6.1.1 - Aromatic compounds

Question 1

Arenes

Answer 1

Aromatic molecules containing 1 or more benzene rings

Question 2

-OH

Answer 2

Hydroxy -

Question 3

NO2

Answer 3

Nitro -

Question 4

F

Answer 4

Fluoro -

Question 5

NH2

Answer 5

-amine

Question 6

Benzoic acid

Answer 6

Benzene ring attached to COOH group

Acid group is always highest priority (1’ priority)

Question 7

Phenyl -

Answer 7

Prefix if attached to an akyl chain w/ more than 6 carbons OR w/ a carbon chain w/ a functional group

Question 8

Kekule structure

Answer 8

Benzene is v. reactive

Diff C-C bond lengths

Question 9

Problems w/ Kekule’s structure

Answer 9

C-C bonds are all same lengths
Benzene does not undergo addn. reactions readily (resistance to reaction)
Enthalpy of hydrogenation 152kJ/mol less exothermic than expected

Question 10

Delocalised electrons

Answer 10

Electrons shared between more than two atoms

Question 11

Which model of benzene do we use now

Answer 11

Delocalised model

Question 12

Using a labelled diagram. disprove Kekule

Answer 12

p-orbitals overlap sideways —> Delocalised pi-system above and below
Planar carbon ring
All bonds 120 degrees

Question 13

Why do electrophiles react more easily w cyclohexane than w/ benzene

Answer 13

In benzene, pi-electrons are delocalised
In cyclohexane, the pi-electrons are localised
So benzene has a lower electron density
And so benzene polarises the electrophile less

Question 14

Priorities

Answer 14

COOH>…> hydrocarbons > F,Cl,Br,I> NO2

Question 15

How did the scientist disprove Kekule

Answer 15

Using diffraction to find the carbon-carbon lengths

Question 16

Why does phenol react more readily w/ electrophiles then benzene does

Answer 16

Lone pair from -OH group in phenol is drawn into the delocalised benzene ring
so electron density increases
and the electrophile is more strongly attracted

Question 17

Electron donating groups

Answer 17

-OH
-NH2
-CH3
2 (6),4 directing

Question 18

Electron withdrawing groups

Answer 18

-NO2
-COOH
3’, (5) directing
Less reactive so only one substitution

Question 19

Why is bromine not attracted to benzoic acid

Answer 19

-COOH is an electron withdrawing group
therefore pi-system becomes less dense
and will not attract electrophiles as strongly

Question 20

Electrophilic substitution w/ bromine and phenols

Answer 20

White ppt

Decolourises bromine water

Question 21

Nitration of benzene (nitrobenzene)

Answer 21

HNO3
H2SO4 (Conc. catalyst)
50 degrees

HNO3 + H2SO4 —> NO2^+ + HSO4 ^- + H2O
H+ + HSO4^- –> H2SO4

Question 22

Dinitrobenzene

Answer 22

2 HNO3
Conc. H2SO4
70 degrees

Question 23

Bromination of benzene

Answer 23

Occurs at RTP (electrophilic subsitution) in the presence of a halogen carrier
Overall: C6H6 + Br2 –> C6H5Br + HBr
Br2 + FeBr3 —> FeBr4^- + Br^+
H^+ + FeBr4^- —> FeBr3 + HBr

Question 24

General formula for electrophilic substitution w/ benzene

Answer 24

AX + C6H6 —> C6H5A + HX
A - Electrophile
X - halogen

Question 25

Halogen carriers

Answer 25

Benzene is non polar and so needs a halogen carrier to gnerate an electrophile e.g. FeBr3/AlBr3

Question 26

The higher the elctronegativity …

Answer 26

The more the electrophile

Question 27

Why isn’t phenol v. soluble

Answer 27

Non polar benzene ring

Question 28

What does phenol dissociate into

Answer 28

Phenoxide ion and H^+

Question 29

Reactions w/ phenol

Answer 29

Only strong bases (no carbonates)
Reactive metals e.g. Na
Br2 –> 2,4,6 tribromophenol

Question 30

Functional group priorities

Answer 30

-COOH (oic acid)
ester (-oate)
acid chloride (-oyl chloride)
-amide
A,K,A
-amine

Question 31

Tests for phenol

Answer 31

pH paper and Na2CO3 (aq) - pH paper turns red, no reaction
FeCl3 (aq) in NH3 sol. - solution turns purple
Bromine water - decolourises, white ppt may form

Question 32

Phenol w/ dilute vs conc HNO3

Answer 32

One substitution vs 3 substitutions