6.1 Aromatic Chemistry Flashcards
What is the simplest aromatic hydrocarbon ?
Benzene (C6H6). Consists of a hexagonal ring of 6 carbon atoms with each carbon atom joined to 2 other carbon atoms and one hydrogen.
What is the Kekule model ?
Suggested that the structure of benzene was a hexagonal ring joined by alternate double bonds.
Why was the Kekule model wrong ?
1 - Benzene did not decolourise bromine water so cannot have any double bonds.
2- All bonds were same length, less than single but more than a double.
3- Should have enthalpy value of -360 for braking 3 double bonds (3x 120). Actual value was -208, so is more stable than Kekule model.
What is the delocalised model of Benzene ?
Planar, cyclic hexagonal hydrocarbon. Each carbon atom has 1 delocalised electron in p-orbital that overlap sideways with each other to create a system of pi bonds.
What type of reaction does benzene undergo ?
Electrophilic substitution.
Nitration benzene ?
Benzene reaction with nitric acid to form nitrobenzene. The reaction is catalysed by concentrated sulfuric acid and heated to 50 degrees. Produces water.
Bromination of benzene ?
Benzene reacts with bromine during electrophilic addition in the presence of a halogen carrier (FeBr3). Produces HBr.
Chlorination of Benzene ?
Reacts in same way of bromine. Used AlCl3. Produces Chlorobenzene and HCl.
Alkylation of benzene ?
Reacting benzene with haloalkane in presence of AlCl3, that acts as a halogen carrier.
Acylation of benzene ?
Benzene reacts with acyl chloride in presence of AlCl 3 to form an aromatic ketone.
Why doesn’t benzene react with bromine ?
This is because benzene has delocalised pi electrons spread above and below the plane. Electron density around 2 carbon atoms is less than that in C=C double bond. When bromine approaches ring, there is insufficient electron density to polarise bromine molecule.
What are phenols ?
Type of organic chemical with hydroxyl functional group. C6H5OH.
Why can phenol act as a weak acid ?
Less soluble in water than alcohols due to non-polar benzene ring. It partially dissociates forming phenoxide ion and proton. More acidic than alcohols but less acidic than carboxylic acids.
How do phenols react with NaOH ?
React to form salt, sodium phenoxide, and water.
Bromination of Phenol ?
Halogen carrier is not required and reaction decolourises bromine water. Bromine is attached at the 2,4 position.
Nitration of Phenol ?
Phenol reacts with dilute nitic acid, where NO2, joins in the 2.4 position.
Why is phenol more reactive than benzene ?
There is a lone pair of electrons from oxygen in OH group being donated into the pi system. The electron density of benzene ring in phenol is increased, so attracts electrophiles more easily.
What is an activating group ?
Group that attaches and activates the ring, as the ring reacts more readily with electrophiles.
What is a deactivating group ?
Group that deactivated the aromatic ring as the ring reacts less readily with electrophiles.
What is the directing effect ?
Different groups can have different directing effects where groups attaching can be directed to attach at different points.
What directing effect does activating groups have ?
They have 2,4 directing, except halogen which are deactivating but still direct in the 2.4 position. Eg. NH2, OH, R
What directing effect do deactivating groups have ?
They are 3 directing, except halogens which are deactivating but direct in the 2,4 position. Eg. NO2, COOH, COOR, CHO.
