Chapter 25 - Aromatic Compounds

benzene

naturally occurring aromatic hydrocarbon

molecular = C6H6

empirical = CH

bond angle = 120

issues with Kekule’s model of benzene/why the delocalised model is more accurate

-benzene is less reactive than alkenes/does not readily react to addition

-enthalpy change of hydrogenation of benzene is less exothermic/more stable than expected

-all the carbon bond lengths are the same

-benzene is less reactive than alkenes/does not readily react to addition

-in benzene pi electrons are delocalised + in alkenes pi electrons are localised

-benzene has a lower electron density than alkenes so induces a weaker dipole in bromine so does not decolourise bromine water

-enthalpy change of hydrogenation of benzene is less exothermic/more stable than expected

-the enthalpy change of hydrogenation of cyclohexene to cyclohexane is -120kJmol-1

-so the theoretical enthalpy change of hydrogenation of benzene would be -120 × 3 due to 3 double bonds = -360kJmol-1

-however the actual enthalpy change was -208kJmol-1 which is 152kJmol-1 more energetically stable than expected

-all the carbon bond lengths are the same

-benzene should contain 2 different bond lengths as single bond length is 0.153nm and double bond length is 0.134nm

-but X-ray crystallography/diffraction revealed that the 6 carbon-carbon bonds were all of length 0.140nm

differences between Kekule’s model + delocalised model

-they both have p-orbitals overlapping to form pi bonds

-but in delocalised model the pi bonds are delocalised + in Kekule’s model the pi bonds are localised

Kekule’s model of benzene (DRAW STRUCTURE + BONDS)

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delocalised model of benzene (DRAW STRUCTURE + BONDS)

the 6 pi electrons in benzene are free to move between all 6 carbon atoms in the ring and are not localised within three distinct double bonds

electrophilic substitution of benzene

an electrophile has substituted hydrogen from the benzene ring

-benzene has a high electron density above and below ring due to delocalised electrons

NITRATION reagents + reaction conditions

-concentrated sulfuric acid + concentrated nitric acid

-reflux at 50C

electrophilic substitution -NITRATION

stage 1 = creating a nitronium ion NO2+

1) H2SO4 + HNO3 ⇌ [H2NO3]+ + HSO4-

2) [H2NO3]+ ⇌ NO2+ + H2O

overall = H2SO4 + HNO3 → NO2+ + HSO4- + H2O

stage 2 = draw

stage 3 = acid catalyst regenerated

H+ + HSO4- → H2SO4

HALOGENATION reagents + reaction conditions

-halogen carrier = AlCl3 or FeCl3, AlBr3 or FeBr3
-is needed as benzene has a lower electron density than an alkene so won’t be able to induce a dipole in bromine

-halogen

-room temp + pressure

electrophilic substitution -HALOGENATION

-bromination or chlorination

stage 1 = creating bromonium or chloronium ion Br+ or Cl+

Br2 + FeBr3 → Br+ + FeBr4-

stage 2 = draw

stage 3 = catalyst regenerated

FeBr4- + H+ → HBr + FeBr3

Friedel-Craft’s ACYLATION

stage 1 = creating acylium ion RCO+

RCOCl + AlCl3 ⇌ RCO+ + AlCl4-

stage 2 = draw

stage 3 = catalyst regenerated

H+ + AlCl4- → AlCl3 + HCl

ACYLATION + ALKYLATION reagents + conditions

-acyl chloride for acylation OR haloalkane for alkylation

-halogen carrier

-reflux at 60C

-anhydrous conditions

Friedel-Craft’s ALKYLATION

stage 1 = RCl + AlCl3 ⇌ R+ + AlCl4-

stage 2 = draw

stage 3 = catalyst regenerated

H+ + AlCl4- → HCl + AlCl3

naming benzene compounds

1) stem + suffix = benzene

2) prefix - should have lowest number combination + in alphabetical order

3) for di- and tri- compounds, the first side chain is given the lowest number possible

side chain names

OH = hydroxy

NO2 = nitro

NH2 = amino

CN = cyano

benzene side chain = phenyl- - when H atom is removed from other functional group and replaced with benzene ring

phenol weak acid properties

1) partially soluble in water - OH group can form hydrogen bonds with water but aromatic group is non-polar so cannot

2) weak acid - orange - partially dissociates - more acidic than alcohols but less acidic than carboxylic acids

3) neutralisation reaction with NaOH + dissolves - can react strong bases such as NaOH to form a soluble salt and water

electrophilic substitution of phenol -DRAW TOO

-orange solution to a white solid precipitate in a colourless solution

-trisubstitution of bromine occurs and forms 2, 4, 6-tribromophenol

reaction conditions = room temp + pressure, halogen carrier NOT required

nitration of phenol -DRAW TOO

-reaction conditions = dilute nitric acid, no sulfuric acid, room temp + pressure

-trisubstitution does not occur = the nitro group (NO2) us a deactivating group

how is phenol more reactive than benzene?

-benzene + phenol contain delocalised pi electrons

-but the lone pair of electrons on oxygen from OH group on phenol is partially delocalised into the ring

-the OH group then activates the aromatic ring so phenol has a greater electron density

-so phenol is able to induce a dipole in bromine + is more susceptible to electrophilic attack while benzene cannot

activating groups

-push electrons into the delocalised ring so increases electron density making molecule more reactive so reacts more readily with electrophiles

-they direct the group to the 2, 4 and 6 positions (2 and 6 are the same)

examples = -NH2, -OH

deactivating groups

-pulls electrons out of the delocalised ring and towards themselves so decreases electron density making molecule less reactive so reacts less readily with electrophiles

-they direct the group to the 3 and 5 positions (3 and 5 are the same)

example = -NO2