Arenes - Benzene Flashcards
Aliphatic
Straight or Branches chain organic substances
Aromatic/Arenee
One or more rings of six carbon atoms that contain delocalised electrons
Why is the Kekule Model incorrect?
The c-c and c=c bonds are different lengths.
In addition when measuring enthaply change of hydrogenation benzene is more stable then the model suggests.
Delocalized model
Each carbon forms 3 sigma bonds (1 to hydrogen and 1 to each carbon). This leaves one electron free on each p orbital of the carbons, forming a ring of Pi bonds above and below the carbon plane as the p orbitals overlap sideways
Enthaply change of hydrogenation
React an alkene with hydrogen gas to add two atoms of hydrogen across the double bond.
Evidence for delocaised model
Benzene is more resistance to hydrogenation then expected, due to the delocalised ring of electrons. The value is less exothermic, hence more energy was required to break the bonds then Kekule. (Delocalisation energy)
Alkenes + Bromine water
Electrophilic Addition
Bromine water discolourises
Why is benzene resistant to electrophillic addition
The delocalized Pi-bonds spread out the negative charge = Less electron dense
Toxicity of benzne
Its a carcinogen
Benzene + Oxygen
Combustion
Smoky flame due to low C:H ratio
C6H6 + 7.5O2 -> 6Co2 + 3H2O
Benzene + Bromine
Electrophillic Substitution
Condition: Iron(III) bromide catalyst
Halogen Carrier
Accepts a lone pair of electrons from a halogen on electophile, increasing the polarisation of the molecule making the electorphile stronger
Formation of electorphiles
AlCl3 + Cl2 -> AlCl4- + Cl+
Nitration of Benzene
Electorphilic Substitution
Regents: Conc Nitric acid + Conc Sulfuric Acid (Cat)
Electrophile: NO2 +
Conditions: Below 55 (For mononitration)
Formation of NO2 + Electrophile
HNO3 + 2H2SO4 -> NO2 + + 2HSO4- + H3O+
Make sure to show reformation of catalyst H2SO4!
Why is NO3- least likely to be in nitrating mixture?
React with the catalyst to form another species
Alkylation of Benzene
Electrophillic Substitution
- R+ attacks Benzene
- Intermediate forms
- Halogen carrier attracted to H, removing it
- R group is left on benzne + HX + AlX3
Show reformation of catalyst!
Heat under reflux
Acylation of Benzne
Electrophillic Substitution
- R-C+=O attacks Benzene
- Intermeidate forms
- Halogen carrier attracted to H, removing it
- Acyl group left + HX + AlX3
Heat under reflux in non-aqueous solvent
Produces Phenylketones
Hydrogenation of Benzene
Electrophillic Subsitution
Nickle Catalyst
200 degress
30 atm
Addition and reduction
Effects of side groups (With no lone pairs) on Benzene
Electron releasing groups (Alkyl/phenols/amines) release electrons into delocalised system increasing electron density hence more attractive to electrophiles.
Effects of side groups with lone pairs on Benzne
Extends the delocalisation of benzne to the lone pair on the side group.
Cl - Causes ring to repel nucelophiles, and C-Cl bond becomes stronger
NH2-Less basic as is delocalised and will accept protons
OH- Delocalisation makes C-O bond stronger O-H weaker, its more acidic then alcohols and doesn’t oxidise.
When is phenol more reactive then benzene?
Phenol will decolourise bromine water, as the OH group makes the ring more attracted to electrophiles.
Increases chances for electrophilic subisition as the lone pair in the p orbital of oxygen overlap with the delocalised elecetrons in the Pi orbital of benzene, becoming partially delocaised into the Pi system itself increasing the electron density.
Bromination of Phenol
Reacts more then once, as bromine is more polarised
2,4,6-Tribromophenol
White solid
Properties of Phenol
Slightly acidic.
Will react with sodium metal and sodium hydroxide, but not sodium carbonate as isn’t strong enough of an acid.
