Aromatics Flashcards
what defines aromatic compounds?
- compounds containing a cyclic ring
- and delocalised π-electrons
what are aliphatic compounds ?
-non-aromatic can be linear, branched or cyclic
what are arenes ?
- type of aromatic specifically with benzene ring
- eg. benzene, phenol
what are aryl groups + 2 examples ?
- groups formed by removing a H from arene
- eg. pheny group = C₆H₅
- eg. benzyl = C₆H₅CH₂
Ortho position
- adjacent to eachother eg. 1,2- positions
Meta position
- separated by a C in the middle of 2 subsitutents
- eg. 1,3-positions
Para position
- opposite eachother
-eg. 1,4- positions
Ipso Position
- one substituent replaced by another on same C
kekules model of benzene ?
- shows alternating double and single bonds
what are the 2 main theories used to describe the covalent bonds ?
- valence bond theory
- molecular orbital theory
what is Molecular Orbital Theory (MOT)?
- covalent bonds = the formation of molecular orbitals that are spread out over the entire molecule
- does not involve hybridisation
what process does Molecular Orbital Theory use to form molecular orbitals?
- Linear Combination of Atomic Orbitals (LCAO)
- combines atomic orbitals to create bonding and anti-bonding
what is Valence Bond Theory (VBT)?
- overlap of atomic orbitals from same atom, forming shared pairs of electrons
- involves hybridisation
- e- are localised
what are the five conditions for bond formation between atomic orbitals (AOs)?
- interacting orbitals must be close in energy.
- If starting with two AOs, we must get two MOs.
- interacting orbitals must overlap efficiently.
- interacting orbitals must have suitable symmetry.
- Any number of orbitals can interact to form an equal number of MOs.
What is the hybridisation state of carbon in benzene + bond angle?
- 120°
- trigonal planar
-sp2
What is the MO description of benzene?
- 6 AO gives 6 Pi MO’s
- delocalisation of e-gives benzenes its stability
How are e- distributed around ring ?
- Evenly
What is the valence bond description of benzene?
- allows for resonance structures of benzene
what is empirical resonance energy?
- difference between expected and actual energy of hydrogenation
what is Huckels rule ?
- if they have (4n+2)pi electrons it is aromatic
Stages of electrophilic aromatic substitution (Ar-SE)?
- initial stage = energy rises as the electrophile (E⁺) approaches the π-system
- leading to first transition state (TS‡) where the π-system is disrupted and a partial σ-bond between E and C.
- progresses to a σ-complex (Wheland intermediate) with sp³ hybridized carbon.
-the reaction then moves to a second transition state (TS‡) featuring a partial bond to H - H leaves and re-aromatization occurs
What is the wheland intermediate/ arenium ion ?
- sigma bond complex
- contains sp3
- has multiple resonance structures
what are the common reaction of arenes ?
- nitration
- sulfonation
- halogentaion
- Friedel-Crafts alkylation
- Friedel-Crafts acylation
Why is nitration important in industry?
- Nitroarenes are used in explosives, pharmaceuticals, and as intermediates to produce aromatic amines for dyes.
reagents for nitration benzene
- concnetrated HNO3 and H2SO4
What is the chemical equation for the formation of the nitronium ion?
- H2SO4 + HNO3 —> NO2+ + H2On+ HSO4-
OR - 2H2SO4 + HNO3 —> NO2+ H3O+ + 2HSO4-
reagent for sulfonation of benzene
- H2SO4 , 150
What is the chemical equation for generating
SO3 from sulfuric acid?
2 H2SO4 ↔SO3 + H3O+ + HSO4 -
Why is sulfonation unique among aromatic substitution reactions?
- It is reversible.
what does sulfonation on benzene produce ?
- benzenesulfonic acid
what is an alternative method for introducing iodine into an aromatic ring?
- Iodine is often introduced through diazonium salts
- or oxidising agent eg. H2O2
3 reagents for Friedel-Crafts alkylation of benzene?
- alkyl halide (AlCl3)
- alkene (HF)
- ## aliphatic alcohol (HF)
What problem can arise with Friedel-Crafts alkylation?
- product is often more reactive than benzene, leading to over-alkylation.
how can over-alkylation be prevented in Friedel-Crafts alkylation?
- using an excess of benzene, which increases the chance of monoalkylation.
reageants and product for Friedel-Crafts acylation?
- acyl chloride or acid anydride
- with lewis acid catalyst
- produces aryl ketone
Why is a full mole of Lewis acid catalyst needed in Friedel-Crafts acylation?
- aryl ketone (product), is more basic than the acyl halide
- forms complex with the Lewis acid
- water and acid added at end to hydrolyse and liberate ketone
What is the electrophile in Friedel-Crafts acylation?
- the acylium ion
- RCO+
Does acylation or alkylation take place faster ?
- acylation
- ace wins the race
what are the final products of the acylation reaction after hydrolysis?
- an aryl ketone (main product)
- HX (e.g., HCl),
- Al(OH)3 (from hydrolysis of
AlCl3) - and additional HCl.
why are two products formed when benzene reacts with propyl chloride in alkylation?
- primary carbocation (CH₃CH₂CH₂⁺) reacts with benzene
- primary carbocation rearranges to a more stable secondary carbocation (CH₃)₂CH⁺, which reacts with benzene
