Unit 4.2 Aromaticity (Sweet B|)

Question 1

Define cyclic

Answer 1

Ring-shaped

Question 2

Define planar

Answer 2

Flat

Question 3

Describe “aromaticity”
(4-way)

Answer 3

• Cyclic & planar molecule
• w/ ring of resonance bonds
• exhibiting more stability than other “arrangements”
• with same set of atoms

Question 4

Define arenes

Answer 4

Ring structures

Question 5

Define carcinogen
(2-way)

Answer 5

• Chemical/agent
• that can potentially cause cancer

Question 6

Describe benzene
(2-way)

Answer 6

• Colourless & flammable liquid
• Has sweet & gasoline-like odour

Question 7

How can benzene (useful chemical substance) be toxic?
(2-way)

Answer 7

• A carcinogen
• Continuous exposure = cancerous effects

Question 8

5 key facts of Benzene
(3 of em are pretty imp)

Answer 8

• C₆H₆
• Planar
• 3 C=C (π) double bonds & 3 C-C (σ) single bonds
• Hexagonal ring
• Trigonal planer about each carbon

Question 9

Draw the 2 versions of benzene

Answer 9

Stage is yours

Question 10

What does the circle in 1 benzene variation represent?
(2-way)

Answer 10

• 3 π bonds
• aka 6 π e^-

Question 11

2 compounds that are aliphatic?

Answer 11

• Alkane
• Alkene

Question 12

How do arenes differ with aliphatics?
(2-way)

Answer 12

• They possess 1/more rings of carbon atoms
• Bonding electrons are delocalised

Question 13

Explain the structure of benzene pt. 1
(3-way)

Answer 13

• Normal sigma bond
• Formed between each pair of carbons
• Each containing 2 electrons

Question 14

Explain the structure of benzene pt. 2
(3-way)

Answer 14

• Remaining 6 electrons (1 from each carbon)
• Occupy p-orbitals
• Which they overlap to form a π bond

Question 15

Hence, draw the explanation of the structure of benzene

Answer 15

Stage is yours

Question 16

3 consequences of benzene and its delocalisation of some bonding electrons?

Answer 16

1. Benzene much more stable than expected
2. Extra stability = benzene less readily undergo addition reactions
3. More loosely held electrons = open to attack by electrophiles
   electrophilic substitution type shi

Question 17

Explain “1.” Benzene much more stable than expected as a consequence?
Pt. 1
(2 inf, 1 + 2-way)

Answer 17

• Actual enthalpy of formation of benzene = 82 kJmol^-1
• Enthalpy of hydrogenation of cyclohexene (C₆H₁₀) = -120 kJmol^-1
• Cyclohexene contains 3 alkene double bonds
• ∴ expected enthalpy of hydrogenation to cyclohexene
• to be “3x”, -360 kJmol^-1

Question 18

Explain “1.” Benzene much more stable than expected as a consequence?
Pt. 2
(4-way)

Answer 18

• Ended up being -210 kJmol^-1
• Difference between expected and actual value = 150 kJmol^-1
• Hence, that’s the stabilisation energy (delocalisation energy) of benzene
• Explains why it’s resistant to addition reactions

Question 19

2 aromatic properties of benzene?

Answer 19

1. Resistance to addition
2. Electrophilic substitution

Question 20

Explain resistance to addition of benzene?
And draw an eqn showing that.

Answer 20

• Extra stable due to delocalistion
• Stage is yours:
  Benzene + X₂ -> Hexagon w/ 2 lines inside and 2 other bonds with X
  which btw, isn’t impossible….

Question 21

Explain electrophilic substitution of benzene?
And draw and eqn showing that.

Answer 21

• Characteristic reaction of benzene is electrophilic substitution
• Stage is yours pal:
  “Benzene”-H + Br₂ -> “Benzene”-Br + HBr

Question 22

What else is a must for benzene to undergo electrophilic substitution?

Answer 22

Use of a halogen carrier

Question 23

Name of Benzene containing OH group?

