Aromatics Flashcards
What is Huckle’s rule of aromaticity?
4n + 2 π electrons
Requires molecule to be: cyclic, planar, and fully conjugated
What is anti-aromatic?
Planar, monocyclic, conjugated hydrocarbons
4n π electrons
How does the MO diagram of aromatic molecules change as their energy increases?
The energy increases as the number of nodes increase
Less stabilisation energy
What is α and β relating to stability of aromatics?
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
What are aromatic and anti-aromatic compounds wrt energy?
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
Why is cyclopentadienone highly reactive?
Resonance form is anti-aromatic
How do halogens affect electrophillic aromatic substitution?
Mildly deactivating - slower rates
Direct ortho and para
Why are halogens o/p directing?
Have lone pairs in high E orbitals which stabilise intermediates for o/p attack
What occurs when attempting to nitrate aniline?
-NMe2 group protonated by the strong acid
Therefore deactivating and meta directing
What form of control is electrophillic aromatic sub based on?
Mostly kinetic control
Exceptions: sulfonation at high T - ipso substitution
What is sulfonation mechanism of benzene at standard T?
Only mono as sulfonation is ewg
What is ipso-substitution?
Electrophile attack at position which already has a non-H substituent
What is nucleophillic aromatic substitution?
Requires halogen and EWGs
What is the RDS of SNAr?
Attack of nucleophile on aromatic ring → bond strength to LG so not important in rate
Where do different groups effect SNAr rates?
EWG - ortho/para to leaving group increase rate
EDG - ortho/para to leaving group decreases rate
meta has less influence
What is vicarious nucleophillic substitution?
Nucleophillic substituent of H
What is the structure of ortho-benzyne?
Alkyne has v strained triple bond → small HOMO-LUMO gap
LUMO v low so v electrophillic (soft)
How are arynes generated?
- Halobenzene + RLi (rate is Br>Cl>F as loss is RDS)
- F- source to attacks -SiMe3 substitutent with -OSO2CF3 LG
Where does a nucleophile attack an aryne?
Attack causes -ve charge to develop at other C
Therefore location which stabilises -ve charge (by induction!) most is major
What is the ene reaction with benzyne?
What is the Bergman Cyclisation?
Formation of para-benzyne from dialkynes
How do primary carbocations rearrange for the friedel-crafts reaction?
1,2-hydride shift
What is the Clemensen reduction?
Reduction of carbonyl to alkane, substrate must be stable to a strong acid (instead of base in Wolff-Kishner)
Reagents: Zn (Hg) and HCl
What is the Fries rearrangement giving ortho or para isomer in Friedel Crafts Acylation?
What is the Sandmeyer Reaction?
Conversion of arylamine to haloaryl
How do diazonium salts react?
- SN1 reaction via carbenium ion (H2O)
- Radical reaction (halide and acid - same result as sn1)
- Electrophillic aromatic substitution (nuc attacks N instead)
What is the mechanism of the Sandmeyer reaction?
Radical reaction pathway
What is the iodination modified sandmeyer reaction?
Aryl diazonium + KI → Aryl-I
No Cu required
What is the Baltz-Schimann reaction?
Fluorination from diazonium salt
What is the halex reaction?
Fluorination via SNAr
What is the bamberger reaction?
Aniline → para-fluoroaniline
Oxidation to N-oxide and then add HF
How can the Baeyer-Villiger Oxidation be used for phenylketones?
Phenylketone → phenylester
What is the Dakin reaction?
Phenolic substrate → 2xOH bonded to benzene
What is the Cumene process?
Benzene + allyl + O2 → Phenol + propanone
What is the Vilsmeier-Haack reaction?
Reaction of a substituted amide with phosphorus oxychloride and an electron-rich arene to produce an aryl aldehyde or ketone
What is the mechanism of the Vilsmeir-Haack reaction?
What is the Gattermann Koch formylation?
Aryl + CO, HCl, CuCl, AlCl3 → aryl formyl
What is the Reimer-Tiemann synthesis?
Phenol + CHCl3 + NaOH → Formylphenol
Via dichlorocarbene intermediate
What is the mechanism for the Reimer-Tiemann reaction?
How can alkyl benzenes be oxidised to carboxylate benzenes?
KMnO4 and heat
What are the properties of pyridine?
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
What is the structure of pyridine?
What methods can be used to synthesise pyridine?
Hantzsch or Hydroxylamine
What is the hydroxylamine route of pyridine synthesis?
How do pyridines become more reactive at the 2 and 4 positions?
Oxidise to N-oxide
What is the Hantzsch synthesis of pyridine?
