Chapter 25.1+25.2: Aromatic Compounds

Question 1

What is benzene?

Answer 1

an aromatic compound that has the molecular formula C6H6
–> colourless, sweet smelling, highly flammable liquid
–> carcinogen

Question 2

Kekule’s model of benzene (+ bonds)

Answer 2

Cyclic structure with alternating double and single bonds
=> 6 sigma bonds and 3 pi bonds ( pi bonds form overlap of p-orbitals)
=> localised electron density ( not very spread out)

Question 3

3 problems with Kekule’s model

Answer 3

Lack of reactivity
Thermodynamic stability
Bond length

Question 4

Problems with Kekule’s model: Lack of reactivity of benzene

Answer 4

Since benzene has double bonds C=C, it should behave like other alkenes and decolorise bromine through an electrophilic addition reaction

However, Benzene does not do this ( => Benzene is not a normal alkene)

Question 5

Problems with Kekule’s model: Thermodynamic stability of benzene

Answer 5

Enthalpy of hydrogenation to cyclohexene is -119 KJmol-1 for ONE C=C bond
(C6H10+ H2 –> C6H12)

Since benzene has 3 C=C bonds, it should have an enthalpy of 3X more:
–> Actual is only -208 KJmol-1 rather than -360 KJmol-1

–> meaning real structure of benzene is more stable

Question 6

Problems with Kekule’s model:
Bond length of benzene

Answer 6

Normally C-C bond is 0.154nm & C=C is 0.134 nm

BUT in benzene, all bond lengths are the same at 0.139nm

Question 7

What was Kekule’s suggestion regarding the problems with his model?

Answer 7

That benzene interconverts between two forms (resonance where the double bonds change positions)
<—> symbol

Question 8

Delocalised model of benzene: What was it

Answer 8

New model which suggested a cyclical structure with a ring of delocalised electron density ABOVE AND BELOW the plane

–> Carbon forms a planar ring ; each carbon contributes an electron to 2 C-C sigma and 1 C-H sigma bond (3/4 electrons used up)

Question 9

Delocalised model of benzene:
What happens to the last electron on the carbon

Answer 9

Remaining electron is in a p orbital at 90 degrees to the ring

–> All 6 p-obitals overlap, causing electrons to be delocalised (spread out)

–> this forms rings of electron density, spreading out electrons and stabilising the molecule

Question 10

Delocalised model of benzene: Why is benzene less reactive?

Answer 10

All C-C bonds are the same length, meaning it polarises other molecules less strongly

Electron density between each carbon is intermediate between alkane and alkene
=> less attraction to electrophiles

Question 11

4 Must learn structures relating to benzene

Answer 11

Benzoic acid: C6H5COOH

Phenylamine: C6H5NH2

Benzaldehyde: C6H5CHO

Phenol: C6H5OH

Question 12

Naming with benzene: parent chain vs substituent

Answer 12

Parent chain: use benzene with the added prefixes e.g. chlorobenzene

Substituent: name phenyl is used if there are more than 7 carbons
e.g. phenylethanone

Question 13

What type of reaction does benzene undergo with it reacts with a compound?

Answer 13

substitution in which H atom on benzene ring is replaced by another atom (typically reacts with electrophiles so electrophilic substitution

C6H6 + (E+) –> C6H5E + (H+)

Question 14

Nitration of benzene: standard reaction

Answer 14

Reactant: Nitric acid (conc) HNO3
Conditions: conc sulfuric acid catalyst (H2SO4) + 50 degrees

C6H6 + HNO3 —> C6H5NO2 (nitrobenzene) + H20

Question 15

Nitration of benzene: excess temperature

Answer 15

C6H6 + 2HN03 —>1,3-dinitrobenzene + 2H2O

conditions: conc H2SO4 + 70 degrees

Question 16

Halogenation of benzene: standard reaction

Answer 16

reactant: chlorine or bromine
Conditions: Halogen carrier e.g. (AlCl3 or FeCl3 catalyst /// AlBr3 or FeBr3) + at room temp and pressure

C6H6 + X2 –> C6H5X + HX
where X is a halogen

Question 17

Alkylation of benzene: standard reaction

Answer 17

substitution of H atom in benzene by an alkyl group

Reactant: Haloalkane
Conditions: AlCl3 catalyst
C6H6 + C2H5Cl —> C6H5C2H5 + HCl

Question 18

Acylation of benzene: standard reactions

Answer 18

Benzene reacts with an acyl chloride to form an aromatic ketone

Reactant: ACYL CHLORIDE
Conditions: AlCl3 catalyst
C6H6 + CH3C0Cl –> phenylethanone + HCl

Question 19

Nitration of benzene: electrophilic substitution steps

Answer 19

1: Electrophile NO2+ is produced by reacting CONC nitric acid + CONC sulfuric acid

2: Electrophile accepts pair of electrons from benzene ring to form a dative covalent bond (organic intermediate is unstable and breaks down to form organic product of nitrobenzene and H+ ion )

3: H+ ion reacts with HS04- ion from step 1 to regenerate catalyst H2SO4

Question 20

Nitration of benzene: electrophilic substitution equations

Answer 20

Step 1: HNO3 + H2SO4 –> NO2+ + HSO4- + H2O (overall equation)

HNO3 + H2SO4 <—> H2NO3+ + HSO4-
H2NO3+ <–> NO2+ + H2O

Step 2: Arrow from inner ring of benzene towards the NO2+ electrophile. Ring is incomplete with a + charge in the middle.

Arrow from middle of C-H bond towards the gap in the ring . Ring is complete and NO2 is a side chain

Step 3: H+ + HSO4- –> H2SO4

Question 21

Halogenation of benzene: electrophilic substitution steps

Answer 21

1: Electrophile Br+ (bromonium ion) is generated when halogen carrier reacts with bromine

2: Bromonium ion accepts pair of electrons from benzene ring to form dative covalent bond. (organic intermediate is unstable and breaks down to form organic product of bromobenzene and H+ ion )

3: H+ formed reacts with FeBr4- to regenerate FeBr3 catalyst

Question 22

Halogenation of benzene: electrophilic substitution equations

Answer 22

Step 1: Br2 + FeBr3 –> FeBr4- + Br+

Step 2: Arrow from inner ring of benzene towards the Br+ electrophile. Ring is incomplete with a + charge in the middle.

Arrow from middle of C-H bond towards the gap in the ring. Ring is complete and Br is a side chain

3: H+ + FeBr4- –> FeBr3 + HBr