Answer 23

Phenol
(Hydroxybenzene)

Question 24

How is phenol more reactive than benzene?
(4-way)

Answer 24

• Oxygen lone pair donated
• into ring system
• Increases electron density
• Hence… voila

Question 25

If phenol more reactive than benzene, what's a "pro" for that? (True 2-way)

Answer 25

- No halogen carrier needed - For electrophilic substitution

Question 26

Name of benzene containing CH₃ group?

Answer 26

Methylbenzene (Toluene)

Question 27

Methylbenzene is also more reactive than benzene, but how so? (4-way)

Answer 27

- No lone pair - BUT, from bonded electrons in CH₃ - Donating additional e^- density into ring - Hence, increases reactivity

Question 28

What are the **4 different reactions** that can take place w/ benzene?

Answer 28

1. Nitration of Benzene 2. Friedel-Crafts alkylation of Benzene 3. Friedel-Crafts acylation of Benzene 4. Halogenation of Benzene

Question 29

What is required for 1. Nitration of Benzene? (2 things)

Answer 29

- Reflux w/ conc. HNO₃ + H₂SO₄ - Must be below 55°C

Question 30

Why must reaction be below 50°C? (1. Nitration of Benzene)

Answer 30

To avoid further substitution onto the ring

Question 31

Draw mechanism of Nitration of Benzene

Answer 31

The #1 stage is yours

Question 32

Which part of mechanism is slow (RDS) (1. Nitration of Benzene)

Answer 32

+ NO₂^{+ part}

Question 33

Which part of mechanism is fast? (1. Nitration of Benzene)

Answer 33

H going into Benzene part

Question 34

What is required for 2. Friedel-Crafts alkylation of Benzene? (2 things)

Answer 34

- Req. formation of **alkyl cation** e.g. CH₃Cl - Req. halogen carrier e.g. AlCl₃

Question 35

Draw mechanism of Friedel-Crafts alkylation of Benzene

Answer 35

The #2 stage is yours

Question 36

What is required for 3. Friedel-Crafts acylation of Benzene? (2 things)

Answer 36

- Req. **acyl group** e.g. R-C(=O)R₁ - Req. halogen carrier (just like 2nd typa rxn)

Question 37

Draw mechanism of Friedel-Crafts acylation of Benzene

Answer 37

The #3 stage is yours

Question 38

What is required for 4. Halogenation of Benzene? (2 things)

Answer 38

- Req. just the **halogens** e.g. Cl₂ - Req. **specific catalyst** for the halogen e.g. AlCl₃

Question 39

Draw mechanism of Halogenation of Benzene

Answer 39

The final #4 stage is yours

Question 40

How to name aromatic compounds with single substituents? (3-way, chill guy type)

Answer 40

- Prefix of such group - Attach to word benzene - e.g. Chlorobenzene or Methylbenzene or Nitrobenzene

Question 41

How to name aromatic compounds with multiple substituents **of the same type**? (4-way, such chill)

Answer 41

- Number at the beginning based on pos. - Additional prefix too - & w/ single substituents naming - e.g. 1, 3 - dichlorobenzene

Question 42

How to name aromatic compounds with multiple substituents **of different types**? (4-way, what a chill guy)

Answer 42

- Number first based on pos. - Additional prefix BUT for one that's multiple - Then single substituent of such - VOILA! e.g. 2,4,6 - trichlorophenol or 2,4,6 - trinitromethylbenzene

Question 43

What's the arylchloride?

Answer 43

Ring bonded w/ Cl

Question 44

What's the alkylchoride?

Answer 44

Straight chain bonded w/ Cl

Question 45

How is Arylchloride > Alkylchloride in terms of bond strength? (2-way + 3 instances)

Answer 45

- Due to lone pair of e^- arylchloride - perp. to plane of molecule - Delocalised into π-e^- system of ring - Bond = stronger + shorter - Less able to be replaced by nucleophiles

Question 46

Congrats on revamping this, now the stage really is yours

Answer 46

... in 4 days??