6.1 Aromatic compounds

Question 1

benzene

Answer 1

is a major feedstock used in many industries:
pharmaceuticals
polymers
dyes
explosives.
Is highly carcinogenic.
Simplest arene with an empirical formula of CH and a molecular formula of C6H6

Question 2

kekule’s model

Answer 2

suggested a cyclic structure for benzene with 3 alternating carbon-carbon double bonds. Proposed benzenes low reactivity was due to a rapid equilibrium between 2 isomers.
The rapidly changing positions of the double bonds makes reactions difficult.

Question 3

problems with kekule’s model

Answer 3

1. bond length C-C and C=C have different bond lengths so benzene would be irregular but imaging shows it has a regular structure.
2. hydrogenation enthalpy. Benzene can be saturated with hydrogen to form cyclohexane just like cyclohexene. ∆H = -120KJmol.Kekule’s model (cyclohex-1.3.5-triene) would expect to have a hydrogenation energy of -360kJmol. Benzene actually has a ∆H of -208kJmol. So its bonds are more stable than alkenes.
3. colour. benzene doesn’t turn bromine water colourless.

Question 4

delocalised model

Answer 4

each carbon creates 3 covalent sigma bonds. The remaining electron is found in a p-orbital at a right angle above and below the carbon atom.
The p-orbitals overlap evenly creating a ring of delocalised electrons above and below the structure.

Question 5

naming arenes

Answer 5

When naming, the position of substituted hydrogens are numbered to give the lowest possible number. 1 group will always occupy position 1. the ring can either be given as the suffix ‘benzene’ or as the suffix ‘phenyl’ if with an alcohol (OH) or amine (-NH2) groups.

Question 6

bromination of benzene

Answer 6

iron catalyst to make bromine more electronegative
2Fe + 3Br2 → 2FeBr3
accepts a lone pair from a bromine
[see physical flashcards for mechanism]

Question 7

nitration of benzene

Answer 7

concentration nitric acid in the presence of concentrated sulfuric acid
produces the electrophile NO2+
HNO3 + 2H2SO4 → NO2+ + HSO4 +H3O+
[see flashcard for mechanism]

Question 8

sulphonation of benzene

Answer 8

electrophile is sulphur trioxide SO3
requires concentration sulfuric acid and heating under reflux
[see physical flashcard for mechanism]

Question 9

chlorination of benzene

Answer 9

chlorine is substituted in a similar way to bromine.
aluminium catalyst s often used
[see physical flashcards for mechanism]

Question 10

alkylation of benzene

Answer 10

requires aluminium chloride catalyst
carried out under reflux
adds to a CH3
[see physical flashcards for mechanism]

Question 11

acylation of benzene

Answer 11

is the same as alkylation but with an acyl group
CH3COCl + AL3 ⇌ CH3CO+ +AlCl4-
[see physical flashcard for structure of acyl]
[see physical flashcard for mechanism]

Question 12

phenols

Answer 12

a type of organic chemical with a hydroxyl -OH group bonded to an aromatic ring
simplest- C6H5OH
[see physical flashcards for structure]

Question 13

phenols as a weak acid

Answer 13

less soluble in water due to the non-polar benzene ring

when dissolved in water, it partially dissociates forming phenoxide and a proton. (partially dissociates_

Question 14

comparing the acidity of phenols to other carbonyls

Answer 14

more acidic than alcohols but less than carboxylic acids
-ethanol doesnt react with sodium hydroxide or sodium carbonate
- phenols and carboxylic acids react with solutions of strong bases (eg NaOH)
- carboxylic acids are strong enough to react with the weak base, sodium carbonate
a reaction with sodium carbon allows to distinguish between a phenol and carboxylic acids

Question 15

reaction of phenol with sodium hydroxide

Answer 15

[see physical flashcards for mechanisms]

reacts to form the salt sodium phenoxide and water in a neutralisation reaction

Question 16

bromination of phenols

Answer 16

aqueous solution to form a white precipitate of 2,4,6-tribromophenol
it decolourises bromine
a halogen carrier catalyst isn’t required
carried out at room temp
[see physical flashcards for mechanisms]

Question 17

nitration of phenol

Answer 17

phenol reacts readily with dilute nitric acid at room temp
a mixture of 2-nitrophenol and 4-nitrophenol forms
[see physical flashcards for mechanism]

Question 18

comparing the reactivity of phenol and benzene

Answer 18

bromine and nitric react more readily with phenol than benzene. phenol is nitrated with dilute nitric acid rather than needing conc nitric or sulphuric acids
the increased reactivity is caused by a lone pair from oxygen p-orbital of -OH group donated into 𝝅- system of phenol
electron density of benzene ring in the phenol is increased which attacks electrophiles more strongly

Question 19

directing groups

Answer 19

with phenylamine, the -NH2 group activates the ring as the aromatic ring reacts more readily with electrophiles. it directs the 2nd substituent to positions 3 or 4
in nitrobenzene, the -NO2 group deactivates the aromatic ring as the ring reacts less readily. it directs the 2nd substituent to position 3

Question 20

chemistry of amines

Answer 20

The cyclic structure of benzene makes it more stable than expected, compared to alternating single and double structure o Kekule’s model.
benzene is disrupted at high temperatures and more vigorous conditions.

Question 21

bromination

Answer 21

[see flashcard for mechanisms]
addition of bromine to alkenes.
Requires an iron catalyst- iron( III) bromide to make bromine more electrophilic ( electrophile- receives electrons). So a catalyst is required.
Its thought that FeBr3 helps polarise the bromine molecule by accepting a lone pair from one of the bromine atoms. Bromine molecule becomes so polarised it splits into Br+ and FeBr4-.

Question 22

nitration

Answer 22

[see flashcard for mechanism]
Concentrated nitric acid in presence of concentrated sulphuric acid produces the electrophile NO2+. Known as the ‘nitrating mixture’.

Question 23

sulphonation

Answer 23

[see flashcard for mechanism]

Electrophile is sulphur trioxide, SO3. requires concentrate sulphuric acid and heating under reflux for several hours

Question 24

chlorination

Answer 24

A chlorine atom may be substituted in a way similar to a bromine atom. Aluminium catalyst is often used. Aluminium chloride reacts violently with water so the reaction must be carried out under anhydrous conditions.
Same mechanism as bromination

Question 25

alkylation by Friedel-Crafts reaction

Answer 25

Requires aluminium chloride catalyst and is carried out under reflux. Chloroalkanes react with the catalyst to form a complex, R+AlCl4-, where R+ is the electrophile.
CH3Cl + AlCl3 → CH3+ + AlCl4-

Question 26

acylation

Answer 26

C H3COCl + AlAl3 ⇌ CH3CO+ + AlAl4-

Question 27

phenol

Answer 27

are aromatic compounds that have an alcohols hydroxyl group attached directly to the ring.
Alcohols containing a benzene ring but don’t have a hydroxyl group attached directly to it are called aromatic alcohols.

Question 28

phenol structure

Answer 28

oxygen atom bonded to the benzene has 2 lone pairs in p-orbitals. As the ring is electron deficient, one of the p-orbitals overlaps into the ring structure to become delocalised.
Thus increasing the rings electrons density, allowing electrophilic substitution reactions to occur easily.

The p-orbitals overlapping into the ring, weakens the O-H bond so phenol can donate H+ and forms stable phenoxide ions