Aromatics

Question 1

what defines aromatic compounds?

Answer 1

• compounds containing a cyclic ring
• and delocalised π-electrons

Question 2

what are aliphatic compounds ?

Answer 2

-non-aromatic can be linear, branched or cyclic

Question 3

what are arenes ?

Answer 3

• type of aromatic specifically with benzene ring
• eg. benzene, phenol

Question 4

what are aryl groups + 2 examples ?

Answer 4

• groups formed by removing a H from arene
• eg. pheny group = C₆H₅
• eg. benzyl = C₆H₅CH₂

Question 5

Ortho position

Answer 5

• adjacent to eachother eg. 1,2- positions

Question 6

Meta position

Answer 6

• separated by a C in the middle of 2 subsitutents
• eg. 1,3-positions

Question 7

Para position

Answer 7

• opposite eachother
  -eg. 1,4- positions

Question 8

Ipso Position

Answer 8

• one substituent replaced by another on same C

Question 9

kekules model of benzene ?

Answer 9

• shows alternating double and single bonds

Question 10

what are the 2 main theories used to describe the covalent bonds ?

Answer 10

• valence bond theory
• molecular orbital theory

Question 11

what is Molecular Orbital Theory (MOT)?

Answer 11

• covalent bonds = the formation of molecular orbitals that are spread out over the entire molecule
• does not involve hybridisation

Question 12

what process does Molecular Orbital Theory use to form molecular orbitals?

Answer 12

• Linear Combination of Atomic Orbitals (LCAO)
• combines atomic orbitals to create bonding and anti-bonding

Question 13

what is Valence Bond Theory (VBT)?

Answer 13

• overlap of atomic orbitals from same atom, forming shared pairs of electrons
• involves hybridisation
• e- are localised

Question 14

what are the five conditions for bond formation between atomic orbitals (AOs)?

Answer 14

1. interacting orbitals must be close in energy.
2. If starting with two AOs, we must get two MOs.
3. interacting orbitals must overlap efficiently.
4. interacting orbitals must have suitable symmetry.
5. Any number of orbitals can interact to form an equal number of MOs.

Question 15

What is the hybridisation state of carbon in benzene + bond angle?

Answer 15

• 120°
• trigonal planar
  -sp2

Question 16

What is the MO description of benzene?

Answer 16

• 6 AO gives 6 Pi MO’s
• delocalisation of e-gives benzenes its stability

Question 17

How are e- distributed around ring ?

Answer 17

• Evenly

Question 18

What is the valence bond description of benzene?

Answer 18

• allows for resonance structures of benzene

Question 19

what is empirical resonance energy?

Answer 19

• difference between expected and actual energy of hydrogenation

Question 20

what is Huckels rule ?

Answer 20

• if they have (4n+2)pi electrons it is aromatic

Question 21

Stages of electrophilic aromatic substitution (Ar-SE)?

Answer 21

• 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

Question 22

What is the wheland intermediate/ arenium ion ?

Answer 22

• sigma bond complex
• contains sp3
• has multiple resonance structures

Question 23

what are the common reaction of arenes ?

Answer 23

• nitration
• sulfonation
• halogentaion
• Friedel-Crafts alkylation
• Friedel-Crafts acylation

Question 24

Why is nitration important in industry?

Answer 24

• Nitroarenes are used in explosives, pharmaceuticals, and as intermediates to produce aromatic amines for dyes.

Question 25

reagents for nitration benzene

Answer 25

• concnetrated HNO3 and H2SO4

Question 26

What is the chemical equation for the formation of the nitronium ion?

Answer 26

• H2SO4 + HNO3 —> NO2+ + H2On+ HSO4-
  OR
• 2H2SO4 + HNO3 —> NO2+ H3O+ + 2HSO4-

Question 27

reagent for sulfonation of benzene

Answer 27

• H2SO4 , 150

Question 28

What is the chemical equation for generating
SO3 from sulfuric acid?

Answer 28

2 H2SO4 ↔SO3 + H3O+ + HSO4 -
​

Question 29

Why is sulfonation unique among aromatic substitution reactions?

Answer 29

• It is reversible.

Question 30

what does sulfonation on benzene produce ?

Answer 30

• benzenesulfonic acid

Question 31

what is an alternative method for introducing iodine into an aromatic ring?

Answer 31

• Iodine is often introduced through diazonium salts
• or oxidising agent eg. H2O2

Question 32

3 reagents for Friedel-Crafts alkylation of benzene?

Answer 32

• alkyl halide (AlCl3)
• alkene (HF)
• ## aliphatic alcohol (HF)

Question 33

What problem can arise with Friedel-Crafts alkylation?

Answer 33

• product is often more reactive than benzene, leading to over-alkylation.

Question 34

how can over-alkylation be prevented in Friedel-Crafts alkylation?

Answer 34

• using an excess of benzene, which increases the chance of monoalkylation.

Question 35

reageants and product for Friedel-Crafts acylation?

Answer 35

• acyl chloride or acid anydride
• with lewis acid catalyst
• produces aryl ketone

Question 36

Why is a full mole of Lewis acid catalyst needed in Friedel-Crafts acylation?

Answer 36

• 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

Question 37

What is the electrophile in Friedel-Crafts acylation?

Answer 37

• the acylium ion
• RCO+

Question 38

Does acylation or alkylation take place faster ?

Answer 38

• acylation
• ace wins the race

Question 39

what are the final products of the acylation reaction after hydrolysis?

Answer 39

• an aryl ketone (main product)
• HX (e.g., HCl),
• Al(OH)3 (from hydrolysis of
  AlCl3)
• and additional HCl.

Question 40

why are two products formed when benzene reacts with propyl chloride in alkylation?

Answer 40

• primary carbocation (CH₃CH₂CH₂⁺) reacts with benzene
• primary carbocation rearranges to a more stable secondary carbocation (CH₃)₂CH⁺, which reacts with benzene