AROMATIC HYDROCARBONS

What is an aliphatic organic chemical?

straight or branched chain organic substances

What is an aromatic organic chemical?

includes one or more ring of 6 carbon atoms with delocalised bonding

What is benzenes structure?

• C6H6

• 6 carbon atoms in a hexagonal ring with 1 hydrogen atom bonded to each carbon atom

• each carbon atom is bonded to 2 other carbon atoms and 1 hydrogen atom by sigma bonds, leaving a delocalised electron in the p orbital

What is the structure and bonding found in benzene?

• planar molecule

• C-C bonds are all the same length and have bond energy between a C-C single and C=C double bond

• bond angle is 120

What is the enthalpy of hydrogenation of benzene?

• theoretically as there are 3 double bonds the enthalpy of hydrogenation should be -360

• actually benzenes is less exothermic

• benzene is more stable due to the delocalised electrons, leading to the delocalisation energy

Why is cyclohexa-1,3-diene more stable than cyclohexa-1,4-diene?

as the pi electrons overlap

What type of reactions does benzene usually undergo?

• not normally addition reactions as these would involve breaking the delocalised system

• electrophilic substitution is usually preferred

What’re the issues with benzene?

benzene is a carcinogen and is banned in schools

Why is methylbenzene used instead of benzene?

• less toxic

• reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles

What occurs in the nitration of benzene?

• forms nitrobenzene

• reagents - concentrated nitric acid in the presence of concentrated sulfuric acid (catalyst)

• mechanism - electrophilic substitution

Why is the nitration of benzene important?

it synthesises useful compounds e.g. TNT and is used in the formation of amines

What is the overall equation for the nitration of benzene?

How is the NO2+ electrophile created?

• in an acid base reaction

• HNO3 + 2H2SO4 → +NO2 + 2HSO4- + H3O+

What is the mechanism for the formation of the electrophile +NO2?

What is the mechanism for the nitration of benzene?

What is friedel crafts acylation?

• forms a phenyl ketone

• reagents - acyl chloride in the presence of anhydrous aluminium chloride catalyst

• conditions - heat under reflux

• mechanism - electrophilic substitution

What is the equation for the formation of the electrophile?

AlCl3 + CH3COCL → [CH3CO]+ + [AlCl4]-

What is the mechanism for friedel crafts acylation?

How does the AlCl4- act as a catalyst?

H+ ion reacts with it to reform AlCl3 catalyst and HCl

How do you reduce a nitroarene into aromatic amines?

reagent - Sn and HCl or Fe and HCl

conditions - heating

mechanism - reduction

or via catalytic hydrogenation

What occurs in the reduction of nitroarene into an aromatic amine?

What is the effect of having a chlorine atom attached to the benzene ring?

• makes it stronger

• halogenoalkane substitution and elimination reactions do not occur

• electron rich benzene will repel nucleophiles

What is the effect of a OH group attached to the benzene ring?

• delocalisation makes the C-O bond stronger and the O-H bond weaker

• phenol does not act as an alcohol, it is more acidic and doesn’t oxidise

What is the effect of having a NH2 group attached to the benzene ring?

• less basic than aliphatic amines as lone pair is delocalised

• less available for accepting a proton