence of a halogen carrier
Why does alkene undergo electrophilic substitution and not benzene
Alkene had localised electrons whereas benzene is delocalised. The electron density in alkenes is higher so it more susceptible to electrophilic attack
Regeneration of an electrophile catalyst
(H+) + AlCl4- —> AlCl3 + HCl
Evidence to support delocalised model
- Need halogen carrier for bromination
- Bond enthalpy for hydrogenation was less exothermic than expected
- C-C bond length are the same
