Aromatic chemistry

Question 1

structure of benzene

Answer 1

6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds

Question 2

bonding in benzene

Answer 2

6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds

Question 3

what forms the π system

Answer 3

This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring causing a π system

Question 4

what is the π system made up of

Answer 4

The π system is made up of two ring shaped clouds of electron density - one above the plane and one below it

Question 5

what type of compounds are aromatic compounds

Answer 5

regular and planar compounds with bond angles of 120o

The delocalisation of electrons means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character

Question 6

what is evidence for the delocalised ring structure of benzene

Answer 6

The bonds all being the same length

Question 7

why is benzene resistant to bromination

Answer 7

In benzene, there are no localised areas of high electron density, preventing it from being able to polarise the bromine molecule

In order for benzene to undergo electrophilic substitution with bromine, a halogen carrier must be present in the reaction e.g. AlBr3

Question 8

benzene reaction with oxygen

Answer 8

C6H6 (l) + 7O2 (g) → 6CO2 (g) + 3H2O (g)

Question 9

given that a large volume of oxygen is required for the reaction of benzene with oxygen what could this cause

Answer 9

incomplete combustion could occur
Therefore unreacted benzene may remain

This would lead to a smokey yellow flame as there would be insufficient oxygen available

Question 10

halogenation of benzene

Answer 10

aromatic compounds will react with halogens in the presence of a metal halide carrier

Question 11

what are the names of the metal halide carriers that are required when benzene reacts with halogens

Answer 11

iron(III) bromide - FeBr3
aluminium chloride - AlCl3

AlCl3 + Cl2 → AlCl4- + Cl+

FeBr3 + Br2 → FeBr4- + Br+

Question 12

explain the nitration of benzene

Answer 12

a nitro (-NO2) group replaces a hydrogen atom on the arene

The benzene is reacted with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)

Question 13

how is friedal-crafts acylation carried out

Answer 13

The delocalised electron ring in benzene can also act as a nucleophile, leading to their nucleophilic attack on acyl chlorides. This reaction is known as Friedel-Crafts acylation.

In order for the reaction to take place, a reactive intermediate must be produced from a reaction between the acyl chloride and an aluminium chloride catalyst.

This reactive intermediate is then attacked by the benzene ring.

At the end of the reaction, the H* ion removed from the ring reacts with the AICI4- ion to reform the aluminium chloride, indicating it to be a catalyst.
The product of this reaction is a phenylketone.

Question 14

what happens during the electrophilic substitution of benzene

Answer 14

one of the H atoms from benzene are replaced
The hydrogen atom is substituted by the electrophile

Question 15

explain the general electrophilic substitution mechanism

Answer 15

THE POSITIVE NATURE OF THE ELECTROPHILE MEANS IT IS ATTRACTED TO THE ELECTRON DENSITY OF THE DELOCALISED π RING IN BENZENE

THIS INTERMEDIATE IS FORMED. THE ELECTROPHILE IS BONDED TO THE BENZENE RING, BUT SO STILL IS THE H ATOM. THE DELOCALISED π SYSTEM IS TEMPORARILY BROKEN, TO LEAVE A PARTIALLY DELOCALISED ELECTRON SYSTEM AND A POSITIVE CHARGE

THE INTERMEDIATE LOSES A PROTON, AND THE ELECTRONS FROM THIS C-H BOND RESTORE THE DELOCALISED π SYSTEM

Question 16

Draw the overall electrophillic substitution mechanism of benzene

Question 17

what are the three steps of electrophillic substitution reaction in arenes

Answer 17

Generation of an electrophile
Electrophilic attack
Regenerating aromaticity

Question 18

draw the mechanism of nitration of benzene

Question 19

how is the electrophile NO2+ generated

Answer 19

by reacting concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)

Question 20

draw the halogenation of benzene

Question 21

draw the friedel craft acylation

Question 22

what is an acyl group

Answer 22

an alkyl group containing a carbonyl, C=O group

Question 23

what is the catalyst during the Friedel-Crafts acylation mechanism

Answer 23

ALCL3

Question 24

why do phenols react more rapidly with electrophiles than benzenes

Answer 24

one of the lone pairs of electrons on the oxygen atom in -OH overlaps with the π bonding system

This increases the electron density of the benzene ring making it more susceptible to electrophilic attack

The -OH group in phenols is activating and directs incoming electrophiles to the 2, 4, and 6 positions

Question 25

explain the bromination of phenols

Answer 25

Phenols also undergo electrophilic substitution reactions when reacted with bromine water at room temperature

Phenol decolourises the orange bromine solution to form a white precipitate of 2,4,6-tribromophenol

Question 26

what type of compounds are arenes

Answer 26

aromatic compounds that contain a benzene ring as part of their structure.

Question 27

why do arenes have high melting points but low boiling points

Answer 27

They have high melting points due to the high stability of the delocalised benzene ring, but low boiling points as they are non-polar molecules and generally cannot be dissolved in water.

Question 28

what is benzene

Answer 28

arene consisting of a ring of six carbon atoms each bonded to one hydrogen atom, giving it the molecular formula CH. This structure means benzene has a ring of delocalised electrons

Question 29

why does benzene not undergo electrophilic addition

Answer 29

this would involve breaking up the stable delocalised ring of electrons, it instead undergoes electrophilic substitution reactions.

Question 30

what is the area of high electron density of benzene

Answer 30

the delocalised ring making it susceptible to attack from electrophile

Question 31

two exceptions when naming aromatic compounds

Answer 31

benzalaldehyde
benzoic acid

Question 32

when naming whats the rule

Answer 32

listed alphabetically with the total lowest possible numbering when you add the positions up

Question 33

how do you measure stability of benzene

Answer 33

compare enthalpy change of hydrogenation in benzene to cyclohexene

Question 34

difference between localised and delocalised

Answer 34

Localized electrons are found between atoms and are confined to a specific region between two atoms, whereas

delocalized electrons are found above and below the atoms and are spread across several atoms.

Question 35

compare and contrast bromination of phenol and benzene

Answer 35

Similarity
* Both electrophilic substitution
(1)

Any two from:
Contrast
* No need of a halogen carrier with phenol
(1)

• oxygen’s lone pair of electrons interacts with the benzene ring of delocalised electrons so electrophilic attack more likely
  (1)
• Tri-substitution of phenol compared to mono for benzene
  (1)
• Bromination of phenol requires bromine in aqueous solution but benzene requires liquid bromine
  (1)
• Bromination of phenol requires room temperature but benzene requires heating (under reflux) / reflux
  (1)

Question 36

why is the enthalpy of hydrogenation of benzene might not be expected to be -360 jkmol-1

Answer 36

delocalisation of electrons in benzene ring results in more energy needed to break bonds in benzene