Aromatics

Question 1

What is Huckle’s rule of aromaticity?

Answer 1

4n + 2 π electrons

Requires molecule to be: cyclic, planar, and fully conjugated

Question 2

What is anti-aromatic?

Answer 2

Planar, monocyclic, conjugated hydrocarbons

4n π electrons

Question 3

How does the MO diagram of aromatic molecules change as their energy increases?

Answer 3

The energy increases as the number of nodes increase

Less stabilisation energy

Question 4

What is α and β relating to stability of aromatics?

Answer 4

α - coulomb integral which is E of an e- in an isolated 2p orbital, this is a stabilising as negative

β - resonance integral, E with e- shared which is -ve and set to 0 for non-adjacent atoms

Question 5

What are aromatic and anti-aromatic compounds wrt energy?

Answer 5

Aromatic: compounds where π-system lower in E than that of acrylic counterpart

Anti-aromatic: compounds where π-system higher in E than that of acrylic counterpart

Question 6

Why is cyclopentadienone highly reactive?

Answer 6

Resonance form is anti-aromatic

Question 7

How do halogens affect electrophillic aromatic substitution?

Answer 7

Mildly deactivating - slower rates

Direct ortho and para

Question 8

Why are halogens o/p directing?

Answer 8

Have lone pairs in high E orbitals which stabilise intermediates for o/p attack

Question 9

What occurs when attempting to nitrate aniline?

Answer 9

-NMe₂ group protonated by the strong acid

Therefore deactivating and meta directing

Question 10

What form of control is electrophillic aromatic sub based on?

Answer 10

Mostly kinetic control

Exceptions: sulfonation at high T - ipso substitution

Question 11

What is sulfonation mechanism of benzene at standard T?

Answer 11

Only mono as sulfonation is ewg

Question 12

What is ipso-substitution?

Answer 12

Electrophile attack at position which already has a non-H substituent

Question 13

What is nucleophillic aromatic substitution?

Answer 13

Requires halogen and EWGs

Question 14

What is the RDS of SNAr?

Answer 14

Attack of nucleophile on aromatic ring → bond strength to LG so not important in rate

Question 15

Where do different groups effect SNAr rates?

Answer 15

EWG - ortho/para to leaving group increase rate

EDG - ortho/para to leaving group decreases rate

meta has less influence

Question 16

What is vicarious nucleophillic substitution?

Answer 16

Nucleophillic substituent of H

Question 17

What is the structure of ortho-benzyne?

Answer 17

Alkyne has v strained triple bond → small HOMO-LUMO gap

LUMO v low so v electrophillic (soft)

Question 18

How are arynes generated?

Answer 18

1. Halobenzene + RLi (rate is Br>Cl>F as loss is RDS)
   
   1. F- source to attacks -SiMe₃ substitutent with -OSO₂CF₃ LG

Question 19

Where does a nucleophile attack an aryne?

Answer 19

Attack causes -ve charge to develop at other C

Therefore location which stabilises -ve charge (by induction!) most is major

Question 20

What is the ene reaction with benzyne?

Answer 20

Question 21

What is the Bergman Cyclisation?

Answer 21

Formation of para-benzyne from dialkynes

Question 22

How do primary carbocations rearrange for the friedel-crafts reaction?

Answer 22

1,2-hydride shift

Question 23

What is the Clemensen reduction?

Answer 23

Reduction of carbonyl to alkane, substrate must be stable to a strong acid (instead of base in Wolff-Kishner)

Reagents: Zn (Hg) and HCl

Question 24

What is the Fries rearrangement giving ortho or para isomer in Friedel Crafts Acylation?

Answer 24

Question 25

What is the Sandmeyer Reaction?

Answer 25

Conversion of arylamine to haloaryl

Question 26

How do diazonium salts react?

Answer 26

1. SN1 reaction via carbenium ion (H₂O) 2. Radical reaction (halide and acid - same result as sn1) 1. Electrophillic aromatic substitution (nuc attacks N instead)

Question 27

What is the mechanism of the Sandmeyer reaction?

Answer 27

Radical reaction pathway

Question 28

What is the iodination modified sandmeyer reaction?

Answer 28

Aryl diazonium + KI → Aryl-I No Cu required

Question 29

What is the Baltz-Schimann reaction?

Answer 29

Fluorination from diazonium salt

Question 30

What is the halex reaction?

Answer 30

Fluorination via SNAr

Question 31

What is the bamberger reaction?

Answer 31

Aniline → para-fluoroaniline Oxidation to N-oxide and then add HF

Question 32

How can the Baeyer-Villiger Oxidation be used for phenylketones?

Answer 32

Phenylketone → phenylester

Question 33

What is the Dakin reaction?

Answer 33

Phenolic substrate → 2xOH bonded to benzene

Question 34

What is the Cumene process?

Answer 34

Benzene + allyl + O₂ → Phenol + propanone

Question 35

What is the Vilsmeier-Haack reaction?

Answer 35

Reaction of a substituted amide with phosphorus oxychloride and an electron-rich arene to produce an aryl aldehyde or ketone

Question 36

What is the mechanism of the Vilsmeir-Haack reaction?

Answer 36

Question 37

What is the Gattermann Koch formylation?

Answer 37

Aryl + CO, HCl, CuCl, AlCl₃ → aryl formyl

Question 38

What is the Reimer-Tiemann synthesis?

Answer 38

Phenol + CHCl₃ + NaOH → Formylphenol Via dichlorocarbene intermediate

Question 39

What is the mechanism for the Reimer-Tiemann reaction?

Answer 39

Question 40

How can alkyl benzenes be oxidised to carboxylate benzenes?

Answer 40

KMnO₄ and heat

Question 41

What are the properties of pyridine?

Answer 41

Polarised due to electronegativity of N lp on N not delocalised into π-system as orthogonal, is the HOMO and the LUMO is an anti-bonding π-orbital Electron poor at C-2 and C-4

Question 42

What is the structure of pyridine?

Answer 42

Question 43

What methods can be used to synthesise pyridine?

Answer 43

Hantzsch or Hydroxylamine

Question 44

What is the hydroxylamine route of pyridine synthesis?

Answer 44

Question 45

How do pyridines become more reactive at the 2 and 4 positions?

Answer 45

Oxidise to N-oxide

Question 46

What is the Hantzsch synthesis of pyridine?

Answer 46