CL 25- aromatic compounds Flashcards
evidence against the Kekule model
3 points
- doesn’t undergo addition reactions as quick as alkenes (needs a halogen carrier)
- enthalpy of hydrogenation of benzene is less exothermic than expected
- C-C lengths and bond angles are all the same in benzene- length between C-C and C=C
shape of benzene
shape of whole molecule, shape and bond angle around each C atom
- hexagonal planar
- triganol planar around each C atom , 120°
describe/explain the shape and bonding of benzene
- each C atom bonded to two other C atoms and one H atom
- each C atom has one electron left over in a p orbital
- these overlap sideways to create a pi-ring system above and below the plane of the molecule
- the electrons are delocalised
why is bromination more difficult in benzene compared to alkenes?
- in benzene, electrons are delocalised
- in alkenes, electrons are localised
- benzene has a lower electron density
- benzene induces a weaker dipole on bromine molecule (electrophile not polarised enough)
what are the prefixes to benzene?
- alkyls (ethylbenzene)
- halogens (chlorobenzene)
- nitro groups (nitrobenzene)
when is benzene a substituent, and what prefix is used?
- when attached to an alkyl chain with a functional group, or the chain is seven or more C long
- phenylethanone, phenyloctane etc.
name of mechanism of benzene
electrophilic substitution
bromination of benzene mechanism
- FeBr₃ + Br₂ –> FeBr₄⁻ + Br⁺
- (a) curly arrow from ring of benzene to Br⁺
(b) open ring in benzene with + charge, H and Br attached on same C, curly arrow from H bond to + charge of ring
(c) product formed with H⁺ - FeBr₄⁻ + H⁺ –> FeBr₃ + HBr
nitration of benzene mechanism
- HNO₃ + H₂SO₄ –> HSO₄⁻ + NO₂⁺ + H₂O
- (a) curly arrow from ring of benzene to N of NO₂⁺
(b) ring open in benzene with + charge, H and NO₂ attached to same C of benzene, curly arrow from H bond to open ring + charge
(c) form product and H⁺ - HSO₄⁻ + H⁺ –> H₂SO₄
what is alkylation?
reactants and conditions
- adding an alkyl chain to benzene
- chloralkane and AlCl₃
what is acylation
reactants, conditions and name of product
- adding an acyl group to benzene
- add acyl chloride and AlCl₃
- forms a phenylketone
what is a phenol?
- benzene with -OH directly attached
solubility of phenol
- sparingly soluble- benzene ring gets in the way (?)
- more soluble than benzene, less than alcohols
pH of phenol
- very weak acid (5-6)
why does phenol react more readily (with electrophiles) than benzene?
- in phenol, the lone pair of electrons on O are partially delocalised into the pi-ring system
- increases the electron density
- electrophile becomes more polarised
phenol and NaOH
observations and products
- phenol + NaOH –> H on phenol replaced with Na, water formed
- colourless solution formed, white crystals reform after acidified
phenol and bromine water
observations and products
- phenol + 3Br₂ –> 2,4,6-tribromophenol + 3HBr
- decolourises bromine water (3HBr) and forms a white precipitate (2,4,6-tribromophenol)
phenol and dilute HNO₃
observations and products
- NO₂ attached to C (2) or (4) with respect to OH phenol group
- yellow precipitate formed
